OPTO−ELECTRONICS REVIEW 18(4), 352–365 DOI: 10.2478/s11772−010−0050−7

Nanophotonic technologies for single-photon devices INVITED PAPER

A.

GERARDINO*1,

M.

FRANCARDI1, N.

A. GAGGERO1, F. MATTIOLI1, R. LEONI1, L. BALET2,3, F. MARSILI2, and A. FIORE2

CHAUVIN2,4,

1CNR−

Istituto di Fotonica e Nanotecnologie, Via Cineto Romano 42, 00156 Rome, Italy Research Institute, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands 3Institute of Photonics and Quantum Electronics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH−1015 Lausanne, Switzerland 4Institut des Nanotechnologies de Lyon, Université de Lyon, CNRS UMR5270, INSA de Lyon, F−69621 Villeurbanne Cedex, France

2COBRA

The progress in nanofabrication has made possible the realization of optic nanodevices able to handle single photons and to exploit the quantum nature of single−photon states. In particular, quantum cryptography (or more precisely quantum key dis− tribution, QKD) allows unconditionally secure exchange of cryptographic keys by the transmission of optical pulses each containing no more than one photon. Additionally, the coherent control of excitonic and photonic qubits is a major step for− ward in the field of solid−state cavity quantum electrodynamics, with potential applications in quantum computing. Here, we describe devices for realization of single photon generation and detection based on high resolution technologies and their physical properties. Particular attention will be devoted to the description of single−quantum dot sources based on photonic crystal microcavites optically and electrically driven: the electrically driven devices is an important result towards the real− ization of single photon source “on demand”. A new class of single photon detectors, based on superconducting nanowires, the superconducting single−photon detectors (SSPDs) are also introduced: the fabrication techniques and the design proposed to obtain large area coverage and photon number−resolving capability are described.

Keywords: photonic crystal microcavities, quantum dots, single photon sources and detectors, superconducting single photon detectors, photon number−resolving detectors.

1. Introduction In the last decades, a huge progress was made in the field of nanotechnologies, opening the path to a new class of experi− ments that involve the nanoscale properties of light and mat− ter [1]. The possibility of creation of nano−structured materi− als on the scale of the wavelength of the radiation, or even smaller, has opened the access to new physical properties involving the quantum nature of both light and matter. The scaling down of electronic device physical dimensions has been at the heart of the exponential growth of semiconduc− tor electronics, allowing a higher level of integration and reduction of fabrication cost [2]. At the same time, the num− ber of electrons used to encode a single bit in transistors and memories has decreased proportionally to the device area, which obviously allows higher speed and reduced power consumption. An analogous phenomenon is taking place also in optical devices, the number of photon involved is in continuous reduction. In fields like quantum information processing (QIP) [3], quantum computing, quantum key *e−mail:

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[email protected]

distribution, and also for fundamental studies on quantum electrodynamics (QED) it would be important to have at our disposal single photon sources and detectors at telecom wavelength [4]. The realization of a single photon source requires a change in the photon number statistics from the Poisson statistics of laser light to a highly non−classical statistics where the photon number is perfectly defined. Single−pho− ton pulses can be obtained by exciting single quantum sys− tems, such as single atoms or molecules, an atom pumped to an excited energy state relaxes to the ground state by emit− ting a single photon if the pump pulse duration is much shorter than the relaxation time. So, a single photon emitter should ideally deliver one photon per excitation pulse. In order to characterise a single photon emitter, quantum crite− ria must be adopted and the most suitable experiment to ve− rify a single photon emission is the Hanbury−Brown and Twiss (HBT) experiment [5]. As it is well known, in this experiment the light is split in two modes 1 and 2 by a 50/50 beam splitter, and two photon detectors are placed in the two exit arms. The autocorrelation function is measured and it can be defined as

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g (2 ) (t ) =

I 1 (t ) I 2 (t + t ) I

2

,

(1)

where Ii(t) is the intensity at the time t at the place i and the brackets denote the average over time. I2 is here the average intensity detected and acts as the normalization factor. In case of single photon emission, the emitted pulse will be detected at the time t by one of the two detectors but never in both of them – in this case g(2)(0) = 0. The zero−time value of the autocorrelation function is thus directly related to the photon number distribution and represents a figure of merit for identifying a suitable single photon source. Single−pho− ton generation from atoms [6] and molecules [7] has been demonstrated even if it is very difficult to isolate and effi− ciently collect light from single emitters. Various types of emitters have been proposed and utilized as sources for sin− gle photons [8,9], among them quantum dots, quantum wells, and colour centres. In particular, quantum well and quantum dots are an example of controlled matter confine− ment, they are defined as being structures confined respec− tively in 1 and in 3 dimensions and hence experience quan− tum mechanical effects due to the confinement at the nano− scale. In QDs, this results in a discrete energy spectrum and in strong carrier correlation effects, making the energy of each electronic configuration (excitons, biexcitons, and multiexcitons) easily distinguishable. While each QD can in practice contain several excitons, and will thus emit several photons after the excitation, by spectrally isolating the pho− ton emitted by the last exciton, a single−photon pulse is obtained [10–13]. QDs can be synthesized by wet chemistry controlling their shape, size, and optical properties like the colloidal QDs that recently are gaining an important role among the single photon sources [14]. However, here our attention will be devoted to semiconductor quantum dots (QDs) obtained by strain−driven self−assembly in conventio− nal semiconductor epitaxy systems, which can be controlled in size, optical properties, and density [15,16]. Single−photon detector development has been rapidly expanding because of the interest in optical quantum infor− mation (QI) applications where photons (individual quan− tum objects) are used to encode and manipulate information [17]. A wide range of time−correlated single−photon count− ing experiment uses conventional single−photon detectors based on avalanche photodiodes and photomultipliers [18, 19], but applications such as quantum key distribution and linear optical quantum computing (LOQC) requires very stringent constraints on single−photon detectors performan− ces [17]. This is the reason why great efforts have been devoted to the optimization of conventional single−photon detectors and development of emerging new photon−count− ing technologies, especially in the application−relevant tele− com wavelength region (1310–1550 nm). Moreover, in many cases, the photon number−resolving (PNR) function− ality (i.e. counting the number of photons in a pulse) is also important for the implementation of many protocols for quantum repeaters and LOQC [20,21]. A linear detector with single−photon sensitivity can also be used for measur− Opto−Electron. Rev., 18, no. 4, 2010

