Superconducting nanowire single-photon detectors (SNSPDs) with high detection efficiency have played significant role in numerous applications for photon detection from visible light to mid-infrared band. However, it is still a challenge to easily promote the sensitivity of SNSPD without compromising its other performance. We report a method to improve the detection sensitivity of devices via suspended SiO2 membrane, and the intrinsic detection efficiency is significantly enhanced from 78% to 100% for a 1550 nm wavelength. The electro-thermal simulation and hotspot relaxation time measurement indicate that a suspended SiO2 membrane can significantly decrease the cooling efficiency between the hotspot and the substrate, and the relaxation time is enlarged twice than before. Meanwhile, we note that the proposed method does not affect other properties of devices, such as dark count rate, detection speed, and timing jitter. Because the thermal dissipation process can be controlled continuously (by varying the size of the suspended area), the proposed method may also help to obtain a deeper understanding of the thermal diffusion and detection process of SNSPDs.
Superconducting nanowire single-photon detectors (SNSPDs) with high detection efficiency have played significant role in numerous applications for photon detection from visible light to mid-infrared band. However, it is still a challenge to easily promote the sensitivity of SNSPD without compromising its other performance. We report a method to improve the detection sensitivity of devices via suspended SiO2 membrane, and the intrinsic detection efficiency is significantly enhanced from 78% to 100% for a 1550 nm wavelength. The electro-thermal simulation and hotspot relaxation time measurement indicate that a suspended SiO2 membrane can significantly decrease the cooling efficiency between the hotspot and the substrate, and the relaxation time is enlarged twice than before. Meanwhile, we note that the proposed method does not affect other properties of devices, such as dark count rate, detection speed, and timing jitter. Because the thermal dissipation process can be controlled continuously (by varying the size of the suspended area), the proposed method may also help to obtain a deeper understanding of the thermal diffusion and detection process of SNSPDs.
Single photon detection advanced rapidly in the last decade. The superconducting nanowire single-photon detector (SNSPD) is an emerging technology, which offers low dark count rates, broadband sensitivity from visible to mid-infrared wavelengths and very high quantum efficiency. All these features are appealing for future emerging applications in remote gas sensing, quantum key distribution and quantum information processing. By enclosing the detector in an optical cavity, tuned to the operating wavelength, the total system detection of the SNSPD is improved. Based on the cavity design, the detector absorption is boosted at the desired experimental wavelength. Devices have been fabricated and tested, in this thesis, with enhanced detection efficiency at near-infrared, infrared and mid-infrared wavelengths. NbTiN detectors fabricated on Ta2O5 / SiO2 distributed Bragg reflectors (DBR) for front-side coupling exhibit a system detection efficiency of ≈ 90% at λ = 940 nm for the best device measured. SNSPDs were also fabricated in NbTiN on metallic mirrors for back-side coupling for 1550 and 2333 nm experimental wavelengths. The first design exhibited a system detection efficiency of ≈ 84.5%, at λ = 1550 nm for the best device measured. The second exhibited low system detection efficiency since the device suffered from unexpectedly high dark count rate mainly due to blackbody radiation at 2.2 K. However, it was possible to measure the timing jitter of this device at 350 mK indicating a FWHM of 84 ps. Another technique to improve the total system detection efficiency of the SNSPD, is by fabricating a hairpin-shaped nanowire atop a single mode waveguide. In this design, photons travel through the waveguide and via evanescent coupling are absorbed in the nanowire. These SNSPD devices have been designed to thread along single mode Si ridge waveguides using a hairpin-shaped nanowire structure, which offers enhanced coupling and absorption efficiency via evanescent coupling. These ultra-thin superconducting MoSi detectors are precisely patterned on top of a single mode Si waveguide (600 nm) with alignment error less than 40 nm. Low temperature electrical and nano-optical characterization was carried out using a two-stage pulse tube (PT) coldhead, using an additional 4He / 3He sorption pump, low vibration characterization of SNSPD devices at 350 mK was carried out. Characterised devices exhibit low dark count rate <5 counts per second and timing jitter of ≈ 83 ps full width half maximum (FWHM) at wavelength (λ) = 1550 nm and at critical current ≈ 30 µA.
Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a highly promising infrared single-photon detector technology. Next-generation devices are being developed with enhanced detection efficiency (DE) at key technological wavelengths via the use of optical cavities. Furthermore, new materials and substrates are being explored for improved fabrication versatility, higher DE, and lower dark counts. We report on the practical performance of packaged NbTiN SNSPDs fabricated on oxidized silicon substrates in the wavelength range from 830 to 1700 nm. We exploit constructive interference from the SiO2/Si interface in order to achieve enhanced front-side fiber-coupled DE of 23.2 % at 1310 nm, at 1 kHz dark count rate, with 60 ps full width half maximum timing jitter.
Ever since superconducting nanowire single-photon detectors (SNSPDs or SSPDs) were demonstrated, they have attracted a considerable attention due to the detectors’ promising performance that includes high detection efficiency (DE), low dark count rate (DCR), broadband sensitivity, and low timing jitter. The system detection efficiency (SDE) has been improved to be over 90% with a wavelength of 1550 nm in a fiber-coupled system by embedding the SNSPD in optical cavities. However, the implementation of optical cavity usually reduces the bandwidth of SNSPDs. As a result, there are some recent works on increasing the bandwidth of SNSPD. Furthermore, the size of the active area of meandered nanowires has to be compatible with the core size of an optical fiber such that good optical coupling can be achieved; this setup requires a long nanowire with a large kinetic inductance, thus resulting in a small counting rate and a large timing jitter, particularly for multi-mode fiber-coupled devices. Nevertheless, a particularly impressive waveguide-coupled SNSPD has been developed in recent years. By placing an SNSPD atop an optical waveguide that has been fabricated on various substrate materials, photons propagating in the waveguide have been found to be evanescently absorbed by nanowires that are only a few tens of microns long. The on-chip DE of this device has been estimated to be over 90%, thus indicating that it has both high absorptance and high intrinsic DE. If the waveguide and the nanowire are correctly dimensioned, the absorptance of this device can reach over 90% for a wavelength range of more than 700 nm. Furthermore, the length of the nanowire is shorter than that of the fiber-coupled SNSPD, thus leading to a higher count rate and lower timing jitter (~18 ps). However, unsatisfactory coupling losses between the optical fiber and the waveguide in such devices can decrease its SDE (~10%), thus limiting its applicability to quantum photonic integrated circuits (QPIC). Recently, we proposed a type of SNSPD that was coupled with microfibers (MFs), and simulations for this SNSPD indicated that its absorption would be as high as 90%. Microfibers and nanofibers, tapered adiabatically from standard fibers, with diameters close to or smaller than the wavelength of the guided light were demonstrated to have low transmittance losses by Tong et al. in 2003. MFs have been shown to have many interesting properties, such as strong evanescent fields, tight optical confinement, wideband abilities, and small masses; these properties are appealing for applications such as optical sensing and coupling. MFs also provide seamless connections with high transmittance from standard fibers; therefore, MF-coupled SNSPDs can achieve both high SDEs and wideband compatibility. We proposed and demonstrated the MF-coupled SNSPDs successfully. The simulation results indicate that with optimal device structure, the optical absorption with efficiency < 90% can be realized over a wavelength range of 350 nm to 2150 nm. The fabricated MF-coupled SNSPD, shows unparalleled broadband system detection efficiencies (SDEs) of more than 50% from 630 nm to 1500 nm. The SDEs reach 66% at 785 nm and 45% at 1550 nm. These results pave the way for ultra-broadband weak light detection with quantum-limit sensitivity.
Superconducting nanowire single photon detector (SNSPD) has been widely used in quantum communication, quantum computing and other fields because of its excellent timing jitter and response speed. However, due to the mutual restraint of the technical parameters of SNSPD nanowires, there are technical challenges to further improve the comprehensive performance of SNSPD, and thus limiting its application on a large scale. Combining high detection efficiency with high timing performance is still an outstanding challenge. In this work, we report the SNSPD with 12-μm small active area, which has high speed, high efficiency, low jitter and broadband absorption. Au/SiO2 membrane cavity, which is determined by finite element analysis simulation, is used to widen the optical response bandwidth. And it is easier to process and improve the alignment accuracy at the same time. The flat substrate is more conducive to the growth of superconducting thin films, so flattening process is introduced. Device package is also optimized to match smaller detector. Self-aligned packaging makes optical alignment more convenient and time-saving. Special optical fibers with small mode-field diameters can reduce the negative effect of the detector on optical coupling. The detector can achieve a maximum SDE of 82% at the central wavelength of 1310 nm and the temperature of 2.2 K, and the SDE of more than 65% in the wavelength range of 1200–1600 nm, with DCR of 70 cps. The detector also exhibits a count rate of 40 MHz@3 dB and a timing jitter of 38 ps, which is significantly improved compared with 23-μm active area detector. Furthermore, the minimum timing jitter of 22 ps can be obtained by using cryogenic amplifier readout. In this work, high comprehensive performance detector is developed, which provides an important technical reference for practical and product SNSPD.
