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Integration of Single-Photon Sources and Detectors on GaAs

Integration of Single-Photon Sources and Detectors on GaAs hv photonics Article Integration of Single-Photon Sources and Detectors on GaAs Giulia Enrica Digeronimo *, Maurangelo Petruzzella, Simone Birindelli, Rosalinda Gaudio, Sartoon Fattah Poor, Frank W.M. van Otten and Andrea Fiore Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; m.petruzzella@tue.nl (M.P.); s.birindelli@tue.nl (S.B.); rosalinda.gaudio@gmail.com (R.G.); sartoon.fattahpoor@gmail.com (S.F.P.); F.W.M.v.Otten@tue.nl (F.W.M.v.O.); a.fiore@tue.nl (A.F.) * Correspondence: g.e.digeronimo@tue.nl; Tel.: +31-402-475-057 Received: 13 September 2016; Accepted: 16 October 2016; Published: 21 October 2016 Abstract: Quantum photonic integrated circuits (QPICs) on a GaAs platform allow the generation, manipulation, routing, and detection of non-classical states of light, which could pave the way for quantum information processing based on photons. In this article, the prototype of a multi-functional QPIC is presented together with our recent achievements in terms of nanofabrication and integration of each component of the circuit. Photons are generated by excited InAs quantum dots (QDs) and routed through ridge waveguides towards photonic crystal cavities acting as filters. The filters with a transmission of 20% and free spectral range 66 nm are able to select a single excitonic line out of the complex emission spectra of the QDs. The QD luminescence can be measured by on-chip superconducting single photon detectors made of niobium nitride (NbN) nanowires patterned on top of a suspended nanobeam, reaching a device quantum efficiency up to 28%. Moreover, two electrically independent detectors are integrated on top of the same nanobeam, resulting in a very compact (2) autocorrelator for on-chip g () measurements. Keywords: quantum photonics integrated circuits; superconducting single photon detectors; quantum dots; photonic crystals cavities 1. Introduction Quantum information processing (QIP) is a well-established scientific field that opens unconventional perspectives for information processing. QIP uses quantum bits (qubits) as units of information. While qubits can be based on atomic and solid state systems, photons have emerged as a promising approach for QIP [1]. However, it is very challenging to implement quantum processing functionalities on a large scale with bulk optics due to the extreme stability requirements and large coupling losses. The concept of quantum photonic integration [2] has been investigated as a scalable approach to photonic QIP, in principle enabling experiments with a few tens of photons. In order to realize a fully-functional quantum photonic integrated circuit (QPIC), all the required quantum optical components, such as single-photon sources, passive circuit elements, and single-photon detectors, must be integrated on a single chip. The use of III-V materials, especially GaAs and InP compounds, is very promising for QPIC [3] because it allows the integration of on-demand single-photon sources. Semiconductor quantum dots (QDs) have been shown to be nearly ideal sources of non-classical light [4] that can be efficiently routed into nanophotonic circuits [5–8]. As compared to integrated single-photon sources based on parametric processes in LiNbO [9] or Si waveguides [10], QDs offer the advantage of much higher efficiency and easier filtering of the pump beam. Moreover, approaches to reproducibly control the exciton and cavity energy on chip, as needed to realize a scalable circuit, have been developed [11–14]. On the other side, superconducting single photon detectors (SSPDs) [15] based on niobium nitride (NbN) nanowires combine high detection efficiency, low dark count rates, Photonics 2016, 3, 55; doi:10.3390/photonics3040055 www.mdpi.com/journal/photonics Photonics 2016, 3, 55 2 of 12 and ultra-fast response, making them promising candidates for QIP [16]. The integration of Photonics 2016, 3, 55 2 of 12 such detectors into nanophotonic circuits has been proved on different platforms: on GaAs with waveguides [17] and QDs [18], and on Si with waveguides [19] and nanobeam cavities [20]. Particularly low dark count rates, and ultra-fast response, making them promising candidates for QIP [16]. The inintegrat [18], the ion possibility of such detec of to on-chip rs into nanop detection hotonic of the circQDs uits hemission as been pro light ved has on di been fferent pr oved; platforhowever ms: on , the demonstrated GaAs with waveguide circuit lacked s [17] an mor d QD e complex s [18], an str d uctur on Si es with (high-finesse waveguides nanocavities, [19] and nanobe beam am cav splitters, ities etc.) [20]. Particularly in [18], the possibility of on-chip detection of the QDs emission light has been needed for photonic QIP. proved; however, the demonstrated circuit lacked more complex structures (high-finesse Here, we present a prototype architecture of a fully integrated photonic quantum circuit on nanocavities, beam splitters, etc.) needed for photonic QIP. a GaAs platform (Figure 1), consisting of InAs QDs, photonic crystal waveguides (PhCWGs) and Here, we present a prototype architecture of a fully integrated photonic quantum circuit on a cavities (PhCCs), and SSPDs patterned on top of suspended nanobeams (SNBs). Thanks to the addition GaAs platform (Figure 1), consisting of InAs QDs, photonic crystal waveguides (PhCWGs) and of PhCCs and PhCWGs, the emission from one or several QDs can be filtered on-chip so that single cavities (PhCCs), and SSPDs patterned on top of suspended nanobeams (SNBs). Thanks to the photons originating from a single excitonic line are funneled to circuits and then measured. We choose addition of PhCCs and PhCWGs, the emission from one or several QDs can be filtered on-chip so to integrate the superconductive nanowires on top of a SNB instead of a ridge waveguide since that single photons originating from a single excitonic line are funneled to circuits and then shorter lengths are sufficient to reach high absorption due to a tighter field confinement [21]. This is measured. We choose to integrate the superconductive nanowires on top of a SNB instead of a ridge expected to reduce the role of wire inhomogeneities [22] and increase efficiency. Moreover, it is waveguide since shorter lengths are sufficient to reach high absorption due to a tighter field possible confinem toent place [21]. two Thelectrically is is expected independent to reduce th detectors e role ofon wire top inh ofom the ogeneitie SNB, such s [22] that and theinc same rease guided mode efficienc is pr y. obed Moreov byer the , it two is poSSPDs ssible to pl [23ace ]. The two autocorr electrically indepe elators together ndent detec with tothe rs on top PhC of filters the S would, NB, for such that the same guided mode is probed by the two SSPDs [23]. The autocorrelators together with example, allow the performance of a Hanbury-Brow-Twiss (HBT) experiment [24] completely on-chip, the PhC filters would, for example, allow the performance of a Hanbury-Brow-Twiss (HBT) which would be a meaningful validation of a fully functional QPIC. However, the integration of these experiment [24] completely on-chip, which would be a meaningful validation of a fully functional different functionalities on the same platform shows significant technological challenges. In this paper, QPIC. However, the integration of these different functionalities on the same platform shows a low-temperature fabrication process that allows for the integration of the different components significant technological challenges. In this paper, a low-temperature fabrication process that allows of the above-mentioned circuit is presented. Further, the characteristics and functionalities of each for the integration of the different components of the above-mentioned circuit is presented. Further, component (SSPDs, QDs and PhC filters) of the circuit fabricated at low temperature are shown in the characteristics and functionalities of each component (SSPDs, QDs and PhC filters) of the circuit detail, and it will be demonstrated how the required functionality and performance is obtained for fabricated at low temperature are shown in detail, and it will be demonstrated how the required each component. functionality and performance is obtained for each component. Figure 1. Sketch of the proposed approach for the quantum photonic integrated circuits (QPIC). Single Figure 1. Sketch of the proposed approach for the quantum photonic integrated circuits (QPIC). photons are produced via spontaneous emission of excitons in quantum dots (QDs), efficiently Single photons are produced via spontaneous emission of excitons in quantum dots (QDs), efficiently funneled in waveguides, filtered by a photonic crystal cavity (PhCC) and detected by waveguide funneled in waveguides, filtered by a photonic crystal cavity (PhCC) and detected by waveguide superconducting single photon detectors (SSPDs). superconducting single photon detectors (SSPDs). 2. Materials and Methods 2. Materials and Methods The fabrication process starts with the sputtering of a thin NbN film on top of the substrate for The fabrication process starts with the sputtering of a thin NbN film on top of the substrate for the the fabrication of the SSPDs. In this step, it is crucial to have a substrate’s surface with very low fabrication roughness of (<0 the .5 nm SSPDs. ), sincIn e th this e interna step,l it qu isantum crucial efto ficie have ncy of a substr the detec ate’s tor surface depends with on th very e quali low ty r and oughness uniformity of the superconducting film [25]; consequently, a rough film could produce an inefficient (<0.5 nm), since the internal quantum efficiency of the detector depends on the quality and uniformity SSPD. Therefore, it is crucial to start from a GaAs/AlGaAs heterostructure featuring very low of the superconducting film [25]; consequently, a rough film could produce an inefficient SSPD. roughness despite the QDs embedded inside. The sample is grown by molecular beam epitaxy on an Therefore, it is crucial to start from a GaAs/AlGaAs heterostructure featuring very low roughness undoped GaAs substrate where a layer of GaAs with a 320 nm thickness including low-density InAs despite the QDs embedded inside. The sample is grown by molecular beam epitaxy on an undoped QDs [26] is grown on top of a 1.5-µ m-thick layer of Al0.7Ga0.3As. At the end of the growth, an extra Photonics 2016, 3, 55 3 of 12 GaAs substrate where a layer of GaAs with a 320 nm thickness including low-density InAs QDs [26] is grown on top of a 1.5-m-thick layer of Al Ga As. At the end of the growth, an extra final step 0.7 0.3 of deposition of a few atomic layers of GaAs, at 20 C above the oxide removal temperature (585 C) under an As flux, yields samples with a smooth surface (root mean square roughness = 0.4 nm). After the growth, a 5-nm-thick NbN film is deposited on top of the wafer by means of direct current reactive magnetron sputtering. A Nb target in Ar + N mixture at total pressure P = 2.3 mTorr 2 TOT is sputtered for the deposition of the film. The deposition is carried out at a nominal temperature of 400 C, with a target current of 250 mA and a target voltage of 380 V, conditions similar to those previously used to fabricate high-performance waveguide SSPDs [22]. This annealing step at 400 C does not significantly change the emission spectra of the QDs, which display only a 5 nm blue shift after the sputtering process. The contact pads and alignment markers, consisting of 14 nm Ti and 140 nm Au layers, are defined through optical lithography, electron beam evaporation, and lift-off. The nanowires are patterned by electron beam lithography on a 100-nm-thick mask of hydrogen silsequioxane (HSQ) and then transferred to the NbN film via reactive ion etching (RIE) in an Ar/SF plasma. Figure 2a shows a scanning electron microscopy (SEM) image of an etched wire (100 nm width, 160 nm pitch, and 20 m length) covered with the HSQ mask. As can be noticed, the design was adjusted in the areas close to the corners in order to avoid current crowding around sharp edges [27]. In order to obtain high quality photonic crystal structures, the pattern has to be etched through a hard mask, which is usually made of a thick SiN film (200–400 nm) deposited at high temperature (300 C) via plasma enhanced chemical vapor deposition (PECVD). However, in this specific case, to preserve the NbN quality film, it is essential to avoid thermal stress, and for this reason it is important that temperatures above 200 C are never reached. A low-temperature recipe (70 C) for the deposition of a 400 nm SiN hard mask has been used based on an induced coupled plasma (ICP) PECVD. A 320-nm-thick layer of ZEP520A as electron beam resist is deposited in the next step, and the design including the RWGs, the PhCs structures, and the SNBs is exposed with electron beam lithography, taking special care on the alignment of the wires with the SNB. Two subsequent RIE steps at room temperature are performed in order to etch through the SiN and GaAs layers. The higher density and different composition of the LT SiN hard mask required an optimization of the composition of the reactive gases; specifically, a CHF /O plasma is used for the SiN etching, and a SiCl plasma for 3 2 4 the GaAs etching. The sacrificial AlGaAs layer is removed via wet etching in an HCl-based solution, which results in a geometrically well-defined etching profile due to different etching rates in different crystallographic orientations in AlGaAs [8]. A final step of dry etching to remove the residual SiN hard mask is usually required in order to open the metal contacts. However, it has been noticed that in this step the nanowires endure significant damage due to a lateral etching that drastically reduces the width of the wire and, correspondingly, the value of the critical current. An intermediate step of optical lithography and SiN etching has been added in order to open only the metal contact, avoiding the SiN etching in the active parts of the circuit. The SiN layer (50 nm) left on top does not seem to significantly modify the optical properties of the PhC structures (Figure 2b). Indeed, the quality factor (Q) of a cavity fabricated with this recipe is around 1000, as expected from the simulation for the chosen design. An example of a complete QPIC is presented in Figure 2 in which its different parts are visible: (a) the nanowire placed on top of a 15-m-long, 1-m-wide nanobeam, the two PhC waveguides with the PhC cavity and (b) the other SNB that is tapered to a RWG with a 5 m width at the cleaved facet. The electro-optical characterization of SSPD is performed at an estimated sample temperature of 2.3 K in a continuous flow helium cryostat. During the electrical measurements, the detector is biased through the DC port of a bias-T with a voltage source connected in series with a 10 W bias resistor. The RF port of the bias-T is instead connected to a 50 W cap. The electrical contact with the device is established with a 50 W microwave probe connected to the circuit through a SMA coaxial cable. During the optical measurement, the 50 W cap is removed and a 4 dB attenuator connected to a series of four amplifiers is now added to the RF port of the bias-T. A continuous wave 1300 nm diode laser is sent through a polarization-maintaining lensed fiber into the waveguides, and the detector Photonics 2016, 3, 55 4 of 12 output signal coming from the amplifiers is then sent to a counter. Both the lensed fiber and the contact probes are mounted on piezoelectric positioners, which are thermally anchored to the cold plate to Photonics 2016, 3, 55 4 of 12 minimize the thermal load to the detector. The QDs and the PhC filters are characterized through through the use of a non-resonant pump laser (780 nm) that is focused by a microscope objective (NA 0.4) the use of a non-resonant pump laser (780 nm) that is focused by a microscope objective (NA 0.4) into into the the samp sample le to to exc excite ite the the QD QDs. s. ThThe e phot photoluminescence oluminescence (PL) (PL) sign signal al is co is ll collected ected from from the the end end facet facet of the RWG through the lensed fiber, sent to a spectrometer (focal length = 1 m) and detected by an of the RWG through the lensed fiber, sent to a spectrometer (focal length = 1 m) and detected by InGaAs array detector. All simulations both for the detectors and the filters are performed using a an InGaAs array detector. All simulations both for the detectors and the filters are performed using finite-element mode solver (COMSOL Multiphysics). a finite-element mode solver (COMSOL Multiphysics). Figure 2. Scanning electron microscopy (SEM) picture of the full QPIC. Zoomed pictures of SSPD (a) Figure 2. Scanning electron microscopy (SEM) picture of the full QPIC. Zoomed pictures of SSPD (a) and PhCC (b). and PhCC (b). 