ing a temporal waveform at extremely low light levels (long−distance optical communications, fluorescence spec− troscopy, and optical time−domain reflectometry). A great number of detectors such as avalanche photodiodes (APDs) [22] and transition edge sensors (TES) [23] have been deve− loped on the purpose. While silicon avalanche photodiodes are not sensitive in telecom wavelength region, InGaAs APDs suffer from large dark count rates and limited speed characteristics [17]. On the other hand, the timing properties of TES detectors are relatively poor, with jitter times of around 100 ns and a thermal time constant of the detector element of the order of 1 μs that affects the dead time of the detector itself [17]. In this framework, nanophotonic tech− nologies can play a major role also in the detection process, the tiny energy carried by a single photon (»10–19 J in the wavelength range of interest) has a larger effect when ab− sorbed in a nanoscale structure, and is therefore more easily detected. The capability of patterning at the nanometric scale has brought to the development of a new class of pho− ton detectors based on superconducting nanowires, called superconducting single−photon detectors (SSPDs) [24]. The SSPDs are promising candidates for all the applications mentioned above because they offer single photon sensitivi− ty from visible to mid−infrared wavelengths [25], low dark counts [25], short recovery time (high frequency operation) [26], and low timing jitter [27]. In this paper, we discuss the application of nanophoto− nics technologies to devices aimed to the generation of sin− gle photons from InAs QDs in GaAs membrane whose emission is in the telecom window (1300 nm) and to their detection by SSPDs. To perform an efficient extraction of the emitted photons from the InAs QDs we have to solve the problem that photons are emitted in a very high refraction index medium (nGaAs »3.5): assuming for simplicity isotro− pic emission, 98% of the photons are totally internally re− flected at a planar GaAs/air interface, and most of the out coupled ones are deviated at angles larger than the typical numerical aperture of collection optics (0.1–0.5). A largely exploited approach to overcome this intrinsic limit is to increase the rate of spontaneous emission (SE) at angles which are easily collected, by inserting the QD in a micro− cavity, manipulating in this way the optical modes at dis− posal of the emitted light. Our work in the last years has been aimed to the realization of single−QD sources optically [29–32] and electrically [33–35] driven based on photonic crystal microcavites. In Sect. 2 and 3, we describe the opti− mization of the fabrication process both in optical and elec− trical injection, respectively. In particular, the electrically driven device is a formidable challenge from the technologi− cal point of view and represents a fundamental step towards achieving an “on demand” single photon source. For what concerns the single−photon detector technology our atten− tion will be devoted to SSPDs based on ultrathin NbN as absorbing superconducting material (NbN is used for its fast photoresponsive properties [28]) patterned by electron beam lithography in a meander shape [24], in order to have

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Nanophotonic technologies for single−photon devices a large area coverage and at the same time line widths of 100 nm or even less. In Sect. 4, together with the SSPD working principle and the fabrication process, designs pro− posed to enhance device active area and to obtain PNR functionality, will be also discussed. This emerging techno− logy has provided a key measurement tool for single−photon science and applications.

2. Photonic crystal microcavities as single photon sources As described in the Introduction, the photon extraction effi− ciency may be increased by integrating an optical nanoca− vity around a QD. This idea is based on the Purcell effect, i.e., the capability to modify the spontaneous emission (SE) rate of an emitter when it is placed inside a cavity. The local density of optical states is modified by the presence of the cavity so allowing the control and modification of the SE rate. The following equation [36] describes the coupling of a two−level system (such as a single exciton) to a given mode of the cavity [37] r r $ (r ) df 3 t free e Dwc2 Q 3l , (2) = r 2 4(w - w ) 2 + Dw 2 t cavity 4p 2 n 3 Veff c c e d where tfree is the emitter SE lifetime in the free space, tcavity is its SE lifetime inside the cavity, n is the refractive index of the medium in which the emitter is embedded, Q is the cavity mode quality factor, Veff is the mode field volume. The first term 3l3Q/4p 2n3Veff is the, so-called, Purcell factor Fp: its value depends on the ratio Q/Veff and controls the SE enhancement factor. The second and third terms represent respectively the spatial, r spectral and polarisation mismatch of the emitter dipole d to the mode field (whose polarisation and spatial dependence are described by the vector field $f ( rr )) at the position of the emitter (rr ), wc is the cavity resoe nant frequency, we is the emitter frequency and Dwc the cavity linewidth. The last two terms are always £ 1 and when they are equal to 1, Eq. (2) reduces to the Purcell factor. Different types of micro and nanocavities have been proposed and investigated [38]. One of the most suitable implementations of optical nanocavities with the high qual− ity factor Q and volume comparable to the wavelength of the emitted light can be accomplished by exploiting the unique characteristics of Photonic Crystal (PhCs). This new class of materials was proposed, starting from an idea of Yablonovitch [39] and John [40], by Joannopoulos et al. [41] and it is based on the possibility of creation of a peri− odic variation in the refractive index of dielectric materials in order to affect the properties of photons, in much the same way that ordinary semiconductor crystals affect the properties of electrons. Photons can be then described in terms of bandstructures, photonic band gaps are opened and ranges of frequencies are forbidden for propagation. The introduction of defect like points and lines allows the cre− ation of nanocavities and waveguides, respectively. Typical

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designs for PhCs are arrays of holes or rods arranged in square or hexagonal lattices in order to perform the index variation in an isotropic way. The fabrication of these mate− rials has been possible only recently, because the periodicity length scale is of the order of the wavelength of light to be confined, so it needs very high resolution (down to tens of nanometers) fabrication techniques. Electron beam lithography is the most suitable techno− logy to transfer the patterning into dielectric materials with high reproducibility and also the etching processes must be calibrated in order to obtain vertical profiles. In fact, in case of nanocavities, their optical properties are strictly depen− dent on the chosen geometry and a precise control during all the steps of the fabrication process is really crucial.

3. Optical excitation Many studies were published in the last decade on the opti− mization of PhC nanocavities [42,43], on the possibility of measuring the Purcell effect by a PhC based nanocavity [44–47] and quite recently on their capability to act as effi− cient single photon sources [48]. An L3 defect cavity is formed by keeping three holes un−etched along the GK direction of a triangular PhC and parallel to a cleaved edge of the GaAs sample. Moreover, the PhC nanocavities are fabricated on a suspended membrane in order to ensure a tri− dimensional confinement of the light, the in−plane confine− ment is due to the triangular lattice based two−dimensional PhC cavities and the vertical one is achieved by the refractive index symmetric interfaces air−semiconductor−air of the slab that in the ideal case produce total internal reflection [49]. In our studies, the point defect nanocavities were fabri− cated on heterostructures grown on a GaAs substrate: a sin− gle layer of high density QDs at the beginning and later of low density QDs emitting at 1300 nm was grown at the cen− tre of a 320−nm–thick GaAs membrane on the top of a 1500 nm−thick Al0.7Ga0.3As sacrificial layer. The low density QD layers were obtained by molecular beam epitaxy at very low growth rates, a technique that allows to reach ultralow areal densities (up to 1–2 dots/um2) and large dot size for emis− sion in the 1300−nm band [50]. A 150−nm–thick SiO2 layer was deposited by ECR−PECVD on the top of the substrate. The PhC pattern was transferred by e−beam lithography at 100 kV on a 200−nm–thick PolyMethyl Methacrylate (PMMA) positive tone electronic resist that acts as a mask for the SiO2 etching by CHF3 based reactive ion etching (RIE). The SiO2 mask was used to transfer the pattern on the GaAs layer by SiCl4/O2/Ar RIE. The GaAs dry etch process is carried out through the membrane and partially into the Al0.7Ga0.3As layer in order to make easier the successive wet etching step. The membrane was then released by a selective etching of the Al0.7Ga0.3As sacrificial layer in a 3%−HF solution for 5 min. This last process step is very delicate and the etching concentration and time have been optimized to get a precise control on the etching rate. A SEM image of a L3 nanocavity at the end of the fabrica− tion process is shown in Fig. 1 together with a SEM image

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© 2010 SEP, Warsaw

of a PhC structure after the HF etching. The release of the membrane is clearly visible. Our studies on PhCs GaAs nanocavities as single photon sources have started with a systematic study of cavities based on the, so−called, “litho− graphic tuning” [51]. The PhCs filling factor has been fixed to 35%. Each PhC structure was then designed for different lattice parameter a spacing between 283 nm and 375 nm (steps of about 10 nm) [29]. This lithographic approach let us to cover all the expected band gap and to compensate the losses due to the non ideal vertical confinement [37,52]. Besides, for each a, we studied L3 cavity behaviour when both lateral displacement of the first holes adjacent to the cavity d L and their radius r were changed following Akahane et al. [42,53]. The displacements dL ranges from 0.125a to 0.225a in 0.025a steps. For each dL value, the radius decreases linearly from r0 to r0/3 in ten steps [ri = r0 (5,6,7,8,9,10,11,12,13,14)/15]. The array of nanocavities covers all the wavelengths in the BandGap and gives us the possibility of understanding the complete mode dynamics. Moreover, the total array was replicated several times in dif− ferent fabrication runs in order to verify the reproducibility of our results. In this way we could identify the best reso−