3. R 3.esu Results lts 3.1. Detectors 3.1. Detectors 3.1.1. Design 3.1.1. Design The chosen design for the detectors on top of a SNB is based on simulations in which The chosen design for the detectors on top of a SNB is based on simulations in which the the absorptances for nanowires on top of either GaAs waveguides or nanobeams are compared. absorptances for nanowires on top of either GaAs waveguides or nanobeams are compared. In the In the first design, an NbN meander (5-nm-thick, 100-nm-wide, and with a 250 nm pitch) was first design, an NbN meander (5-nm-thick, 100-nm-wide, and with a 250 nm pitch) was placed on top placed on top of a GaAs/Al Ga As waveguide (1.85-m-wide and 250-nm-deep). Assuming 0.7 0.3 of a GaAs/Al0.7Ga0.3As waveguide (1.85-µm-wide and 250-nm-deep). Assuming a complex NbN a complex NbN reflective index ñ = 4.35 i4.65, the absorption coefficients for TE and TM NbN reflective index ñ NbN = 4.35 − i4.65, the absorption coefficients for TE and TM polarizations provide a polarizations provide a total absorptance of 84% and 82% for a 50-m-long waveguide, respectively [21]. total absorptance of 84% and 82% for a 50-µm-long waveguide, respectively [21]. While the While the theoretical calculations suggest a high absorptance, the experimental device detection theoretical calculations suggest a high absorptance, the experimental device detection efficiency for efficiency for this detector ’s design does not go above 20% [17], indicating that the inhomogeneities [22] this detector’s design does not go above 20% [17], indicating that the inhomogeneities [22] could play could play a major role on limiting the detector ’s efficiency. Therefore, a new design, with a NbN a major role on limiting the detector’s efficiency. Therefore, a new design, with a NbN meander on meander on top of a GaAs SNB surrounded by air, has been proposed as a possible improvement [21]. top of a GaAs SNB surrounded by air, has been proposed as a possible improvement [21]. As the As the index contrast between GaAs and the bottom cladding layer is higher as compared to the index contrast between GaAs and the bottom cladding layer is higher as compared to the previous previous design, the field is pushed towards the top GaAs/NbN interface and the absorptance is design, the field is pushed towards the top GaAs/NbN interface and the absorptance is improved. improved. For the simulation, a NbN meander (5-nm-thick, 100-nm-wide nanowire with a 160 nm For the simulation, a NbN meander (5-nm-thick, 100-nm-wide nanowire with a 160 nm pitch) on top pitch) on top of a 1-m-wide and 300-nm-thick SNB is considered. A 100-nm-thick HSQ film of a 1-µm-wide and 300-nm-thick SNB is considered. A 100-nm-thick HSQ film (amorphous silicon (amorphous silicon oxide) remaining on top of the nanowires after processing, as well as a residual oxide) remaining on top of the nanowires after processing, as well as a residual 50-nm-thick SiN layer, 50-nm-thick SiN layer, are taken into account during the simulation. The absorption coefficients for TE are taken into account during the simulation. The absorption coefficients for TE and TM polarizations 1 1 and TM polarizations at 1310 nm are calculated as = 1166 cm and = 1399 cm , by using −1 TE −1 TM at 1310 nm are calculated as αTE = 1166 cm and αTM = 1399 cm , by using the same ñ NbN as in [21]. the same ñ as in [21]. The fundamental TE mode, the only interesting one in terms of integration NbN The fundamental TE mode, the only interesting one in terms of integration with the QD sources, is with the QD sources, is presented in Figure 3. From the calculation, a 15 m NbN nanowire on top of presented in Figure 3. From the calculation, a 15 µ m NbN nanowire on top of a 7.5 µ m SNB is already long enough to guarantee that 90% of the input light is absorbed, resulting in a lower probability of suffering of inhomogeneities. Photonics 2016, 3, 55 5 of 12 a 7.5 m SNB is already long enough to guarantee that 90% of the input light is absorbed, resulting in a lower probability of suffering of inhomogeneities. Photonics 2016, 3, 55 5 of 12 Photonics 2016, 3, 55 5 of 12 Figure 3. Simulated amplitude (V/m) and direction of the electric field for the fundamental mode of Figure 3. Simulated amplitude (V/m) and direction of the electric field for the fundamental mode of a Figure suspende 3. Si d m n ulated anobeam am pli (SN tud B)e with (V/m a) top and nio dir bium ection nitrid of the e ( NbN electri ) c wfie ire. ld for the fundamental mode of a suspended nanobeam (SNB) with a top niobium nitride (NbN) wire. a suspended nanobeam (SNB) with a top niobium nitride (NbN) wire. 3.1.2. Performance 3.1.2. Performance 3.1.2. Performance NbN SSPDs were fabricated on top of GaAs SNBs following the design in Figure 3 and the NbN SSPDs were fabricated on top of GaAs SNBs following the design in Figure 3 and the fabricNbN ation S pSPD roces du were re pr e fabri sentc eat d ed in S on ectto iop n 2 of . In GaAs whatS fNB ollo s w foll s, tow heiing r peth rfo e rm design ance ain t a F lo igure w tem 3 p and erat u th re e fabrication procedure presented in Section 2. In what follows, their performance at a low temperature (2.2 fabr K) ica in tioter n p m ro sc of edquantu ure prem sen efficienc ted in Sy, ect dead ion 2.time In w , an had t fjitt oller ow is s, dis the cius r p se er d. foA rm pr an el cieminar at a lo y w mea tem surement perature (2.2 K) in terms of quantum efficiency, dead time, and jitter is discussed. A preliminary measurement (2.2 K) in terms of quantum efficiency, dead time, and jitter is discussed. A preliminary measurement to quickly judge the quality of an SSPD is the current-voltage (I-V) characteristic. The inset of Figure 4a to quickly judge the quality of an SSPD is the current-voltage (I-V) characteristic. The inset of Figure 4a to quickly judge the quality of an SSPD is the current-voltage (I-V) characteristic. The inset of Figure 4a displays the current-voltage characteristic measured for a 20-µ m-long wire, showing a critical current (Ic) displays display the s thcurr e cur ent-voltage rent-voltagecharacteristic characteristic m measur easured ed fofor r a 2 a020- -µ m-m-long long wirwir e, sh e,oshowing wing a cria ticcritical al currecurr nt (Icent ) of 8 µ A. (I )of of88 µ A. A. (a) (b) (a) (b) Figure 4. (a) Device quantum efficiency (DQE) (red) of a 20 µm-long SSPD on top of SNB under il Figure luminati 4. on (a) at D evi 1300 ce nm quan , and tum dark effici co ency unt (D rate QE (bla ) (re ck) d) as of a a fun 20 ct µm io- n loof ngthe SS PD norm on ali top zed of b iSN as B curr un ent der . Figure 4. (a) Device quantum efficiency (DQE) (red) of a 20 m-long SSPD on top of SNB under Ins illum et: c inati urrent on -at vol 1300 tage c nm har , and acte ris dark tic. ( co b) His unt ra tote gram (bla of ck) the ji as att fun er time ction ofof the the d etec nortor mali bzed iase d b i94% as curr Ic. ent. illumination at 1300 nm, and dark count rate (black) as a function of the normalized bias current. Inset: current-voltage characteristic. (b) Histogram of the jitter time of the detector biased 94% Ic. Inset: current-voltage characteristic. (b) Histogram of the jitter time of the detector biased 94% I . The device quantum efficiency (DQE) is plotted in red Figure 4a as a function of the normalized The device quantum efficiency (DQE) is plotted in red Figure 4a as a function of the normalized bias current Ib/Ic, and it is defined as the number of counts minus the dark counts (in black) divided The device quantum efficiency (DQE) is plotted in red Figure 4a as a function of the normalized b bia y ts hc eurrent numbe Ir b/ o Icf, p and hoto it nis s c defi oup n le ed d t as o tth he e w numb aveger uid of e. c W ount e no s tminu e thats tth hee ddark ark c o co uun nts ts s h (io n w bl nack) in F id givided ure 4a bias current I /I , and it is defined as the number of counts minus the dark counts (in black) divided have by the been num m be ea rc s oured f pho in ton a s cryostat coupled w toith the an w a op vetica guil dw e. iW ndow e no. te Th they at t ar he e d th aerefor rk coue nt mo s sh stly own pro ind Fu ig ced ureby 4a by have the number been mof eas photons ured in a coupled cryostat tow the ith waveguide. an optical wW indow e note . Th that ey ar the e th dark erefor counts e moshown stly proin du Figur ced by e 4a the absorption of infrared thermal photons and not indicative of the detector’s performance in an the absorption of infrared thermal photons and not indicative of the detector’s performance in an have optimized been measur environm ed in enta . T cryostat he numbwith er of ph anot optical ons couwindow pled into . th They e ridgar e w e aveguide thereforeis mostly determpr ined oduced by optimized environment. The number of photons coupled into the ridge waveguide is determined by bytthe ransabsorption mission meaof sur infrar ement ed s: tthermal he ratio photons of fiber-in and to fnot iber- indicative out averagof ed the ovedetector r a 2 nm’s sp performance ectra around in 1300 trans nm mis s gi io ve n s m a ev aalue surem of e n Tt s = : 0.06 the % ra, ti me o o an f ing fiber th -in at t th o e ficouplin ber-out g av effic erag iency ed o v from er a th 2 e nfi m ber sp to ec tth rae acent roun er d an optimized environment. The number of photons coupled into the ridge waveguide is determined 1300 nm gives a value of T = 0.06%, meaning that the coupling efficiency from the fiber to the center of the RWG is 𝜂 = √𝑇 = 2.5%, including the loss. Taking this ηc into account, the maximum value by transmission measurements: the ratio of fiber-in to fiber-out averaged over a 2 nm spectra around rea of th ched e RWG for th is e 𝜂 DQ = E √ is 𝑇 28 = %. 2. 5% Wh, ile incl th ud e couplin ing the g loss effi . c Tiency aking is th ris el atiηc vely into account low, due , th toe th maximum e unoptim vialue zed 1300 nm gives a value of T = 0.06%, meaning that the coupling efficiency from the fiber to the center of mo readal ched m for atchi th ng e DQ betE ween is 28th %. e Wh lensed ile th fie be couplin r and th g e ef wav ficiency eguide, is rth ele ati D vely QE is low, the d rel ue e vant to thfi e g unopt ure of im merit ized the RWG is h = T = 2.5%, including the loss. Taking this h into account, the maximum value C c for mointegrated dal matchi experiment ng between sth where t e lensed he ph fibeot r and ons a th re e pro wav duced w eguide, th ithin e D the chip. QE is the relevant figure of merit reached for the DQE is 28%. While the coupling efficiency is relatively low, due to the unoptimized for integrated experiments where the photons are produced within the chip. The timing resolution (jitter) of a detector can be determined illuminating the detector with a The timing resolution (jitter) of a detector can be determined illuminating the detector with a pulsed laser (λ = 965 nm, 100 ps pulse width) and sending the output pulse to a correlation card pulsed laser (λ = 965 nm, 100 ps pulse width) and sending the output pulse to a correlation card together with the laser trigger signal to record the coincidences counts. Figure 4b shows the together with the laser trigger signal to record the coincidences counts. Figure 4b shows the Photonics 2016, 3, 55 6 of 12 modal matching between the lensed fiber and the waveguide, the DQE is the relevant figure of merit for integrated experiments where the photons are produced within the chip. The timing resolution (jitter) of a detector can be determined illuminating the detector with a pulsed laser (l = 965 nm, 100 ps pulse width) and sending the output pulse to a correlation card Photonics 2016, 3, 55 6 of 12 together with the laser trigger signal to record the coincidences counts. Figure 4b shows the coincidence peak using a bias current of 94% I . The fit with a Gaussian distribution gives a full width half coincidence peak using a bias current of 94% Ic. The fit with a Gaussian distribution gives a full width maximum (FWHM) value of 127 8 ps. This value is much shorter than the QD carrier lifetime (~1 ns), half maximum (FWHM) value of 127 ± 8 ps. This value is much shorter than the QD carrier lifetime so correlation measurements are not going to be significantly affected by the detector jitter. The analysis (~1 ns), so correlation measurements are not going to be significantly affected by the detector jitter. of the SSPD output pulse provides a value of FWHM = 3.4 ns and a 1/e decay time of 3.7 ns, which The analysis of the SSPD output pulse provides a value of FWHM = 3.4 ns and a 1/e decay time of 3.7 ns, corrwhich esponds corre very spo well nds very to the well expected to the time expect constant ed time  con = L stant/ τR = for LKa in/value R for a ofvalue wire kinetic of wire inductance kinetic Kin inductance LKin = 180 nH, calculated from the kinetic inductance per square of the NbN wire L = 180 nH, calculated from the kinetic inductance per square of the NbN wire L = 90 pH/ [28]. Kin L = 90 pH/□ [28]. 