nant mode cavity for a L3 cavity, a = 311 nm, dL = 0.125 a r = 0.53r0 and a Q = 3100. On the basis of these results, we carried on the same process on heterostructures with lower areal density of QDs down to a single layer of low density (5–7 dots/μm2) self−assembled InAs QDs emitting at 1.3 μm at low temperature [31]. In this case, we varied the a value only from 300 nm to 340 nm with 5−nm step. For each a value, twelve L3 point defect nanocavities were fabri− cated with different radii r and lateral displacement dL of the first holes adjacent to the cavity to increase the mode cavity Q. The optical characterization of the nanocavities was per− formed in a confocal microphotoluminescence (μPL) setup at 10 K, under pumping with a pulsed laser at 750 nm. In Fig. 2, a μPL spectrum of a modified L3 cavity is shown. In this case, the QDs density is 5–7 dots/μm2 and the cavity Q is about 15000, indicating the high quality of the fabrication process. The Purcell factor can be estimated as ~1500, assu− ming Veff = 0.73( l c n) 3 [42]. In the showed spectrum, are clearly visible the cavity mode and two QD emissions. From the power and temperature dependence of these last lines we can surely conclude that they arise from excitons or charged excitons (as opposed to cavity modes), but we did not perform detailed measurements allowing us to deter− mine the charge state. According to Eq. (2), the ideal SE en− hancement needs high Purcell factor but also the fulfilment of both spectral and spatial resonance between the cavity mode and the QD signal. As regards the spatial resonance, our QDs are randomly distributed inside the membrane and we expect to work on a weak coupling regime. Several groups are studying spatial matching using nanofabrication techniques. For example, growth on pre−patterned surfaces can provide positional control of QDs [54–58]. On the other hand, the PhC cavity can be fabricated aligned with a ran− domly nucleated QD and its resonant frequency matched to the QD emission by post−growth processing [47]. In litera− ture, an intense activity dedicated to study the spectral cou− pling between emitter and cavity, is also reported [59,60].

Fig. 1. (a) SEM image of a PhC nanocavity: in this case a= 340 nm, dL= 0.15a, r = 8/15r0, (b) SEM image of a cleaved PhC structure af− ter the HF etching of the AlGaAs layer and

Fig. 2. μPL spectrum of a nanocavity mode in case of a low density sample (5–7 dots/m2). The Q cavity mode is 15000. QD1 and QD2 are emissions from two separate QDs.

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Nanophotonic technologies for single−photon devices Tuning has been attempted so far with various methods, in− cluding temperature sensitivity [52], digital etching [47,61], liquid crystal infiltration [62–64], adsorption of gas mole− cules [65,66] and nano−oxidation [67]. Another interesting technique is the reversible tuning by SNOM glass tip [46,32]. In our experiment we exploited the reversible shift due to residual gas molecules frozen on the sample surface, as in cryopumping systems, which decrease the filling factor of the PhC and modify the resonant energy of the mode [65]. In this way we were able to couple the cavity mode to the QD emission labelled as QD1. We performed time−resolved measurements by coupling the PL signal to a fiber−coupled superconducting single photon detector (SSPD) [68,19], of the same type as those described in the following Sect. 4, and measuring the histo− gram of delays between the laser pulse and the detector out− put. The SSPD has a quantum efficiency of 10% and a dark count rate of 8 Hz at a bias current of 21 μA. The temporal resolution of the system is around 150 ps and deconvolution was performed to extract the emitter lifetime. The PL signal was spectrally selected by a fiber−coupled thin film optical tuneable filter with 0.8 nm bandwidth. The sample was excited with ~5 nW incident power at 20 MHz. In this regime, the intensity of the mode is much smaller than the QD emission. Figure 3(a) presents the temporal dynamics measured on a QD ensemble located outside the PhC region (black squares) and the line from QD1 at 2.5 nm detuning (blue squares). In Fig. 3(b), it is shown the QD1 line on res− onance (red triangles). The measured lifetime of the QDs located outside the PhC region is tbulk = 1.2 ns in good agreement with previously reported values for these QDs [69]. Instead, the PL decay time of the QD in resonance clearly shows two time constants. The faster is t1QD1= 150 ps. This decrease in the decay time as compared with tbulk is attributed to the Purcell effect and indicates an 8−fold rate enhancement for QD1. The longer time is t2QD1 = 1.9 ns. It is believed to come from out of resonance emission coupled to the optical bandwidth of our interference filter or from the background coupled to the mode. In contrast, when QD1 is detuned from the cavity mode, a lifetime of toff QD1 =3.6 ns is measured [Fig. 3(a), blue squares]. From the off and on resonance lifetimes of QD1, the coupling efficiency into the mode defined as b = 1 – (t1QD1/toff QD1) is 96%. This result is very promising. As most photons are emitted into the cav− ity mode, very large collection efficiency can be obtained by optimizing the far field of this mode. These results encouraged us to design a possible configuration to pump our cavities also by electrical injection.

4. Electrical excitation The implementation of electrical injection in a low−loss, ultra small cavity poses tremendous fabrication and experi− mental challenges [70]. On one hand, electrical contacts must be integrated in an »μm scale device without signifi− cantly increasing the optical losses (e.g., from free−carrier

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Fig. 3. (a) Temporal dynamics measured on a QD ensemble located outside the PhC region (black squares) and the line from QD1 at 2.5 nm detuning (blue squares) and (b) temporal dynamics of the QD1 emission coupled to the cavity mode.

absorption in the metal or in doped layers). On the other hand, the experimental demonstration of enhanced sponta− neous emission requires an ultrafast (sub−ns) electrical prob− ing, requiring a careful control of device parasitics. Evi− dence of spontaneous emission control in electrolumines− cence has been recently deduced from the characteristics of metallic−coated nanolasers [71] and of electrically−contac− ted micropillars [72,74] We developed a very challenging fabrication process based again on e−beam lithography and thin−film processes [34]. It consists of several lithographic steps and each new layer needs to be aligned to the previous one with an accu− racy £ 500 nm. The new sample is grown by molecular beam epitaxy and it is a 370−nm thick GaAs membrane on the top of a 1.5−μm Al0.7Ga0.3As sacrificial layer. The mem− brane is p−doped in the upper part and n−doped in the lower one. A single layer of low areal density InAs QDs, with emission at 1.3 μm at liquid helium temperature, is embed−

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ded in the middle of the membrane. Mesas with diameter 8 and 10 μm are wet−etched in diluted H3PO4/H2O2/ H2O solution to a height of about 300 nm using a SiO2 mask (e−beam exposure, SiO2 deposition and lift−off). It must be underlined that this wet etching is extremely critical, due to the very thin n−doped GaAs layer (80 nm) on which it has to be stopped. To pattern the n−contact at the bottom of the mesa, an alloy of Ni/Ge/Au/Ni/Au has been deposited and annealed (400 C for 30 min) to obtain a better electrical behaviour. A 200−nm thick Si3N4 layer is then deposited to isolate the n and p contacts and then removed from the n−contact and from the top of the mesa. The p contact pat− terns are exposed on a 1−μm thick UVIII resist layer and then patterned by lift−off in Acetone of a 110−nm Cr/Au layer. Three tilted Cr/Au evaporations are performed in order to obtain a continuous film over the mesa lateral edge. In the same evaporation step, the annular top p−contact and the bottom n−contact are connected to ground−signal−ground coplanar electrodes on the top of the Si3N4 surface. Finally, an annular gold cover is evaporated around the entire mesa edge in order to filter out light scattered by the mesa sidewalls. The LED structure is then completed. The last step con− sists of the integration of the PhC nanocavity on the top of it. A 150−nm thick layer of SiO2 is deposited by ECR−PECVD and the PhC nanocavities are transferred by e−beam lithog− raphy on PMMA resist on the top of the mesa. Successive etching of the SiO2 (CHF3/Ar based RIE) and of the GaAs membrane (SiCl4/O2/Ar RIE) allows the transfer of the PhC cavity. The membrane is then released in diluted HF. The chosen cavity is again an L3 type with three different lattice parameters. The air filling factor was measured to be around 27%. The first holes on each side are shifted outwards by 15% and rescaled to 61% of the unperturbed holes diame− ters, following the optimization process described before. In Fig. 4(a), it is shown a global view of the device (optical microscope) and in Fig. 4(b), a particular of the PhC nano− cavity and of the n and p contact on the mesa (SEM image). The electro−optical characterization of the LED has been performed in a cryogenic probe station coupled with a mi− croelectroluminescence (μEL) setup, see Ref. 35 for the details. IV−curves were recorded, showing clear diode behaviour and a relatively low forward−bias (reverse−bias) threshold voltage of 2 V (–7 V), which proves the high qual− ity of the process and very good electrical contacts. The EL spectrum at I = 1.25 mA is shown in Fig. 5 (grey line). A clear cavity mode is observed around 1329 nm with a quality factor Q = 4600 (magnified in the inset), superim− posed on a broad emission line corresponding to ground− −state emission from the 10–20 QDs emitting within the col− lection area. The emission spectrum under optical pumping, measured on the same open−circuited device in a different μPL setup, presents a very similar Q factor of 4000 (blue line). A small redshift 5 nm is observed in the EL spectrum, likely due to higher device temperature under electrical pumping. Moreover, in the PL spectrum, a QD emission line is visible at ~1275 nm that it is not visible in case of EL Opto−Electron. Rev., 18, no. 4, 2010

Fig. 4. Optical microscope image of the PhC LED: (a) together with a SEM image (b) of the mesa region. The n contact is visible in the upper part and so the p contact and the anular ring around the mesa. The PhC nanocavity is positioned in the centre of the mesa.

spectrum due to the different resolution of the μPL setups and possibly to the different temperature of the active area. For this reason, we performed time resolved measurements of the cavity peak pumped by the QD ensemble emission to demonstrate a direct evidence of enhanced SE. Time resolved measurements have been performed on an EL cavity mode (on resonance) at 1319 nm and off reso− nance at 1310 nm. The falling edge of a TTL signal, with

Fig. 5. EL spectrum (grey) and μPl spectrum (blue) of a PhC nanocavity integrated on our LED. The measured cavity Q is 4600.

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Nanophotonic technologies for single−photon devices frequency of 40 MHz and 0.6 V amplitude, has been used to drive the LED and thus, to measure the decay time of its emission. The EL signal is filtered by band pass filter with FWHM 0.2 nm (on resonance) and 0.8 nm (off resonance) and then sent to the SSPD. The SSPD output pulse and the TTL trigger are sent to a correlation card to produce a histo− gram of photon arrival times. The results are plotted in Fig. 6. Blue diamonds describe the temporal decay of the wetting layer emission, whose lifetime is expected to be well below 100 ps. The measured value (220 ps) is an esti− mation of the temporal resolution of the experiment and demonstrates that device parasitics are very low. The tem− poral decay of cavity mode (on resonance) is plotted as squares and the one out resonance as dots. Fitting the data by the convolution of a mono−exponential curve with the time response of the experimental set−up, recombination lifetimes of 380 ps and 580 ps are obtained leading to an enhancement factor (Purcell factor) of 1.5 [35]. In our knowledge, this is the first evidence of a Purcell effect in a PhC LED. It must be noted that the out resonance emis− sion is faster than the typical exciton lifetime on similar QDs in bulk (about 1 ns) [69] and that we are not in the ideal case. By reducing the injection and/or the collection area, it should be possible to isolate the EL of single QDs and achieve higher Purcell factors. The relevance of this result is twofold: On one hand, it opens the way to the fabrication of LEDs and diode lasers with a modulation speed and turn−on delay not limited by the free−space spontaneous emission time – for example, large−signal modulation at frequencies above 100 GHz is possible in a PhC laser [74]. Also the efficiency and maxi− mum rate of electrically−pumped single−photon sources could be much improved using this structure. On the other hand, the integration of electrical contacts with a coupled QD−cavity system allows the electrical control of the cou− pling. For example, by reverse−biasing the same devices described here, we have recently demonstrated [75] the con− trol of the charge state around the QDs and the suppression of its background emission, which is important for improv− ing the purity of QD−based single−photon sources. Addi− tionally, the ultrafast control of the QD−cavity detuning,

Fig. 6. Time resolved EL of the wetting layer (blue diamonds), mode (open red squares), and detuned from the mode ||open turquoise cir− cles), under 40 MHz pulsed excitation.