3.1.3. Autocorrelator Performance 3.1.3. Autocorrelator Performance As already mentioned, the addition of a second independent detector on top of the SNB provides As already mentioned, the addition of a second independent (2) detector on top of the SNB provides a very compact autocorrelator and opens the way to on-chip g () measurements. In order to test the (2) a very compact autocorrelator and opens the way to on-chip g (τ) measurements. In order to test the performance of the autocorrelators, two electrically independent detectors have been fabricated on performance of the autocorrelators, two electrically independent detectors have been fabricated on top of a SNB that is tapered to a RWG. The close packing of the nanowires could in principle produce top of a SNB that is tapered to a RWG. The close packing of the nanowires could in principle produce electrical, magnetic, or thermal coupling between the two detectors. In the following, we prove the electrical, magnetic, or thermal coupling between the two detectors. In the following, we prove the absence of any crosstalk, either static or dynamic, between the two detectors on a single nanobeam, absence of any crosstalk, either static or dynamic, between the two detectors on a single nanobeam, following a similar approach used in [23]. An initial series of tests is performed in static conditions to following a similar approach used in [23]. An initial series of tests is performed in static conditions to determine whether the bias condition of one detector has an influence on the electrical response of the determine whether the bias condition of one detector has an influence on the electrical response of other. Figure 5a shows the I-V characteristic of one detector (A) when the other one (B) is biased above the other. Figure 5a shows the I-V characteristic of one detector (A) when the other one (B) is biased the I above t or when he Ic it or is when not biased it is notat bia all, sed and at al vice l, and versa. vice versa. (a) (b) Figure 5. (a) I-V curve of Detector A while Detector B is unbiased or in resistive state; I-V curve of Figure 5. (a) I-V curve of Detector A while Detector B is unbiased or in resistive state; I-V curve of Detector B while Detector A is unbiased or in resistive state. Inset: sketch of the two independent Detector B while Detector A is unbiased or in resistive state. Inset: sketch of the two independent SSPDs on top of SNB. (b) Coincidence rate under illumination with a 80 MHz pulsed laser at 965 nm SSPDs on top of SNB. (b) Coincidence rate under illumination with a 80 MHz pulsed laser at 965 nm and detectors biased at 97% Ic. and detectors biased at 97% I . Biasing the other detector in the resistive region is a stringent test to prove the absence of static crosstalk. Indeed, the heat dissipated due to Joule effect could thermally affect the other detector, as Biasing the other detector in the resistive region is a stringent test to prove the absence of static has been observed for other substrates like sapphire [29]. Instead, it can be clearly seen how the I-V crosstalk. Indeed, the heat dissipated due to Joule effect could thermally affect the other detector, curve are completely superposed in every electrical regime of the device, proving the absence of static as has been observed for other substrates like sapphire [29]. Instead, it can be clearly seen how the I-V crosstalk. The difference between the Ic values (2.7 µ A) of the two nominally identical detectors is an curve are completely superposed in every electrical regime of the device, proving the absence of static indication that, for 20-µm-long wires, the inhomogeneities still play a role [22]. We then tested the crosstalk. The difference between the I values (2.7 A) of the two nominally identical detectors is existence of dynamic crosstalk. In this test, the temporal variation of the detection probability of one an indication that, for 20-m-long wires, the inhomogeneities still play a role [22]. We then tested detector due to the firing of the other detector is investigated. If the probability to click after a the existence of dynamic crosstalk. In this test, the temporal variation of the detection probability (2) detection event on the other wire was increased or decreased, g (τ) would show a peak or a dip at τ = 0 of one detector due to the firing of the other detector is investigated. If the probability to click after even in the case of laser illumination. To this end, we measured the intensity correlation function (2) a detection (2) event on the other wire was increased or decreased, g () would show a peak or a dip g (τ) of a pulsed laser (λ = 965 nm, 80 MHz repetition rate) coupled to the waveguide. The detectors were biased at Ib = 0.97Ic, and the coincidence counts were measured by sending their outputs to the inputs of a correlation card (PicoHarp 300) using a line with a known delay. In Figure 5b, the coincidence counts are shown as a function of the delay time between the start and stop channels, and as expected coincidences are observed only at delays multiple of the repetition period of the laser Photonics 2016, 3, 55 7 of 12 at  = 0 even in the case of laser illumination. To this end, we measured the intensity correlation (2) function g () of a pulsed laser ( = 965 nm, 80 MHz repetition rate) coupled to the waveguide. The detectors were biased at I = 0.97I , and the coincidence counts were measured by sending their outputs to the inputs of a correlation card (PicoHarp 300) using a line with a known delay. In Figure 5b, Photonics 2016, 3, 55 7 of 12 the coincidence counts are shown as a function of the delay time between the start and stop channels, and as expected coincidences are observed only at delays multiple of the repetition period of the laser (12.5 ns). The coincidence counts are normalized to the average of the peak’s maximum presented in (12.5 ns). The coincidence counts are normalized to the average of the peak’s maximum presented in Figure 5b and show a standard deviation of 0.03 due to statistical fluctuation. Considering that the Figure 5b and show a standard deviation of 0.03 due to statistical fluctuation. Considering that the peak at zero delay has 1.00 normalized coincidences, it is fair to conclude that no sign of dynamic peak at zero delay has 1.00 normalized coincidences, it is fair to conclude that no sign of dynamic crosstalk was observed. crosstalk was observed. 3.2. QD Emission 3.2. QD Emission On-Chip On-Chip Detection Detection The next step toward the demonstration of a fully functional QPIC is to prove that the SSPDs are The next step toward the demonstration of a fully functional QPIC is to prove that the SSPDs ar abl e e able to detec to detect t only only the the ligh light t com coming ing from from the the QDQDs s and and notnot from from other other source sour sces such such as as the the direct direct or scattered light of the laser pump. For this low-temperature (2.2 K) measurement, a 750 nm gain or scattered light of the laser pump. For this low-temperature (2.2 K) measurement, a 750 nm gain switched switched diode diode llaser aser with with a arepet repetition ition rrate ate of of 80 80 M MHz Hz w was as used used toto excite excite ththe e QDs QDs anand d use use them them as an as internal light source. The SSPD on SNB was biased at 0.94Ic, and the output was sent to the correlation an internal light source. The SSPD on SNB was biased at 0.94I , and the output was sent to card together with the laser trigger to record the coincidences counts. In the first measurement, the the correlation card together with the laser trigger to record the coincidences counts. In the first instrument response function (IRF) of the SSPD under direct laser illumination at normal incidence measurement, the instrument response function (IRF) of the SSPD under direct laser illumination at with 30 nW of power was measured. The IRF (Figure 6; blue curve) shows a main peak and a normal incidence with 30 nW of power was measured. The IRF (Figure 6; blue curve) shows a main shoulder, which may be due to a relaxation oscillation of the laser. A Gaussian fit to the main peak peak and a shoulder, which may be due to a relaxation oscillation of the laser. A Gaussian fit to the has a FWHM of 134 ± 4 ps, which corresponds very well to the value of jitter previously measured main peak has a FWHM of 134  4 ps, which corresponds very well to the value of jitter previously for the SSPD on a sample without QDs. At long delay, it was possible to recognize a shoulder due to measured for the SSPD on a sample without QDs. At long delay, it was possible to recognize a shoulder the response to QD emission. The exponential decay fit of the shoulder (green curve) yielded a decay due to the response to QD emission. The exponential decay fit of the shoulder (green curve) yielded time value τd = 0.94 ± 0.01 ns, which corresponds to the typical spontaneous emission lifetime of these a decay time value  = 0.94 0.01 ns, which corresponds to the typical spontaneous emission lifetime InAs quantum dots [30]. In the second measurement, an ensemble of QDs at the edge of the ridge of these InAs quantum dots [30]. In the second measurement, an ensemble of QDs at the edge of the waveguide (0.5 mm away from the SSPD) was excited non-resonantly with the same power, and the ridge waveguide (0.5 mm away from the SSPD) was excited non-resonantly with the same power, on-chip time resolved photoluminescence (PL) data were recorded (Figure 6; black curve). The and the on-chip time resolved photoluminescence (PL) data were recorded (Figure 6; black curve). exponential fit of the decay time yielded a value τd = 0.92 ± 0.01 ns, in good agreement with the decay The exponential fit of the decay time yielded a value  = 0.92  0.01 ns, in good agreement with time observed in the shoulder on the IRF curve. This proves that, whenever the excitation spot is far the decay time observed in the shoulder on the IRF curve. This proves that, whenever the excitation enough from the detector, the laser is completely absorbed in the GaAs so that only the QD emission spot is far enough from the detector, the laser is completely absorbed in the GaAs so that only the QD is detected. emission is detected. Figure 6. Instrument response function (IRF) of the SSPD (blue) and on-chip time resolve QD Figure 6. Instrument response function (IRF) of the SSPD (blue) and on-chip time resolve QD photoluminescence measurement (black). The red line represents the Gaussian fit of the IRF and the photoluminescence measurement (black). The red line represents the Gaussian fit of the IRF and green lines the exponential fit of the decay time of the QD emission. the green lines the exponential fit of the decay time of the QD emission. 3.3. Filters 3.3.1. Design In order to obtain single-photon emission from QDs, a single excitonic line must be filtered out of the complex emission spectrum of one or more QDs. A coupled PhCC-PhCWG system has been theoretically and experimentally investigated and proposed as a system capable of realizing the filtering functionality [31,32]. Indeed, when the wavelength of the input light matches the wavelength Photonics 2016, 3, 55 8 of 12 3.3. Filters 3.3.1. Design In order to obtain single-photon emission from QDs, a single excitonic line must be filtered Photonics 2016, 3, 55 8 of 12 out of the complex emission spectrum of one or more QDs. A coupled PhCC-PhCWG system has been theoretically and experimentally investigated and proposed as a system capable of realizing the of the cavity, light can pass through the cavity via resonant tunneling. The performance of this filter filtering functionality [31,32]. Indeed, when the wavelength of the input light matches the wavelength can be described by coupled-mode theory [33]. The coupling of the cavity to the PhCWG and to the of the cavity, light can pass through the cavity via resonant tunneling. The performance of this leaky radiation modes can be characterized by quality factors QWG and QR, where the total quality filter can be described by coupled-mode theory [33]. The coupling of the cavity to the PhCWG factor QT is obtained from 1⁄𝑄 = 1⁄𝑄 + 1⁄𝑄 , and the transmission is calculated as and to the leaky radiation modes 𝑇can be characterized 𝑊𝐺 𝑅 by quality factors Q and Q , where the WG R 𝑇 = [1⁄(1 + 𝑄 ⁄𝑄 )] . This means that for a fixed QR (which depends on the design and fabrication total quality factor𝑅 Q is obtained from 1/Q = 1/Q + 1/Q , and the transmission is calculated T T WG R of as the T cav = ity) [1/, (incre 1 + Q asing /Q the )] transm . This is means sion requir that for es a a fixed decreQ ase (which of the Q depends WG, whion ch the will design increas and e the WG R R bandwidth fabrication of ofth the e filter cavity), , thus incr deter easing iorthe ating transmission the filtering requir function es a decr ality. ease Thof erefor the e, Q to ,awhich chievewill good WG increase the bandwidth of the filter, thus deteriorating the filtering functionality. Therefore, to achieve filtering performances, a trade-off between the transmission and the bandwidth is required. As the good filtering performances, a trade-off between the transmission and the bandwidth is required. typical spectral separation between lines when few QDs are pumped is in the order of a few nm, we As the typical spectral separation between lines when few QDs are pumped is in the order of a few nm, aimed at QT ≈ 1000. Together with a QR about equal to 10,000, this should ensure good filtering and we aimed at Q  1000. Together with a Q about equal to 10,000, this should ensure good filtering high transmission T . Another important par R ameter is the free spectral range (FSR) around the and high transmission. Another important parameter is the free spectral range (FSR) around the transmission peak: all the QDs with a wavelength different from the cavity must be filtered, so a large transmission peak: all the QDs with a wavelength different from the cavity must be filtered, so a large FSR is necessary (at least 40 nm). Designs with different coupling between the PhCC and the PhCWG FSR is necessary (at least 40 nm). Designs with different coupling between the PhCC and the PhCWG have been simulated. A direct coupling (in-line) of an L3 PhCC (obtained by removing the first three have been simulated. A direct coupling (in-line) of an L3 PhCC (obtained by removing the first three holes from the center of the PhC) with a PhCWG, and a two-hole barrier in between, was found to be holes from the center of the PhC) with a PhCWG, and a two-hole barrier in between, was found to be the design that best fulfills the requirements (Figure 7 inset). Moreover, the diameter of the external the design that best fulfills the requirements (Figure 7 inset). Moreover, the diameter of the external hole of the barrier was reduced by 10%, compared with the others holes, to increase Q. Figure 7 shows hole of the barrier was reduced by 10%, compared with the others holes, to increase Q. Figure 7 the simulated transmissions spectra: both the fundamental and second mode peaks are quite narrow shows the simulated transmissions spectra: both the fundamental and second mode peaks are quite with Q = 1170 and Q = 1230, respectively, and are well separated, resulting in a FSR of 75 nm. The narrow with Q = 1170 and Q = 1230, respectively, and are well separated, resulting in a FSR of 75 nm. fundamental mode has a transmission of 58%, while the second mode a transmission of 78%. The fundamental mode has a transmission of 58%, while the second mode a transmission of 78%. Figure 7. Simulated transmission spectra. Inset: sketch of the chosen design. Figure 7. Simulated transmission spectra. Inset: sketch of the chosen design. 