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through the Stark tuning of the exciton energy in the PhC diode, may allow the coherent manipulation of excitonic and photonic qubits, a major step forward in the field of solid−state CQED, with potential application in quantum computing.

5. Single-photon detectors An overview of the current state of the art in single photon technologies is given in Ref. 76. The development of practi− cal quantum communications and quantum cryptography has created a need for photon−counting detectors that oper− ate reliably within the telecommunication window of the optical spectrum. As already briefly discussed in the intro− duction, commercial silicon APDs have been used for most of the experiments in quantum optics. They have a peak intrinsic efficiency of ~70% and (like photomultipliers) can− not discriminate between one or more photons. In the tele− com window (1310–1550 nm), InGaAs avalanche photo− diodes can be used, however, the practical implementation of InGaAs APDs has been problematic because of a high level of spontaneous noise (dark count rates ~10 kHz) that triggers false avalanches. False avalanches are also associ− ated with the, so−called, after pulsing: charge trapped when an intense avalanche current runs through the device will be spontaneously released later, over a period of several micro− seconds. False avalanches triggered by noise or after−puls− ing without the detection of a photon, create errors during quantum key distribution protocols. Nanowire superconducting single−photon detectors have emerged as a promising alternative approach [68]. SSPDs offer an appealing alternative to traditional photon counting, especially in the near infrared where they outperform pho− tomultiplier tubes and APDs. SSPDs are promising candi− dates for application such as QKD [76], characterization of quantum emitters [77,19], circuit testing [78], high speed optical communication [79], and time of flight laser ranging [80]. In the characterisation of single−photon sources in the telecom band they have become a key tool due to their unprecedented sensitivity, temporal resolution (jitter < 100 ps) and possibility of operating at much higher rates (> 80 MHz) than commercial APDs. For the first time, the autoco− rrelation function g(2)(t) of single QDs emitting at 1300 nm has been measured using SSPDs [19]. The sensing mechanism of the device is based on the combination of the superconducting properties and of the submicrometric width of a wire, generally made of NbN. The device is biased close to its critical current where, due to superconductivity, the voltage across the device is zero. When a single photon is adsorbed in the wire, a resistive “hot spot” is created. This normal region, of about 20 nm size for 850 nm wavelength photons, forces the current to flow in the side regions of the wire. Due to the small width of the wire, about 100 nm, in this region the current exceeds the critical current of the material, a superconducting to nor− mal transition takes place so that in a time of the order of 10 ps, the whole region near the hot spot becomes normal, cre−

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© 2010 SEP, Warsaw

ating a resistive barrier across the entire cross−section of the nanowire. This, in turn gives rise to a voltage pulse. The SSPD reset mechanism is due to the low impedance of the electromagnetic environment. As the superconducting film is very thin (3–5 nm), the normal part of the NbN nanowire has a high resistance, about 450–550 W/sq. During the superconducting−to−normal transition, the device follows a properly designed load line and reaches a metastable re− gion, the bias current is diverted from the active part of the device to the 50−W coaxial transmission line and the 50−W load, so that the power dissipated in the device is not enough to sustain a stable resistive region. The device self reset to its superconducting state and the current through it recovers with the time constant Lkin/50 W, where Lkin is the kinetic inductance of the nanowire [83]. Superconducting detectors based on NbN nanowires operate at 2–4 K temperatures and are capable of very fast counting rates (up to GHz), low dark counts (< 1 Hz), and sensitivity from visible wavelengths to far into the infrared [68,81–84]. While other superconducting detectors (TES), based on the bolometer principle, can also achieve low noise and high efficiency (88%) [85], they require cooling to much lower temperatures (~100 mK) and are much slo− wer (kHz−MHz). In contrast, NbN nanowire detectors can be operated also in a commercial cryo−cooler [86]. Detec− tion efficiencies of 67% have been achieved by incorporat− ing the detector into an integrated optical cavity [87]. Most frequently used SSPDs have so far been fabricated on sapphire [68], MgO [88], and Si [89] substrates, which are not suitable for the integration with sources. The fabrica− tion of SSPDs also on GaAs would enable integration with all the circuitry required for photonic quantum information processing, since GaAs readily lends itself to the large−scale production of single−photon sources [102], waveguides, in− terferometers and phase modulators. Additionally, the inte− gration of NbN nanowires with GaAs−based waveguides and microcavities [87] can be used to increase the absorp− tion in the thin film, leading to quantum efficiencies poten− tially approaching 100%. However, the use of GaAs as a substrate for NbN deposition poses significant challenges for the SSPDs fabrication because the mismatch between the substrate and NbN film lattice parameters is higher (~28%) than with the other substrates used so far and the best NbN films quality is usually obtained with deposition temperatures (> 800°C) incompatible with GaAs process− ing. By optimizing the deposition and fabrication process we recently demonstrated significant progress in the deposi− tion of high−quality thin NbN films on GaAs [91,92] obtai− ning also efficient SSPDs on GaAs substrates after film pa− tterning [93]. As an example of SSPD fabrication process and their optical characterization, we will describe the fabrication of the devices on NbN thin films deposited under optimal con− ditions on MgO substrates at 400°C [88]. NbN thin (4–5 nm thick) films are deposited by DC magnetron sputtering of an Nb target in a gas mixture of N2 and Ar. As mentioned Opto−Electron. Rev., 18, no. 4, 2010

above, several substrate temperatures are needed for the growth, depending on the substrates material. The thickness of the films is then measured by an atomic force microscope (AFM) scan over a 1−μm wide NbN stripe defined on the films by photolithography and etched in CHF3 + O2 plasma. The nanolithography steps needed to fabricate the structure have been carried out by using an e−beam lithography sys− tem equipped with a field emission gun (acceleration volt− age 100 kV, 20 nm resolution) and RIE [93]. In the first step, e−beam lithography is used to define electric contacts (patterned as a 50−W coplanar transmission line) and align− ment markers on PMMA, of about 450−nm thick. The sam− ple is then coated with a film made of 10 nm Ti and 60 nm Au using an e−gun evaporator, and then lift−offed to remove Ti/Au from unpatterned areas. In the second step, an hydro− gen silsesquioxane (HSQ FOX−14, a negative tone elec− tronic resist) diluted in MIBK to obtain a 160−nm thick resist mask, is defined reproducing the meander pattern. The alignment between the different layers is ensured by the markers realized in the first lithography step. All the un− wanted material, i.e., the material not covered by the HSQ mask and the Ti/Au film, is removed by using a fluorine based RIE (CHF3/SF6/Ar gas mixture is used). Detectors fabricated with this process are 5×5 μm2 in size, and made of nanowires ranging from 60 nm to 100 nm in width (w), folded in a meander pattern with filling factors (the ratio f between the area covered by the absorbing patterned film and the whole device area) ranging from 40% to 60%. The superconducting critical temperature TC and the transition width DTC of the patterned SSPD were found to be the same as those of the original NbN films, which confirms that the fabrication process does not affect their superconducting properties. Figure 7 shows an example of a device obtained on an NbN film sputtered on an MgO substrate. If needed, as in the case of photon−number−resolving PNR SSPD detectors (see below), integrated resistors, aligned with the two previous layers, are fabricated by an e−gun evaporation of 10 nm Ti and 85 nm AuPd alloy, 50%−each in weight and by lift off using a PMMA stencil mask.

Fig. 7. Scanning electron microscope (SEM) image of an SSPD. The inset shows an ultra−high resolution image of six stripes. The nanowire width is w = 60 nm. The mean width variation was esti− mated to be »10 nm.

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Nanophotonic technologies for single−photon devices The performances of SSPDs on MgO were optically probed using 50 ps wide, 25 MHz repetition rate pulses at 1.3 μm wavelength from a fiber−pigtailed, gain−switched laser diode. The photons were fed to the SSPDs through a single−mode optical fiber coupled with a 3−mm focal length aspheric lens, which was placed 7 cm from the plane of the chip in order to insure uniform illumination of the devices. The average number of incident photons per optical pulse was estimated to be »0.5 with an error of 5%. Dark counts rate DK was determined as the number of counts re− gistered in one second when the SSPD optical input was blocked. The quantum efficiency QE at a fixed bias current IB was calculated as QE = (Nc – DK)/Nph, where Nc is the number of detection events registered by the counter in one second, Nph is the number of photons incident on the device area in the same time and DK is the dark counts rate at IB. The best performance was exhibited by a w = 100 nm, f = 40%, t = 4 nm meander, which reaches QE = 20% for 1.3 μm wavelength light (Fig. 8) before saturation. Recent studies have highlighted the challenges in creat− ing large area nanowire devices [93,94]. Defects or constric− tions arising from the basic film or during processing are believed to limit the device yield. To date, the presence of constrictions has been inferred indirectly from critical cur− rent [93,94] and inductance measurements [83]. Optical characterization can offer a direct measurement but has typ− ically only been carried out using a focal spot significantly larger than the wire width [83,87,95,77]. To achieve reaso− nable area coverage very long meandered nanowires have been used, but this detector configuration is not well suited to meet demands of higher energy sensitivity or large area coverage because it looses speed, due to the large kinetic inductance Lkin. A series connection of blocks made of pa− rallel nanowires can be used to significant increase the area coverage with no loss in key detector characteristics such as efficiency and speed [96]. The parallel SSPDs working

Fig. 8. Quantum efficiency, QE (squares) and dark counts, DK (cir− cles) as a function of the normalized bias current for the single pho− ton detection regime of a 5×5 μm2 SSPD: w = 100 nm, f = 40%, t = 4 nm. The incident photon wavelength was 1.3 μm. Temperature was 4.2 K.