3.3.2 3.3.2. . Per Performance formance Filters according to the chosen design were fabricated and the transmission spectra were Filters according to the chosen design were fabricated and the transmission spectra were experimentally measured at room temperature using two techniques. First, a non-resonant laser experimentally measured at room temperature using two techniques. First, a non-resonant laser (780 nm) was applied from the top on the two sides of the cavity (Figure 8a; red and blue arrows) and (780 nm) was applied from the top on the two sides of the cavity (Figure 8a; red and blue arrows) and directly on top of it (black arrow). The PL coupled to the PhCWG (green arrow) was collected from the directly on top of it (black arrow). The PL coupled to the PhCWG (green arrow) was collected from RWG through a lensed fiber. Figure 8b shows side-collection spectra of the PL signal when the laser the RWG through a lensed fiber. Figure 8b shows side-collection spectra of the PL signal when the was focused on top of the cavity and on the two sides. The blue curve (after cavity) shows a broad laser was focused on top of the cavity and on the two sides. The blue curve (after cavity) shows a broad emission with several peaks in the slow-light part of the dispersion curve of the PhCWG mode [12], while in the other two it is possible to observe the filtered cavity mode (1266 nm) located around 20 nm away from the slow-light dispersion edge. When the pump was applied on top of the PhCC, some additional peaks were observed, compared with the case where the laser was applied before the PhCC, due to the excitation of the PhCWG section around the cavity (since the laser spot is bigger than the area of the cavity). From the Lorentz fit of the cavity mode, a Q of 1000 was obtained, a value close to the one predicted by the simulation. In the spectra before the cavity (red curve) for the full range of wavelengths (1200–1350 nm; not shown), no other cavity peak was observed, indicating that a FSR of at least 66 nm was obtained. 𝑊𝐺 Photonics 2016, 3, 55 9 of 12 emission with several peaks in the slow-light part of the dispersion curve of the PhCWG mode [12], while in the other two it is possible to observe the filtered cavity mode (1266 nm) located around 20 nm away from the slow-light dispersion edge. When the pump was applied on top of the PhCC, some additional peaks were observed, compared with the case where the laser was applied before the PhCC, due to the excitation of the PhCWG section around the cavity (since the laser spot is bigger than the area of the cavity). From the Lorentz fit of the cavity mode, a Q of 1000 was obtained, a value close to the one predicted by the simulation. In the spectra before the cavity (red curve) for the full range of wavelengths (1200–1350 nm; not shown), no other cavity peak was observed, indicating that a FSR of Photonics 2016, 3, 55 9 of 12 at least 66 nm was obtained. (a) (b) Figure 8. (a) Sketch of the design B filter: the red (before cavity), black (on cavity), and blue (after Figure 8. (a) Sketch of the design B filter: the red (before cavity), black (on cavity), and blue (after cavity) cavity) arrows indicate the different position of the top excitation, while the green the direction of the arrows indicate the different position of the top excitation, while the green the direction of the side side collection. collectio (bn ).PL (bspectra ) PL spe forcthe tra thr for ee dif the f er thr ent ee excit differen ation posi t exci tions tatiwit on hpositions color coding wit corr h c esponding olor coding to (a). The blue spectra has been reduced by a factor of 5 for greater clarity. corresponding to (a). The blue spectra has been reduced by a factor of 5 for greater clarity. The transmission spectrum shown in black in Figure 9 was calculated as the ratio between the The transmission spectrum shown in black in Figure 9 was calculated as the ratio between the PL PL spectra acquired from the two sides of the cavity, therefore before and after filtering. The peaks spectra acquired from the two sides of the cavity, therefore before and after filtering. The peaks were were modulated modulat by ed Fabry-Per by Fabry ot-Perot fringes frdue inges to due reflections to reflect at the ions cleaved at the facet cleaved and at facet the an interface d at thbetween e interface bet Rween WG and RW SNB. G and A SN clear B. A transmission clear transmiss peakion at the peak same at th wavelength e same wavelen of the gth cavity of thmode e cavity was measily ode was distinguishable and well isolated, confirming a very large FSR, although the transmission was only easily distinguishable and well isolated, confirming a very large FSR, although the transmission was 21%  2%. Secondly, a direct measure of transmission was taken using a PhCC-PhCWG coupled with only 21 ± 2%. Secondly, a direct measure of transmission was taken using a PhCC-PhCWG coupled a RWG on both sides, performing a fiber-in:fiber-out measurement with a tunable laser. Considering with a RWG on both sides, performing a fiber-in:fiber-out measurement with a tunable laser. a coupling efficiency of h = 2.5%, the obtained transmission for this measurement was equal to Considering a coupling efficiency of ηc = 2.5%, the obtained transmission for this measurement was 18%  2%. equal to 18 ± 2%. Figure 9. Experimental transmission spectra: black curve indirectly calculated from the ratio between the photoluminescence (PL) spectra before and after filtering, red curve from direct fiber-in:fiber-out measurement. Photonics 2016, 3, 55 9 of 12 (a) (b) Figure 8. (a) Sketch of the design B filter: the red (before cavity), black (on cavity), and blue (after cavity) arrows indicate the different position of the top excitation, while the green the direction of the side collection. (b) PL spectra for the three different excitation positions with color coding corresponding to (a). The blue spectra has been reduced by a factor of 5 for greater clarity. The transmission spectrum shown in black in Figure 9 was calculated as the ratio between the PL spectra acquired from the two sides of the cavity, therefore before and after filtering. The peaks were modulated by Fabry-Perot fringes due to reflections at the cleaved facet and at the interface between RWG and SNB. A clear transmission peak at the same wavelength of the cavity mode was easily distinguishable and well isolated, confirming a very large FSR, although the transmission was only 21 ± 2%. Secondly, a direct measure of transmission was taken using a PhCC-PhCWG coupled with a RWG on both sides, performing a fiber-in:fiber-out measurement with a tunable laser. Considering a coupling efficiency of ηc = 2.5%, the obtained transmission for this measurement was Photonics 2016, 3, 55 10 of 12 equal to 18 ± 2%. Figure 9. Experimental transmission spectra: black curve indirectly calculated from the ratio Figure 9. Experimental transmission spectra: black curve indirectly calculated from the ratio between between the photol theum photoluminescence inescence (PL) spectra (PL) before spectra and after befor fie lterin and g, after red cur filtering, ve from di red rectcur fiber ve -in fr :fib om er-dir out ect measurement. fiber-in:fiber-out measurement. Photonics 2016, 3, 55 10 of 12 3.3.3. Filtering of Single QD Lines 3.3.3. Filtering of Single QD Lines Finally, the capability of the on-chip filter to isolate a single QD excitonic line is presented. Finally, the capability of the on-chip filter to isolate a single QD excitonic line is presented. In In Figure 10, two PL spectra measured at low temperature (2.2 K) are shown for side collection when Figure 10, two PL spectra measured at low temperature (2.2 K) are shown for side collection when the laser is focused either on top of the cavity or on top of the SNB before the cavity. To be able to better the laser is focused either on top of the cavity or on top of the SNB before the cavity. To be able to discriminate the single QD excitonic emission line, the pump laser power was strongly attenuated better discriminate the single QD excitonic emission line, the pump laser power was strongly (20 nW). In this particular case, a QD was emitting (1194 nm) exactly within the bandwidth of the attenuated (20 nW). In this particular case, a QD was emitting (1194 nm) exactly within the bandwidth of the cavity mode; therefore, a single excitonic line was passing through the filter while all the other cavity mode; therefore, a single excitonic line was passing through the filter while all the other QDs QDs emission lines at different wavelengths were suppressed. However, due to the lower Q = 200 of emission lines at different wavelengths were suppressed. However, due to the lower Q = 200 of this this specific cavity, some residual QD emission close to the filtered line was not yet well suppressed. specific cavity, some residual QD emission close to the filtered line was not yet well suppressed. Figure Figure 10. 10 PL . PL spectra spectra on two on two diff er diff ent erent excitation excitati position on positi following on following the pr the evious previo color us co coding. lor codi A ng single . A single QD excitonic line is clearly visible on the red curve. QD excitonic line is clearly visible on the red curve. 4. Discussion 4. Discussion The low temperature fabrication process, developed in order to integrate single-photon sources The low temperature fabrication process, developed in order to integrate single-photon sources and detectors on a GaAs platform, has shown an ability to preserve high performance for each and detectors on a GaAs platform, has shown an ability to preserve high performance for each component of the proposed QPIC. In particular, the SSPD patterned on top of a SNB presents a DQE component of the proposed QPIC. In particular, the SSPD patterned on top of a SNB presents a DQE of of 28% and a jitter of 127 ± 8 ps. The simulation of the electric field mode guided inside the SNB 28% and a jitter of 127  8 ps. The simulation of the electric field mode guided inside the SNB showed showed the possibility of using shorter wires (15-µm-long), preserving high absorptance (90%) and the possibility of using shorter wires (15-m-long), preserving high absorptance (90%) and thereby thereby reducing the inhomogeneities problem. Even if the experimental DQE did not improve as reducing the inhomogeneities problem. Even if the experimental DQE did not improve as much as much as expected, a new record for the DQE on the GaAs platform has been established. expected, a new record for the DQE on the GaAs platform has been established. Autocorrelators Autocorrelators based on two electrically independent SSPDs on top of a SNB did not present any based on two electrically independent SSPDs on top of a SNB did not present any static or dynamic static or dynamic crosstalk; this, together with a jitter much shorter than the QD lifetime, shows that crosstalk; this, together with a jitter much shorter than the QD lifetime, shows that the integrated the integrated autocorrelators are suitable for the on-chip measurement of single-photon emission. On the other hand, the PhCC-PhCWG system showed the expected filtering functionality of a single- excitonic line. From the experimental transmission spectra, a FSR ≥ 66 nm and Q = 1000, as estimated from the simulation, were found; however, the transmission was 21% lower than expected, presumably due to disorder-induced optical losses. Even with such transmission, it has been possible to prove that single excitonic lines can be isolated and transmitted through the filter. Moreover, it has been proved that the QD emission can be detected by the on-chip SSPDs, whenever the laser is focused at a sufficiently long distance from the detector. In conclusion, a platform able to combine QD sources, filters, detectors, and autocorrelators has been developed and successfully tested, paving the way for numerous QIP applications. Acknowledgments: This research is financially supported by the European Commission, Initial Training Network PICQUE (No. 608062), by the Dutch Technology Foundation STW, Applied Science Division of NWO, the Technology Program of the Ministry of Economic Affairs under projects No. 10380, 12662 and by NanoNextNL, a micro and nanotechnology program of the Dutch Ministry of Economic Affairs, Agriculture and Innovation (EL&I) and 130 partners. Photonics 2016, 3, 55 11 of 12 autocorrelators are suitable for the on-chip measurement of single-photon emission. On the other hand, the PhCC-PhCWG system showed the expected filtering functionality of a single-excitonic line. From the experimental transmission spectra, a FSR  66 nm and Q = 1000, as estimated from the simulation, were found; however, the transmission was 21% lower than expected, presumably due to disorder-induced optical losses. Even with such transmission, it has been possible to prove that single excitonic lines can be isolated and transmitted through the filter. Moreover, it has been proved that the QD emission can be detected by the on-chip SSPDs, whenever the laser is focused at a sufficiently long distance from the detector. In conclusion, a platform able to combine QD sources, filters, detectors, and autocorrelators has been developed and successfully tested, paving the way for numerous QIP applications. Acknowledgments: This research is financially supported by the European Commission, Initial Training Network PICQUE (No. 608062), by the Dutch Technology Foundation STW, Applied Science Division of NWO, the Technology Program of the Ministry of Economic Affairs under projects No. 10380, 12662 and by NanoNextNL, a micro and nanotechnology program of the Dutch Ministry of Economic Affairs, Agriculture and Innovation (EL&I) and 130 partners. Author Contributions: G.E.D., M.P. and A.F. conceived and designed the experiment, F.W.M.v.O. grew the wafer and G.E.D. with the contribution of R.G. and M.P. fabricated the devices. M.P. with the contribution of S.F.P. developed the simulation. G.E.D., M.P. and S.B. performed the experiments. G.E.D. analyzed the data and wrote the paper. A.F. supervised the project. Conflicts of Interest: The authors declare no conflict of interest. References 1. O’Brien, J.L. Optical quantum computing. Science 2007, 318, 1567–1570. [CrossRef] [PubMed] 2. 