360

principle is based on the cascade switch mechanism [97]. As for a meander SSPD, in a parallel SSPD all wires are biased with a current slightly below their critical current. In this configuration, when a photon is absorbed in a nanowire of a block, the current is immediately diverted out from it and suddenly it is redistributed between the other nanowire in parallel, which are still in the superconductive state. If the bias current is near enough to the critical value, this mecha− nism of current redistribution induces a current assisted transition to the normal state of all the other parallel nano− wires of the block (cascade switch mechanism). Only when all the wires of a parallel block becomes normal, the voltage pulse takes place as the whole current is diverted in the external load: The signal is n times higher (if n wires are put in parallel in each block) because the current is n times that of a single wire. The reduced power dissipated when the current is diverted out of the device, provides the reset mechanism as for standard SSPDs. Different parallel nanowire geometry has also been used to obtain photon number−resolving (PNR) detector with an ultrashort response time [98,99]. The photon number reso− lution regime can be achieved mainly in two ways. First, certain single−photon detector types (such as superconduct− ing transition edge sensors) intrinsically produce a pulse proportional to the number of photons absorbed [100]. The second method multiplexes SSPD detectors by combining the output signals of a parallel−connected array of detectors (spatial multiplexing) that has a compact footprint and is compatible with light delivery to its surface by a single opti− cal fibre [98]. When an optical pulse, consisting of several photons arrives at the device, it can trigger several detectors, producing an electrical pulse with amplitude that is directly proportional to the number of photons absorbed in the array. In detail, the structure of a PNR detector is the parallel con− nection of N superconducting nanowires, each of which is connected in series to the resistor R0 [Fig. 9(b)]. The time evolution of the device after photon absorption can be simu− lated using the equivalent circuit of Fig. 9(a). Each section is modelled as the series connection of a switch that opens on the hotspot resistance Rhs simulating the absorption of a photon, of the inductance Lkin, accounting for kinetic in− ductance and of the resistor R0. The device is connected through the bias T to the bias voltage source VB and to the input resistance of the preamplifier Rout. The n firing sec− tions, in pink, all carry the same current If and the N−n still superconducting sections (unfiring), in green, all carry the same current Iu. Iout is the current flowing through Rout. Be− cause of the sudden increase in the resistance of the firing nanowire, its current If is then redistributed between the other N−1 unfiring branches and Rout. The device shows photon number−resolving capability if the height of the cur− rent pulse through Rout for n firing stripes is n times higher than the pulse for one, i.e., if the leakage current drained by each of the unfiring nanowires is negligible with respect to IB. The leakage current is also undesirable because it lowers the signal available for amplification and temporarily in− creases the current flowing through the still superconduct−

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© 2010 SEP, Warsaw

Fig. 9. (a) Circuit equivalent of an N section PNR superconducting detector. The n firing sections, in pink, all carry the same current If and the N−n still superconducting sections (unfiring) in green, all carry the same current Iu Iout is the current flowing through the input resistance Rout of the preamplifier. (b) Scanning electron microscope (SEM) image of a PNR superconducting detector with N = 8 fabricated on a 4−nm thick NbN film on MgO. The nanowire width is w = 100 nm and the meander fill factor is f = 40%. The devices are contacted through 70−nm thick Au−Ti pads. The nanowires are connected in series with 0.5−μm wide Au−Pd resistors (light grey).

ing (unfiring) sections, eventually driving them to the nor− mal state as in the case of the cascade switching mechanism [97]. The leakage current can be reduced by engineering the dimensions of the nanowire (thus, its kinetic inductance Lkin) and of the series resistor (Ro). Figure 10 shows a sin− gle−shot oscilloscope trace of the photoresponse of a 4 sec− tion PNR under laser illumination (l = 1.3 μm, 100−ps pulses from a laser diode. The average photon number per

pulse was 1.5×104). Pulses with four different amplitudes can be observed, corresponding to the transition of one to four sections. The measured 80−MHz counting rate repre− sents an improvement in three orders of magnitude over most of the PNR detectors at telecom wavelength, with the only exception of the SSPD array [101]. The PNR super− conducting detector approach benefits from all the advan− tages associated with superconducting nanowire technolo− gy, such as a high speed of detection, low jitter and the absence of after−pulsing. It provides a PNR detector that enables direct characterization of photon−number statistics of light from fast sources, including quantum dots, at tele− com wavelengths.

6. Conclusions

Fig. 10. Single−shot oscilloscope trace during photodetection by a 4 section PNR. The device was tested under uniform illumination in a cryogenic dipstick dipped in a liquid He bath at 4.2 K with a laser diode at 1.3 μm and 100−ps long pulses. The average photon number per pulse was 1.5×104. Opto−Electron. Rev., 18, no. 4, 2010

The common feature of the technologies here described is that structures with »100 nm critical dimensions and tole− rances in the nm−range, are needed to achieve control over optical energies at the single−photon level. In this review we show our progresses in the development of nanophotonic devices to be applied in the generation and detection of sin− gle photons. Accurate optimisation of the fabrication pro− cesses is fundamental in order to achieve the desired perfor− mances both in single photon sources and detectors. With respect to single photon sources, we successfully realize the electrical pumping of a PhC nanocavity, opening the way to the fabrication of efficient and fast electrically−pumped sin− gle−photon sources and to the electrical control of the QD− −cavity coupling. Moreover, to meet the demands of new quantum information applications, optical detectors with single−photon sensitivity in the near−infrared, excellent tim− ing resolution and a high signal−to−noise ratio, based on superconducting nanowires have been developed. We be−

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Nanophotonic technologies for single−photon devices lieve that these new single photon detector technologies will have impact in a range of fields far beyond that of quantum information, like astronomy, laser ranging, remote sensing, classical communications and biomedical imaging.

Acknowledgements The authors wish to thank Sergio Pagano, Roberto Cristiano and Mikkel Ejrnaes for the valuable discussions regarding parallel single photon detectors.

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