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hv photonics Article Integration of Single-Photon Sources and Detectors on GaAs Giulia Enrica Digeronimo *, Maurangelo Petruzzella, Simone Birindelli, Rosalinda Gaudio, Sartoon Fattah Poor, Frank W.M. van Otten and Andrea Fiore Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands; m.petruzzella@tue.nl (M.P.); s.birindelli@tue.nl (S.B.); rosalinda.gaudio@gmail.com (R.G.); sartoon.fattahpoor@gmail.com (S.F.P.); F.W.M.v.Otten@tue.nl (F.W.M.v.O.); a.fiore@tue.nl (A.F.) * Correspondence: g.e.digeronimo@tue.nl; Tel.: +31-402-475-057 Received: 13 September 2016; Accepted: 16 October 2016; Published: 21 October 2016 Abstract: Quantum photonic integrated circuits (QPICs) on a GaAs platform allow the generation, manipulation, routing, and detection of non-classical states of light, which could pave the way for quantum information processing based on photons. In this article, the prototype of a multi-functional QPIC is presented together with our recent achievements in terms of nanofabrication and integration of each component of the circuit. Photons are generated by excited InAs quantum dots (QDs) and routed through ridge waveguides towards photonic crystal cavities acting as filters. The filters with a transmission of 20% and free spectral range 66 nm are able to select a single excitonic line out of the complex emission spectra of the QDs. The QD luminescence can be measured by on-chip superconducting single photon detectors made of niobium nitride (NbN) nanowires patterned on top of a suspended nanobeam, reaching a device quantum efficiency up to 28%. Moreover, two electrically independent detectors are integrated on top of the same nanobeam, resulting in a very compact (2) autocorrelator for on-chip g () measurements. Keywords: quantum photonics integrated circuits; superconducting single photon detectors; quantum dots; photonic crystals cavities 1. Introduction Quantum information processing (QIP) is a well-established scientific field that opens unconventional perspectives for information processing. QIP uses quantum bits (qubits) as units of information. While qubits can be based on atomic and solid state systems, photons have emerged as a promising approach for QIP [1]. However, it is very challenging to implement quantum processing functionalities on a large scale with bulk optics due to the extreme stability requirements and large coupling losses. The concept of quantum photonic integration [2] has been investigated as a scalable approach to photonic QIP, in principle enabling experiments with a few tens of photons. In order to realize a fully-functional quantum photonic integrated circuit (QPIC), all the required quantum optical components, such as single-photon sources, passive circuit elements, and single-photon detectors, must be integrated on a single chip. The use of III-V materials, especially GaAs and InP compounds, is very promising for QPIC [3] because it allows the integration of on-demand single-photon sources. Semiconductor quantum dots (QDs) have been shown to be nearly ideal sources of non-classical light [4] that can be efficiently routed into nanophotonic circuits [5–8]. As compared to integrated single-photon sources based on parametric processes in LiNbO [9] or Si waveguides [10], QDs offer the advantage of much higher efficiency and easier filtering of the pump beam. Moreover, approaches to reproducibly control the exciton and cavity energy on chip, as needed to realize a scalable circuit, have been developed [11–14]. On the other side, superconducting single photon detectors (SSPDs) [15] based on niobium nitride (NbN) nanowires combine high detection efficiency, low dark count rates, Photonics 2016, 3, 55; doi:10.3390/photonics3040055 www.mdpi.com/journal/photonics Photonics 2016, 3, 55 2 of 12 and ultra-fast response, making them promising candidates for QIP [16]. The integration of Photonics 2016, 3, 55 2 of 12 such detectors into nanophotonic circuits has been proved on different platforms: on GaAs with waveguides [17] and QDs [18], and on Si with waveguides [19] and nanobeam cavities [20]. Particularly low dark count rates, and ultra-fast response, making them promising candidates for QIP [16]. The inintegrat [18], the ion possibility of such detec of to on-chip rs into nanop detection hotonic of the circQDs uits hemission as been pro light ved has on di been fferent pr oved; platforhowever ms: on , the demonstrated GaAs with waveguide circuit lacked s [17] an mor d QD e complex s [18], an str d uctur on Si es with (high-finesse waveguides nanocavities, [19] and nanobe beam am cav splitters, ities etc.) [20]. Particularly in [18], the possibility of on-chip detection of the QDs emission light has been needed for photonic QIP. proved; however, the demonstrated circuit lacked more complex structures (high-finesse Here, we present a prototype architecture of a fully integrated photonic quantum circuit on nanocavities, beam splitters, etc.) needed for photonic QIP. a GaAs platform (Figure 1), consisting of InAs QDs, photonic crystal waveguides (PhCWGs) and Here, we present a prototype architecture of a fully integrated photonic quantum circuit on a cavities (PhCCs), and SSPDs patterned on top of suspended nanobeams (SNBs). Thanks to the addition GaAs platform (Figure 1), consisting of InAs QDs, photonic crystal waveguides (PhCWGs) and of PhCCs and PhCWGs, the emission from one or several QDs can be filtered on-chip so that single cavities (PhCCs), and SSPDs patterned on top of suspended nanobeams (SNBs). Thanks to the photons originating from a single excitonic line are funneled to circuits and then measured. We choose addition of PhCCs and PhCWGs, the emission from one or several QDs can be filtered on-chip so to integrate the superconductive nanowires on top of a SNB instead of a ridge waveguide since that single photons originating from a single excitonic line are funneled to circuits and then shorter lengths are sufficient to reach high absorption due to a tighter field confinement [21]. This is measured. We choose to integrate the superconductive nanowires on top of a SNB instead of a ridge expected to reduce the role of wire inhomogeneities [22] and increase efficiency. Moreover, it is waveguide since shorter lengths are sufficient to reach high absorption due to a tighter field possible confinem toent place [21]. two Thelectrically is is expected independent to reduce th detectors e role ofon wire top inh ofom the ogeneitie SNB, such s [22] that and theinc same rease guided mode efficienc is pr y. obed Moreov byer the , it two is poSSPDs ssible to pl [23ace ]. The two autocorr electrically indepe elators together ndent detec with tothe rs on top PhC of filters the S would, NB, for such that the same guided mode is probed by the two SSPDs [23]. The autocorrelators together with example, allow the performance of a Hanbury-Brow-Twiss (HBT) experiment [24] completely on-chip, the PhC filters would, for example, allow the performance of a Hanbury-Brow-Twiss (HBT) which would be a meaningful validation of a fully functional QPIC. However, the integration of these experiment [24] completely on-chip, which would be a meaningful validation of a fully functional different functionalities on the same platform shows significant technological challenges. In this paper, QPIC. However, the integration of these different functionalities on the same platform shows a low-temperature fabrication process that allows for the integration of the different components significant technological challenges. In this paper, a low-temperature fabrication process that allows of the above-mentioned circuit is presented. Further, the characteristics and functionalities of each for the integration of the different components of the above-mentioned circuit is presented. Further, component (SSPDs, QDs and PhC filters) of the circuit fabricated at low temperature are shown in the characteristics and functionalities of each component (SSPDs, QDs and PhC filters) of the circuit detail, and it will be demonstrated how the required functionality and performance is obtained for fabricated at low temperature are shown in detail, and it will be demonstrated how the required each component. functionality and performance is obtained for each component. Figure 1. Sketch of the proposed approach for the quantum photonic integrated circuits (QPIC). Single Figure 1. Sketch of the proposed approach for the quantum photonic integrated circuits (QPIC). photons are produced via spontaneous emission of excitons in quantum dots (QDs), efficiently Single photons are produced via spontaneous emission of excitons in quantum dots (QDs), efficiently funneled in waveguides, filtered by a photonic crystal cavity (PhCC) and detected by waveguide funneled in waveguides, filtered by a photonic crystal cavity (PhCC) and detected by waveguide superconducting single photon detectors (SSPDs). superconducting single photon detectors (SSPDs). 2. Materials and Methods 2. Materials and Methods The fabrication process starts with the sputtering of a thin NbN film on top of the substrate for The fabrication process starts with the sputtering of a thin NbN film on top of the substrate for the the fabrication of the SSPDs. In this step, it is crucial to have a substrate’s surface with very low fabrication roughness of (<0 the .5 nm SSPDs. ), sincIn e th this e interna step,l it qu isantum crucial efto ficie have ncy of a substr the detec ate’s tor surface depends with on th very e quali low ty r and oughness uniformity of the superconducting film [25]; consequently, a rough film could produce an inefficient (<0.5 nm), since the internal quantum efficiency of the detector depends on the quality and uniformity SSPD. Therefore, it is crucial to start from a GaAs/AlGaAs heterostructure featuring very low of the superconducting film [25]; consequently, a rough film could produce an inefficient SSPD. roughness despite the QDs embedded inside. The sample is grown by molecular beam epitaxy on an Therefore, it is crucial to start from a GaAs/AlGaAs heterostructure featuring very low roughness undoped GaAs substrate where a layer of GaAs with a 320 nm thickness including low-density InAs despite the QDs embedded inside. The sample is grown by molecular beam epitaxy on an undoped QDs [26] is grown on top of a 1.5-µ m-thick layer of Al0.7Ga0.3As. At the end of the growth, an extra Photonics 2016, 3, 55 3 of 12 GaAs substrate where a layer of GaAs with a 320 nm thickness including low-density InAs QDs [26] is grown on top of a 1.5-m-thick layer of Al Ga As. At the end of the growth, an extra final step 0.7 0.3 of deposition of a few atomic layers of GaAs, at 20 C above the oxide removal temperature (585 C) under an As flux, yields samples with a smooth surface (root mean square roughness = 0.4 nm). After the growth, a 5-nm-thick NbN film is deposited on top of the wafer by means of direct current reactive magnetron sputtering. A Nb target in Ar + N mixture at total pressure P = 2.3 mTorr 2 TOT is sputtered for the deposition of the film. The deposition is carried out at a nominal temperature of 400 C, with a target current of 250 mA and a target voltage of 380 V, conditions similar to those previously used to fabricate high-performance waveguide SSPDs [22]. This annealing step at 400 C does not significantly change the emission spectra of the QDs, which display only a 5 nm blue shift after the sputtering process. The contact pads and alignment markers, consisting of 14 nm Ti and 140 nm Au layers, are defined through optical lithography, electron beam evaporation, and lift-off. The nanowires are patterned by electron beam lithography on a 100-nm-thick mask of hydrogen silsequioxane (HSQ) and then transferred to the NbN film via reactive ion etching (RIE) in an Ar/SF plasma. Figure 2a shows a scanning electron microscopy (SEM) image of an etched wire (100 nm width, 160 nm pitch, and 20 m length) covered with the HSQ mask. As can be noticed, the design was adjusted in the areas close to the corners in order to avoid current crowding around sharp edges [27]. In order to obtain high quality photonic crystal structures, the pattern has to be etched through a hard mask, which is usually made of a thick SiN film (200–400 nm) deposited at high temperature (300 C) via plasma enhanced chemical vapor deposition (PECVD). However, in this specific case, to preserve the NbN quality film, it is essential to avoid thermal stress, and for this reason it is important that temperatures above 200 C are never reached. A low-temperature recipe (70 C) for the deposition of a 400 nm SiN hard mask has been used based on an induced coupled plasma (ICP) PECVD. A 320-nm-thick layer of ZEP520A as electron beam resist is deposited in the next step, and the design including the RWGs, the PhCs structures, and the SNBs is exposed with electron beam lithography, taking special care on the alignment of the wires with the SNB. Two subsequent RIE steps at room temperature are performed in order to etch through the SiN and GaAs layers. The higher density and different composition of the LT SiN hard mask required an optimization of the composition of the reactive gases; specifically, a CHF /O plasma is used for the SiN etching, and a SiCl plasma for 3 2 4 the GaAs etching. The sacrificial AlGaAs layer is removed via wet etching in an HCl-based solution, which results in a geometrically well-defined etching profile due to different etching rates in different crystallographic orientations in AlGaAs [8]. A final step of dry etching to remove the residual SiN hard mask is usually required in order to open the metal contacts. However, it has been noticed that in this step the nanowires endure significant damage due to a lateral etching that drastically reduces the width of the wire and, correspondingly, the value of the critical current. An intermediate step of optical lithography and SiN etching has been added in order to open only the metal contact, avoiding the SiN etching in the active parts of the circuit. The SiN layer (50 nm) left on top does not seem to significantly modify the optical properties of the PhC structures (Figure 2b). Indeed, the quality factor (Q) of a cavity fabricated with this recipe is around 1000, as expected from the simulation for the chosen design. An example of a complete QPIC is presented in Figure 2 in which its different parts are visible: (a) the nanowire placed on top of a 15-m-long, 1-m-wide nanobeam, the two PhC waveguides with the PhC cavity and (b) the other SNB that is tapered to a RWG with a 5 m width at the cleaved facet. The electro-optical characterization of SSPD is performed at an estimated sample temperature of 2.3 K in a continuous flow helium cryostat. During the electrical measurements, the detector is biased through the DC port of a bias-T with a voltage source connected in series with a 10 W bias resistor. The RF port of the bias-T is instead connected to a 50 W cap. The electrical contact with the device is established with a 50 W microwave probe connected to the circuit through a SMA coaxial cable. During the optical measurement, the 50 W cap is removed and a 4 dB attenuator connected to a series of four amplifiers is now added to the RF port of the bias-T. A continuous wave 1300 nm diode laser is sent through a polarization-maintaining lensed fiber into the waveguides, and the detector Photonics 2016, 3, 55 4 of 12 output signal coming from the amplifiers is then sent to a counter. Both the lensed fiber and the contact probes are mounted on piezoelectric positioners, which are thermally anchored to the cold plate to Photonics 2016, 3, 55 4 of 12 minimize the thermal load to the detector. The QDs and the PhC filters are characterized through through the use of a non-resonant pump laser (780 nm) that is focused by a microscope objective (NA 0.4) the use of a non-resonant pump laser (780 nm) that is focused by a microscope objective (NA 0.4) into into the the samp sample le to to exc excite ite the the QD QDs. s. ThThe e phot photoluminescence oluminescence (PL) (PL) sign signal al is co is ll collected ected from from the the end end facet facet of the RWG through the lensed fiber, sent to a spectrometer (focal length = 1 m) and detected by an of the RWG through the lensed fiber, sent to a spectrometer (focal length = 1 m) and detected by InGaAs array detector. All simulations both for the detectors and the filters are performed using a an InGaAs array detector. All simulations both for the detectors and the filters are performed using finite-element mode solver (COMSOL Multiphysics). a finite-element mode solver (COMSOL Multiphysics). Figure 2. Scanning electron microscopy (SEM) picture of the full QPIC. Zoomed pictures of SSPD (a) Figure 2. Scanning electron microscopy (SEM) picture of the full QPIC. Zoomed pictures of SSPD (a) and PhCC (b). and PhCC (b). 3. R 3.esu Results lts 3.1. Detectors 3.1. Detectors 3.1.1. Design 3.1.1. Design The chosen design for the detectors on top of a SNB is based on simulations in which The chosen design for the detectors on top of a SNB is based on simulations in which the the absorptances for nanowires on top of either GaAs waveguides or nanobeams are compared. absorptances for nanowires on top of either GaAs waveguides or nanobeams are compared. In the In the first design, an NbN meander (5-nm-thick, 100-nm-wide, and with a 250 nm pitch) was first design, an NbN meander (5-nm-thick, 100-nm-wide, and with a 250 nm pitch) was placed on top placed on top of a GaAs/Al Ga As waveguide (1.85-m-wide and 250-nm-deep). Assuming 0.7 0.3 of a GaAs/Al0.7Ga0.3As waveguide (1.85-µm-wide and 250-nm-deep). Assuming a complex NbN a complex NbN reflective index ñ = 4.35 i4.65, the absorption coefficients for TE and TM NbN reflective index ñ NbN = 4.35 − i4.65, the absorption coefficients for TE and TM polarizations provide a polarizations provide a total absorptance of 84% and 82% for a 50-m-long waveguide, respectively [21]. total absorptance of 84% and 82% for a 50-µm-long waveguide, respectively [21]. While the While the theoretical calculations suggest a high absorptance, the experimental device detection theoretical calculations suggest a high absorptance, the experimental device detection efficiency for efficiency for this detector ’s design does not go above 20% [17], indicating that the inhomogeneities [22] this detector’s design does not go above 20% [17], indicating that the inhomogeneities [22] could play could play a major role on limiting the detector ’s efficiency. Therefore, a new design, with a NbN a major role on limiting the detector’s efficiency. Therefore, a new design, with a NbN meander on meander on top of a GaAs SNB surrounded by air, has been proposed as a possible improvement [21]. top of a GaAs SNB surrounded by air, has been proposed as a possible improvement [21]. As the As the index contrast between GaAs and the bottom cladding layer is higher as compared to the index contrast between GaAs and the bottom cladding layer is higher as compared to the previous previous design, the field is pushed towards the top GaAs/NbN interface and the absorptance is design, the field is pushed towards the top GaAs/NbN interface and the absorptance is improved. improved. For the simulation, a NbN meander (5-nm-thick, 100-nm-wide nanowire with a 160 nm For the simulation, a NbN meander (5-nm-thick, 100-nm-wide nanowire with a 160 nm pitch) on top pitch) on top of a 1-m-wide and 300-nm-thick SNB is considered. A 100-nm-thick HSQ film of a 1-µm-wide and 300-nm-thick SNB is considered. A 100-nm-thick HSQ film (amorphous silicon (amorphous silicon oxide) remaining on top of the nanowires after processing, as well as a residual oxide) remaining on top of the nanowires after processing, as well as a residual 50-nm-thick SiN layer, 50-nm-thick SiN layer, are taken into account during the simulation. The absorption coefficients for TE are taken into account during the simulation. The absorption coefficients for TE and TM polarizations 1 1 and TM polarizations at 1310 nm are calculated as = 1166 cm and = 1399 cm , by using −1 TE −1 TM at 1310 nm are calculated as αTE = 1166 cm and αTM = 1399 cm , by using the same ñ NbN as in [21]. the same ñ as in [21]. The fundamental TE mode, the only interesting one in terms of integration NbN The fundamental TE mode, the only interesting one in terms of integration with the QD sources, is with the QD sources, is presented in Figure 3. From the calculation, a 15 m NbN nanowire on top of presented in Figure 3. From the calculation, a 15 µ m NbN nanowire on top of a 7.5 µ m SNB is already long enough to guarantee that 90% of the input light is absorbed, resulting in a lower probability of suffering of inhomogeneities. Photonics 2016, 3, 55 5 of 12 a 7.5 m SNB is already long enough to guarantee that 90% of the input light is absorbed, resulting in a lower probability of suffering of inhomogeneities. Photonics 2016, 3, 55 5 of 12 Photonics 2016, 3, 55 5 of 12 Figure 3. Simulated amplitude (V/m) and direction of the electric field for the fundamental mode of Figure 3. Simulated amplitude (V/m) and direction of the electric field for the fundamental mode of a Figure suspende 3. Si d m n ulated anobeam am pli (SN tud B)e with (V/m a) top and nio dir bium ection nitrid of the e ( NbN electri ) c wfie ire. ld for the fundamental mode of a suspended nanobeam (SNB) with a top niobium nitride (NbN) wire. a suspended nanobeam (SNB) with a top niobium nitride (NbN) wire. 3.1.2. Performance 3.1.2. Performance 3.1.2. Performance NbN SSPDs were fabricated on top of GaAs SNBs following the design in Figure 3 and the NbN SSPDs were fabricated on top of GaAs SNBs following the design in Figure 3 and the fabricNbN ation S pSPD roces du were re pr e fabri sentc eat d ed in S on ectto iop n 2 of . In GaAs whatS fNB ollo s w foll s, tow heiing r peth rfo e rm design ance ain t a F lo igure w tem 3 p and erat u th re e fabrication procedure presented in Section 2. In what follows, their performance at a low temperature (2.2 fabr K) ica in tioter n p m ro sc of edquantu ure prem sen efficienc ted in Sy, ect dead ion 2.time In w , an had t fjitt oller ow is s, dis the cius r p se er d. foA rm pr an el cieminar at a lo y w mea tem surement perature (2.2 K) in terms of quantum efficiency, dead time, and jitter is discussed. A preliminary measurement (2.2 K) in terms of quantum efficiency, dead time, and jitter is discussed. A preliminary measurement to quickly judge the quality of an SSPD is the current-voltage (I-V) characteristic. The inset of Figure 4a to quickly judge the quality of an SSPD is the current-voltage (I-V) characteristic. The inset of Figure 4a to quickly judge the quality of an SSPD is the current-voltage (I-V) characteristic. The inset of Figure 4a displays the current-voltage characteristic measured for a 20-µ m-long wire, showing a critical current (Ic) displays display the s thcurr e cur ent-voltage rent-voltagecharacteristic characteristic m measur easured ed fofor r a 2 a020- -µ m-m-long long wirwir e, sh e,oshowing wing a cria ticcritical al currecurr nt (Icent ) of 8 µ A. (I )of of88 µ A. A. (a) (b) (a) (b) Figure 4. (a) Device quantum efficiency (DQE) (red) of a 20 µm-long SSPD on top of SNB under il Figure luminati 4. on (a) at D evi 1300 ce nm quan , and tum dark effici co ency unt (D rate QE (bla ) (re ck) d) as of a a fun 20 ct µm io- n loof ngthe SS PD norm on ali top zed of b iSN as B curr un ent der . Figure 4. (a) Device quantum efficiency (DQE) (red) of a 20 m-long SSPD on top of SNB under Ins illum et: c inati urrent on -at vol 1300 tage c nm har , and acte ris dark tic. ( co b) His unt ra tote gram (bla of ck) the ji as att fun er time ction ofof the the d etec nortor mali bzed iase d b i94% as curr Ic. ent. illumination at 1300 nm, and dark count rate (black) as a function of the normalized bias current. Inset: current-voltage characteristic. (b) Histogram of the jitter time of the detector biased 94% Ic. Inset: current-voltage characteristic. (b) Histogram of the jitter time of the detector biased 94% I . The device quantum efficiency (DQE) is plotted in red Figure 4a as a function of the normalized The device quantum efficiency (DQE) is plotted in red Figure 4a as a function of the normalized bias current Ib/Ic, and it is defined as the number of counts minus the dark counts (in black) divided The device quantum efficiency (DQE) is plotted in red Figure 4a as a function of the normalized b bia y ts hc eurrent numbe Ir b/ o Icf, p and hoto it nis s c defi oup n le ed d t as o tth he e w numb aveger uid of e. c W ount e no s tminu e thats tth hee ddark ark c o co uun nts ts s h (io n w bl nack) in F id givided ure 4a bias current I /I , and it is defined as the number of counts minus the dark counts (in black) divided have by the been num m be ea rc s oured f pho in ton a s cryostat coupled w toith the an w a op vetica guil dw e. iW ndow e no. te Th they at t ar he e d th aerefor rk coue nt mo s sh stly own pro ind Fu ig ced ureby 4a by have the number been mof eas photons ured in a coupled cryostat tow the ith waveguide. an optical wW indow e note . Th that ey ar the e th dark erefor counts e moshown stly proin du Figur ced by e 4a the absorption of infrared thermal photons and not indicative of the detector’s performance in an the absorption of infrared thermal photons and not indicative of the detector’s performance in an have optimized been measur environm ed in enta . T cryostat he numbwith er of ph anot optical ons couwindow pled into . th They e ridgar e w e aveguide thereforeis mostly determpr ined oduced by optimized environment. The number of photons coupled into the ridge waveguide is determined by bytthe ransabsorption mission meaof sur infrar ement ed s: tthermal he ratio photons of fiber-in and to fnot iber- indicative out averagof ed the ovedetector r a 2 nm’s sp performance ectra around in 1300 trans nm mis s gi io ve n s m a ev aalue surem of e n Tt s = : 0.06 the % ra, ti me o o an f ing fiber th -in at t th o e ficouplin ber-out g av effic erag iency ed o v from er a th 2 e nfi m ber sp to ec tth rae acent roun er d an optimized environment. The number of photons coupled into the ridge waveguide is determined 1300 nm gives a value of T = 0.06%, meaning that the coupling efficiency from the fiber to the center of the RWG is 𝜂 = √𝑇 = 2.5%, including the loss. Taking this ηc into account, the maximum value by transmission measurements: the ratio of fiber-in to fiber-out averaged over a 2 nm spectra around rea of th ched e RWG for th is e 𝜂 DQ = E √ is 𝑇 28 = %. 2. 5% Wh, ile incl th ud e couplin ing the g loss effi . c Tiency aking is th ris el atiηc vely into account low, due , th toe th maximum e unoptim vialue zed 1300 nm gives a value of T = 0.06%, meaning that the coupling efficiency from the fiber to the center of mo readal ched m for atchi th ng e DQ betE ween is 28th %. e Wh lensed ile th fie be couplin r and th g e ef wav ficiency eguide, is rth ele ati D vely QE is low, the d rel ue e vant to thfi e g unopt ure of im merit ized the RWG is h = T = 2.5%, including the loss. Taking this h into account, the maximum value C c for mointegrated dal matchi experiment ng between sth where t e lensed he ph fibeot r and ons a th re e pro wav duced w eguide, th ithin e D the chip. QE is the relevant figure of merit reached for the DQE is 28%. While the coupling efficiency is relatively low, due to the unoptimized for integrated experiments where the photons are produced within the chip. The timing resolution (jitter) of a detector can be determined illuminating the detector with a The timing resolution (jitter) of a detector can be determined illuminating the detector with a pulsed laser (λ = 965 nm, 100 ps pulse width) and sending the output pulse to a correlation card pulsed laser (λ = 965 nm, 100 ps pulse width) and sending the output pulse to a correlation card together with the laser trigger signal to record the coincidences counts. Figure 4b shows the together with the laser trigger signal to record the coincidences counts. Figure 4b shows the Photonics 2016, 3, 55 6 of 12 modal matching between the lensed fiber and the waveguide, the DQE is the relevant figure of merit for integrated experiments where the photons are produced within the chip. The timing resolution (jitter) of a detector can be determined illuminating the detector with a pulsed laser (l = 965 nm, 100 ps pulse width) and sending the output pulse to a correlation card Photonics 2016, 3, 55 6 of 12 together with the laser trigger signal to record the coincidences counts. Figure 4b shows the coincidence peak using a bias current of 94% I . The fit with a Gaussian distribution gives a full width half coincidence peak using a bias current of 94% Ic. The fit with a Gaussian distribution gives a full width maximum (FWHM) value of 127 8 ps. This value is much shorter than the QD carrier lifetime (~1 ns), half maximum (FWHM) value of 127 ± 8 ps. This value is much shorter than the QD carrier lifetime so correlation measurements are not going to be significantly affected by the detector jitter. The analysis (~1 ns), so correlation measurements are not going to be significantly affected by the detector jitter. of the SSPD output pulse provides a value of FWHM = 3.4 ns and a 1/e decay time of 3.7 ns, which The analysis of the SSPD output pulse provides a value of FWHM = 3.4 ns and a 1/e decay time of 3.7 ns, corrwhich esponds corre very spo well nds very to the well expected to the time expect constant ed time  con = L stant/ τR = for LKa in/value R for a ofvalue wire kinetic of wire inductance kinetic Kin inductance LKin = 180 nH, calculated from the kinetic inductance per square of the NbN wire L = 180 nH, calculated from the kinetic inductance per square of the NbN wire L = 90 pH/ [28]. Kin L = 90 pH/□ [28]. 3.1.3. Autocorrelator Performance 3.1.3. Autocorrelator Performance As already mentioned, the addition of a second independent detector on top of the SNB provides As already mentioned, the addition of a second independent (2) detector on top of the SNB provides a very compact autocorrelator and opens the way to on-chip g () measurements. In order to test the (2) a very compact autocorrelator and opens the way to on-chip g (τ) measurements. In order to test the performance of the autocorrelators, two electrically independent detectors have been fabricated on performance of the autocorrelators, two electrically independent detectors have been fabricated on top of a SNB that is tapered to a RWG. The close packing of the nanowires could in principle produce top of a SNB that is tapered to a RWG. The close packing of the nanowires could in principle produce electrical, magnetic, or thermal coupling between the two detectors. In the following, we prove the electrical, magnetic, or thermal coupling between the two detectors. In the following, we prove the absence of any crosstalk, either static or dynamic, between the two detectors on a single nanobeam, absence of any crosstalk, either static or dynamic, between the two detectors on a single nanobeam, following a similar approach used in [23]. An initial series of tests is performed in static conditions to following a similar approach used in [23]. An initial series of tests is performed in static conditions to determine whether the bias condition of one detector has an influence on the electrical response of the determine whether the bias condition of one detector has an influence on the electrical response of other. Figure 5a shows the I-V characteristic of one detector (A) when the other one (B) is biased above the other. Figure 5a shows the I-V characteristic of one detector (A) when the other one (B) is biased the I above t or when he Ic it or is when not biased it is notat bia all, sed and at al vice l, and versa. vice versa. (a) (b) Figure 5. (a) I-V curve of Detector A while Detector B is unbiased or in resistive state; I-V curve of Figure 5. (a) I-V curve of Detector A while Detector B is unbiased or in resistive state; I-V curve of Detector B while Detector A is unbiased or in resistive state. Inset: sketch of the two independent Detector B while Detector A is unbiased or in resistive state. Inset: sketch of the two independent SSPDs on top of SNB. (b) Coincidence rate under illumination with a 80 MHz pulsed laser at 965 nm SSPDs on top of SNB. (b) Coincidence rate under illumination with a 80 MHz pulsed laser at 965 nm and detectors biased at 97% Ic. and detectors biased at 97% I . Biasing the other detector in the resistive region is a stringent test to prove the absence of static crosstalk. Indeed, the heat dissipated due to Joule effect could thermally affect the other detector, as Biasing the other detector in the resistive region is a stringent test to prove the absence of static has been observed for other substrates like sapphire [29]. Instead, it can be clearly seen how the I-V crosstalk. Indeed, the heat dissipated due to Joule effect could thermally affect the other detector, curve are completely superposed in every electrical regime of the device, proving the absence of static as has been observed for other substrates like sapphire [29]. Instead, it can be clearly seen how the I-V crosstalk. The difference between the Ic values (2.7 µ A) of the two nominally identical detectors is an curve are completely superposed in every electrical regime of the device, proving the absence of static indication that, for 20-µm-long wires, the inhomogeneities still play a role [22]. We then tested the crosstalk. The difference between the I values (2.7 A) of the two nominally identical detectors is existence of dynamic crosstalk. In this test, the temporal variation of the detection probability of one an indication that, for 20-m-long wires, the inhomogeneities still play a role [22]. We then tested detector due to the firing of the other detector is investigated. If the probability to click after a the existence of dynamic crosstalk. In this test, the temporal variation of the detection probability (2) detection event on the other wire was increased or decreased, g (τ) would show a peak or a dip at τ = 0 of one detector due to the firing of the other detector is investigated. If the probability to click after even in the case of laser illumination. To this end, we measured the intensity correlation function (2) a detection (2) event on the other wire was increased or decreased, g () would show a peak or a dip g (τ) of a pulsed laser (λ = 965 nm, 80 MHz repetition rate) coupled to the waveguide. The detectors were biased at Ib = 0.97Ic, and the coincidence counts were measured by sending their outputs to the inputs of a correlation card (PicoHarp 300) using a line with a known delay. In Figure 5b, the coincidence counts are shown as a function of the delay time between the start and stop channels, and as expected coincidences are observed only at delays multiple of the repetition period of the laser Photonics 2016, 3, 55 7 of 12 at  = 0 even in the case of laser illumination. To this end, we measured the intensity correlation (2) function g () of a pulsed laser ( = 965 nm, 80 MHz repetition rate) coupled to the waveguide. The detectors were biased at I = 0.97I , and the coincidence counts were measured by sending their outputs to the inputs of a correlation card (PicoHarp 300) using a line with a known delay. In Figure 5b, Photonics 2016, 3, 55 7 of 12 the coincidence counts are shown as a function of the delay time between the start and stop channels, and as expected coincidences are observed only at delays multiple of the repetition period of the laser (12.5 ns). The coincidence counts are normalized to the average of the peak’s maximum presented in (12.5 ns). The coincidence counts are normalized to the average of the peak’s maximum presented in Figure 5b and show a standard deviation of 0.03 due to statistical fluctuation. Considering that the Figure 5b and show a standard deviation of 0.03 due to statistical fluctuation. Considering that the peak at zero delay has 1.00 normalized coincidences, it is fair to conclude that no sign of dynamic peak at zero delay has 1.00 normalized coincidences, it is fair to conclude that no sign of dynamic crosstalk was observed. crosstalk was observed. 3.2. QD Emission 3.2. QD Emission On-Chip On-Chip Detection Detection The next step toward the demonstration of a fully functional QPIC is to prove that the SSPDs are The next step toward the demonstration of a fully functional QPIC is to prove that the SSPDs ar abl e e able to detec to detect t only only the the ligh light t com coming ing from from the the QDQDs s and and notnot from from other other source sour sces such such as as the the direct direct or scattered light of the laser pump. For this low-temperature (2.2 K) measurement, a 750 nm gain or scattered light of the laser pump. For this low-temperature (2.2 K) measurement, a 750 nm gain switched switched diode diode llaser aser with with a arepet repetition ition rrate ate of of 80 80 M MHz Hz w was as used used toto excite excite ththe e QDs QDs anand d use use them them as an as internal light source. The SSPD on SNB was biased at 0.94Ic, and the output was sent to the correlation an internal light source. The SSPD on SNB was biased at 0.94I , and the output was sent to card together with the laser trigger to record the coincidences counts. In the first measurement, the the correlation card together with the laser trigger to record the coincidences counts. In the first instrument response function (IRF) of the SSPD under direct laser illumination at normal incidence measurement, the instrument response function (IRF) of the SSPD under direct laser illumination at with 30 nW of power was measured. The IRF (Figure 6; blue curve) shows a main peak and a normal incidence with 30 nW of power was measured. The IRF (Figure 6; blue curve) shows a main shoulder, which may be due to a relaxation oscillation of the laser. A Gaussian fit to the main peak peak and a shoulder, which may be due to a relaxation oscillation of the laser. A Gaussian fit to the has a FWHM of 134 ± 4 ps, which corresponds very well to the value of jitter previously measured main peak has a FWHM of 134  4 ps, which corresponds very well to the value of jitter previously for the SSPD on a sample without QDs. At long delay, it was possible to recognize a shoulder due to measured for the SSPD on a sample without QDs. At long delay, it was possible to recognize a shoulder the response to QD emission. The exponential decay fit of the shoulder (green curve) yielded a decay due to the response to QD emission. The exponential decay fit of the shoulder (green curve) yielded time value τd = 0.94 ± 0.01 ns, which corresponds to the typical spontaneous emission lifetime of these a decay time value  = 0.94 0.01 ns, which corresponds to the typical spontaneous emission lifetime InAs quantum dots [30]. In the second measurement, an ensemble of QDs at the edge of the ridge of these InAs quantum dots [30]. In the second measurement, an ensemble of QDs at the edge of the waveguide (0.5 mm away from the SSPD) was excited non-resonantly with the same power, and the ridge waveguide (0.5 mm away from the SSPD) was excited non-resonantly with the same power, on-chip time resolved photoluminescence (PL) data were recorded (Figure 6; black curve). The and the on-chip time resolved photoluminescence (PL) data were recorded (Figure 6; black curve). exponential fit of the decay time yielded a value τd = 0.92 ± 0.01 ns, in good agreement with the decay The exponential fit of the decay time yielded a value  = 0.92  0.01 ns, in good agreement with time observed in the shoulder on the IRF curve. This proves that, whenever the excitation spot is far the decay time observed in the shoulder on the IRF curve. This proves that, whenever the excitation enough from the detector, the laser is completely absorbed in the GaAs so that only the QD emission spot is far enough from the detector, the laser is completely absorbed in the GaAs so that only the QD is detected. emission is detected. Figure 6. Instrument response function (IRF) of the SSPD (blue) and on-chip time resolve QD Figure 6. Instrument response function (IRF) of the SSPD (blue) and on-chip time resolve QD photoluminescence measurement (black). The red line represents the Gaussian fit of the IRF and the photoluminescence measurement (black). The red line represents the Gaussian fit of the IRF and green lines the exponential fit of the decay time of the QD emission. the green lines the exponential fit of the decay time of the QD emission. 3.3. Filters 3.3.1. Design In order to obtain single-photon emission from QDs, a single excitonic line must be filtered out of the complex emission spectrum of one or more QDs. A coupled PhCC-PhCWG system has been theoretically and experimentally investigated and proposed as a system capable of realizing the filtering functionality [31,32]. Indeed, when the wavelength of the input light matches the wavelength Photonics 2016, 3, 55 8 of 12 3.3. Filters 3.3.1. Design In order to obtain single-photon emission from QDs, a single excitonic line must be filtered Photonics 2016, 3, 55 8 of 12 out of the complex emission spectrum of one or more QDs. A coupled PhCC-PhCWG system has been theoretically and experimentally investigated and proposed as a system capable of realizing the of the cavity, light can pass through the cavity via resonant tunneling. The performance of this filter filtering functionality [31,32]. Indeed, when the wavelength of the input light matches the wavelength can be described by coupled-mode theory [33]. The coupling of the cavity to the PhCWG and to the of the cavity, light can pass through the cavity via resonant tunneling. The performance of this leaky radiation modes can be characterized by quality factors QWG and QR, where the total quality filter can be described by coupled-mode theory [33]. The coupling of the cavity to the PhCWG factor QT is obtained from 1⁄𝑄 = 1⁄𝑄 + 1⁄𝑄 , and the transmission is calculated as and to the leaky radiation modes 𝑇can be characterized 𝑊𝐺 𝑅 by quality factors Q and Q , where the WG R 𝑇 = [1⁄(1 + 𝑄 ⁄𝑄 )] . This means that for a fixed QR (which depends on the design and fabrication total quality factor𝑅 Q is obtained from 1/Q = 1/Q + 1/Q , and the transmission is calculated T T WG R of as the T cav = ity) [1/, (incre 1 + Q asing /Q the )] transm . This is means sion requir that for es a a fixed decreQ ase (which of the Q depends WG, whion ch the will design increas and e the WG R R bandwidth fabrication of ofth the e filter cavity), , thus incr deter easing iorthe ating transmission the filtering requir function es a decr ality. ease Thof erefor the e, Q to ,awhich chievewill good WG increase the bandwidth of the filter, thus deteriorating the filtering functionality. Therefore, to achieve filtering performances, a trade-off between the transmission and the bandwidth is required. As the good filtering performances, a trade-off between the transmission and the bandwidth is required. typical spectral separation between lines when few QDs are pumped is in the order of a few nm, we As the typical spectral separation between lines when few QDs are pumped is in the order of a few nm, aimed at QT ≈ 1000. Together with a QR about equal to 10,000, this should ensure good filtering and we aimed at Q  1000. Together with a Q about equal to 10,000, this should ensure good filtering high transmission T . Another important par R ameter is the free spectral range (FSR) around the and high transmission. Another important parameter is the free spectral range (FSR) around the transmission peak: all the QDs with a wavelength different from the cavity must be filtered, so a large transmission peak: all the QDs with a wavelength different from the cavity must be filtered, so a large FSR is necessary (at least 40 nm). Designs with different coupling between the PhCC and the PhCWG FSR is necessary (at least 40 nm). Designs with different coupling between the PhCC and the PhCWG have been simulated. A direct coupling (in-line) of an L3 PhCC (obtained by removing the first three have been simulated. A direct coupling (in-line) of an L3 PhCC (obtained by removing the first three holes from the center of the PhC) with a PhCWG, and a two-hole barrier in between, was found to be holes from the center of the PhC) with a PhCWG, and a two-hole barrier in between, was found to be the design that best fulfills the requirements (Figure 7 inset). Moreover, the diameter of the external the design that best fulfills the requirements (Figure 7 inset). Moreover, the diameter of the external hole of the barrier was reduced by 10%, compared with the others holes, to increase Q. Figure 7 shows hole of the barrier was reduced by 10%, compared with the others holes, to increase Q. Figure 7 the simulated transmissions spectra: both the fundamental and second mode peaks are quite narrow shows the simulated transmissions spectra: both the fundamental and second mode peaks are quite with Q = 1170 and Q = 1230, respectively, and are well separated, resulting in a FSR of 75 nm. The narrow with Q = 1170 and Q = 1230, respectively, and are well separated, resulting in a FSR of 75 nm. fundamental mode has a transmission of 58%, while the second mode a transmission of 78%. The fundamental mode has a transmission of 58%, while the second mode a transmission of 78%. Figure 7. Simulated transmission spectra. Inset: sketch of the chosen design. Figure 7. Simulated transmission spectra. Inset: sketch of the chosen design. 3.3.2 3.3.2. . Per Performance formance Filters according to the chosen design were fabricated and the transmission spectra were Filters according to the chosen design were fabricated and the transmission spectra were experimentally measured at room temperature using two techniques. First, a non-resonant laser experimentally measured at room temperature using two techniques. First, a non-resonant laser (780 nm) was applied from the top on the two sides of the cavity (Figure 8a; red and blue arrows) and (780 nm) was applied from the top on the two sides of the cavity (Figure 8a; red and blue arrows) and directly on top of it (black arrow). The PL coupled to the PhCWG (green arrow) was collected from the directly on top of it (black arrow). The PL coupled to the PhCWG (green arrow) was collected from RWG through a lensed fiber. Figure 8b shows side-collection spectra of the PL signal when the laser the RWG through a lensed fiber. Figure 8b shows side-collection spectra of the PL signal when the was focused on top of the cavity and on the two sides. The blue curve (after cavity) shows a broad laser was focused on top of the cavity and on the two sides. The blue curve (after cavity) shows a broad emission with several peaks in the slow-light part of the dispersion curve of the PhCWG mode [12], while in the other two it is possible to observe the filtered cavity mode (1266 nm) located around 20 nm away from the slow-light dispersion edge. When the pump was applied on top of the PhCC, some additional peaks were observed, compared with the case where the laser was applied before the PhCC, due to the excitation of the PhCWG section around the cavity (since the laser spot is bigger than the area of the cavity). From the Lorentz fit of the cavity mode, a Q of 1000 was obtained, a value close to the one predicted by the simulation. In the spectra before the cavity (red curve) for the full range of wavelengths (1200–1350 nm; not shown), no other cavity peak was observed, indicating that a FSR of at least 66 nm was obtained. 𝑊𝐺 Photonics 2016, 3, 55 9 of 12 emission with several peaks in the slow-light part of the dispersion curve of the PhCWG mode [12], while in the other two it is possible to observe the filtered cavity mode (1266 nm) located around 20 nm away from the slow-light dispersion edge. When the pump was applied on top of the PhCC, some additional peaks were observed, compared with the case where the laser was applied before the PhCC, due to the excitation of the PhCWG section around the cavity (since the laser spot is bigger than the area of the cavity). From the Lorentz fit of the cavity mode, a Q of 1000 was obtained, a value close to the one predicted by the simulation. In the spectra before the cavity (red curve) for the full range of wavelengths (1200–1350 nm; not shown), no other cavity peak was observed, indicating that a FSR of Photonics 2016, 3, 55 9 of 12 at least 66 nm was obtained. (a) (b) Figure 8. (a) Sketch of the design B filter: the red (before cavity), black (on cavity), and blue (after Figure 8. (a) Sketch of the design B filter: the red (before cavity), black (on cavity), and blue (after cavity) cavity) arrows indicate the different position of the top excitation, while the green the direction of the arrows indicate the different position of the top excitation, while the green the direction of the side side collection. collectio (bn ).PL (bspectra ) PL spe forcthe tra thr for ee dif the f er thr ent ee excit differen ation posi t exci tions tatiwit on hpositions color coding wit corr h c esponding olor coding to (a). The blue spectra has been reduced by a factor of 5 for greater clarity. corresponding to (a). The blue spectra has been reduced by a factor of 5 for greater clarity. The transmission spectrum shown in black in Figure 9 was calculated as the ratio between the The transmission spectrum shown in black in Figure 9 was calculated as the ratio between the PL PL spectra acquired from the two sides of the cavity, therefore before and after filtering. The peaks spectra acquired from the two sides of the cavity, therefore before and after filtering. The peaks were were modulated modulat by ed Fabry-Per by Fabry ot-Perot fringes frdue inges to due reflections to reflect at the ions cleaved at the facet cleaved and at facet the an interface d at thbetween e interface bet Rween WG and RW SNB. G and A SN clear B. A transmission clear transmiss peakion at the peak same at th wavelength e same wavelen of the gth cavity of thmode e cavity was measily ode was distinguishable and well isolated, confirming a very large FSR, although the transmission was only easily distinguishable and well isolated, confirming a very large FSR, although the transmission was 21%  2%. Secondly, a direct measure of transmission was taken using a PhCC-PhCWG coupled with only 21 ± 2%. Secondly, a direct measure of transmission was taken using a PhCC-PhCWG coupled a RWG on both sides, performing a fiber-in:fiber-out measurement with a tunable laser. Considering with a RWG on both sides, performing a fiber-in:fiber-out measurement with a tunable laser. a coupling efficiency of h = 2.5%, the obtained transmission for this measurement was equal to Considering a coupling efficiency of ηc = 2.5%, the obtained transmission for this measurement was 18%  2%. equal to 18 ± 2%. Figure 9. Experimental transmission spectra: black curve indirectly calculated from the ratio between the photoluminescence (PL) spectra before and after filtering, red curve from direct fiber-in:fiber-out measurement. Photonics 2016, 3, 55 9 of 12 (a) (b) Figure 8. (a) Sketch of the design B filter: the red (before cavity), black (on cavity), and blue (after cavity) arrows indicate the different position of the top excitation, while the green the direction of the side collection. (b) PL spectra for the three different excitation positions with color coding corresponding to (a). The blue spectra has been reduced by a factor of 5 for greater clarity. The transmission spectrum shown in black in Figure 9 was calculated as the ratio between the PL spectra acquired from the two sides of the cavity, therefore before and after filtering. The peaks were modulated by Fabry-Perot fringes due to reflections at the cleaved facet and at the interface between RWG and SNB. A clear transmission peak at the same wavelength of the cavity mode was easily distinguishable and well isolated, confirming a very large FSR, although the transmission was only 21 ± 2%. Secondly, a direct measure of transmission was taken using a PhCC-PhCWG coupled with a RWG on both sides, performing a fiber-in:fiber-out measurement with a tunable laser. Considering a coupling efficiency of ηc = 2.5%, the obtained transmission for this measurement was Photonics 2016, 3, 55 10 of 12 equal to 18 ± 2%. Figure 9. Experimental transmission spectra: black curve indirectly calculated from the ratio Figure 9. Experimental transmission spectra: black curve indirectly calculated from the ratio between between the photol theum photoluminescence inescence (PL) spectra (PL) before spectra and after befor fie lterin and g, after red cur filtering, ve from di red rectcur fiber ve -in fr :fib om er-dir out ect measurement. fiber-in:fiber-out measurement. Photonics 2016, 3, 55 10 of 12 3.3.3. Filtering of Single QD Lines 3.3.3. Filtering of Single QD Lines Finally, the capability of the on-chip filter to isolate a single QD excitonic line is presented. Finally, the capability of the on-chip filter to isolate a single QD excitonic line is presented. In In Figure 10, two PL spectra measured at low temperature (2.2 K) are shown for side collection when Figure 10, two PL spectra measured at low temperature (2.2 K) are shown for side collection when the laser is focused either on top of the cavity or on top of the SNB before the cavity. To be able to better the laser is focused either on top of the cavity or on top of the SNB before the cavity. To be able to discriminate the single QD excitonic emission line, the pump laser power was strongly attenuated better discriminate the single QD excitonic emission line, the pump laser power was strongly (20 nW). In this particular case, a QD was emitting (1194 nm) exactly within the bandwidth of the attenuated (20 nW). In this particular case, a QD was emitting (1194 nm) exactly within the bandwidth of the cavity mode; therefore, a single excitonic line was passing through the filter while all the other cavity mode; therefore, a single excitonic line was passing through the filter while all the other QDs QDs emission lines at different wavelengths were suppressed. However, due to the lower Q = 200 of emission lines at different wavelengths were suppressed. However, due to the lower Q = 200 of this this specific cavity, some residual QD emission close to the filtered line was not yet well suppressed. specific cavity, some residual QD emission close to the filtered line was not yet well suppressed. Figure Figure 10. 10 PL . PL spectra spectra on two on two diff er diff ent erent excitation excitati position on positi following on following the pr the evious previo color us co coding. lor codi A ng single . A single QD excitonic line is clearly visible on the red curve. QD excitonic line is clearly visible on the red curve. 4. Discussion 4. Discussion The low temperature fabrication process, developed in order to integrate single-photon sources The low temperature fabrication process, developed in order to integrate single-photon sources and detectors on a GaAs platform, has shown an ability to preserve high performance for each and detectors on a GaAs platform, has shown an ability to preserve high performance for each component of the proposed QPIC. In particular, the SSPD patterned on top of a SNB presents a DQE component of the proposed QPIC. In particular, the SSPD patterned on top of a SNB presents a DQE of of 28% and a jitter of 127 ± 8 ps. The simulation of the electric field mode guided inside the SNB 28% and a jitter of 127  8 ps. The simulation of the electric field mode guided inside the SNB showed showed the possibility of using shorter wires (15-µm-long), preserving high absorptance (90%) and the possibility of using shorter wires (15-m-long), preserving high absorptance (90%) and thereby thereby reducing the inhomogeneities problem. Even if the experimental DQE did not improve as reducing the inhomogeneities problem. Even if the experimental DQE did not improve as much as much as expected, a new record for the DQE on the GaAs platform has been established. expected, a new record for the DQE on the GaAs platform has been established. Autocorrelators Autocorrelators based on two electrically independent SSPDs on top of a SNB did not present any based on two electrically independent SSPDs on top of a SNB did not present any static or dynamic static or dynamic crosstalk; this, together with a jitter much shorter than the QD lifetime, shows that crosstalk; this, together with a jitter much shorter than the QD lifetime, shows that the integrated the integrated autocorrelators are suitable for the on-chip measurement of single-photon emission. On the other hand, the PhCC-PhCWG system showed the expected filtering functionality of a single- excitonic line. From the experimental transmission spectra, a FSR ≥ 66 nm and Q = 1000, as estimated from the simulation, were found; however, the transmission was 21% lower than expected, presumably due to disorder-induced optical losses. Even with such transmission, it has been possible to prove that single excitonic lines can be isolated and transmitted through the filter. Moreover, it has been proved that the QD emission can be detected by the on-chip SSPDs, whenever the laser is focused at a sufficiently long distance from the detector. In conclusion, a platform able to combine QD sources, filters, detectors, and autocorrelators has been developed and successfully tested, paving the way for numerous QIP applications. Acknowledgments: This research is financially supported by the European Commission, Initial Training Network PICQUE (No. 608062), by the Dutch Technology Foundation STW, Applied Science Division of NWO, the Technology Program of the Ministry of Economic Affairs under projects No. 10380, 12662 and by NanoNextNL, a micro and nanotechnology program of the Dutch Ministry of Economic Affairs, Agriculture and Innovation (EL&I) and 130 partners. Photonics 2016, 3, 55 11 of 12 autocorrelators are suitable for the on-chip measurement of single-photon emission. On the other hand, the PhCC-PhCWG system showed the expected filtering functionality of a single-excitonic line. From the experimental transmission spectra, a FSR  66 nm and Q = 1000, as estimated from the simulation, were found; however, the transmission was 21% lower than expected, presumably due to disorder-induced optical losses. Even with such transmission, it has been possible to prove that single excitonic lines can be isolated and transmitted through the filter. Moreover, it has been proved that the QD emission can be detected by the on-chip SSPDs, whenever the laser is focused at a sufficiently long distance from the detector. In conclusion, a platform able to combine QD sources, filters, detectors, and autocorrelators has been developed and successfully tested, paving the way for numerous QIP applications. Acknowledgments: This research is financially supported by the European Commission, Initial Training Network PICQUE (No. 608062), by the Dutch Technology Foundation STW, Applied Science Division of NWO, the Technology Program of the Ministry of Economic Affairs under projects No. 10380, 12662 and by NanoNextNL, a micro and nanotechnology program of the Dutch Ministry of Economic Affairs, Agriculture and Innovation (EL&I) and 130 partners. Author Contributions: G.E.D., M.P. and A.F. conceived and designed the experiment, F.W.M.v.O. grew the wafer and G.E.D. with the contribution of R.G. and M.P. fabricated the devices. M.P. with the contribution of S.F.P. developed the simulation. G.E.D., M.P. and S.B. performed the experiments. G.E.D. analyzed the data and wrote the paper. A.F. supervised the project. Conflicts of Interest: The authors declare no conflict of interest. References 1. O’Brien, J.L. Optical quantum computing. Science 2007, 318, 1567–1570. [CrossRef] [PubMed] 2. 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Published: Oct 21, 2016

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