The hotspot dynamics is one of the key process involved in the detection mechanism in superconducting nanowires single photon detectors (SNSPDs). The energy delivered by the photon absorption creates a non-equilibrium region where Cooper pairs break into quasi-particles, which undergo a thermalization process involving hot-electron–phonon interactions within the superconducting film. Eventually, phonons dissipate the excess energy into the substrate and relax to the base temperature. Hence, intrinsic reset time, count rate and possibly other parameters are conceivably affected by thermal coupling between the substrates and the superconducting film. Thus, a detailed understanding of the thermal boundary conductance is crucial for achieving the best possible performances. Even though recent studies have demonstrated that the substrate plays an important role in defining the hotspot relaxation time, no systematic study on the influence on detector performance metrics has been carried out yet. In this work, we investigated the effect of different substrates on SNSPDs performance. Meandered detectors and single nanowires with various dimensions have been successfully fabricated on ultrathin NbN film sputtered on six substrates: SiO2, Al2O3, SiC, SiN, AlN and GaN. We characterized the deposited films measuring Tc, ρ, ΔTc and RRR. Lattice mismatch is known to play an important role in determining the resulting film quality. SiO2 and SiN samples, in which the mismatch is pronounced, showed lower Tc and higher ρ compared to the other substrates. We extracted the critical and retrapping current densities (Jc, Jr respectively) for all the nanowire widths, observing a peculiar behavior in SiO2 and SiN samples which needs further investigation. Optical response was characterized in terms of reset time (τ), dark count rate (DCR) and photon count rate (PCR) with a 1550 nm wavelength laser at 3K. Single photon regime was verified in all the measured detectors. Reset time has been confirmed to be limited by kinetic inductance Lk for all the substrates. DCR behavior appeared to be correlated to the difference in film properties. Only detector fabricated on SiN and SiO2 substrates showed saturation in the photon count rate, while the highest count rate was achieved by Al2O3 sample. The different behavior of photon counts response on the substrates suggests a correlation with the measured thermal conductance β and needs further study. Nevertheless, the comparison between the extracted thermal boundary conductances and the theoretical ones highlighted relevant discrepancies between theory and experimental data. In conclusion, devices fabricated on NbN deposited on different substrates showed a peculiar detector photo-response behavior. A deeper evaluation of NbN disorder grade and a better thermal characterization of the substrate-thin film interface are needed as future work.
Superconducting nanowire single-photon detectors (SNSPDs) are indispensable in fields such as quantum science and technology, astronomy, and biomedical imaging, where high detection efficiency, low dark count rates, and high timing accuracy are required. Recently, helium (He) ion irradiation was shown to be a promising method to enhance SNSPD performance. Here, we study how changes in the underlying superconducting NbTiN film and the SiO2/Si substrate affect device performance. While irradiated and unirradiated NbTiN films show similar crystallinity, we observe He bubble formation below the SiO2/Si interface and an amorphization of the Si substrate. Both reduce the thermal conductance between the superconducting thin film and the substrate from 210 to 70 W m−2 K−4 after irradiation with 2000 ions nm−2. This effect, combined with the lateral straggle of He ions in the substrate, allows the modification of the superconductor-to-substrate thermal conductance of an SNSPD by selectively irradiating only the regions around the nanowire. With this approach, we achieved a broader bias current range (9.8 μA vs 3.7 µA) in which the detector operates at its maximum detection efficiency, which is beneficial for reducing dark counts while maintaining high sensitivity. Moreover, the photon-assisted critical current remained similar to that of the unirradiated reference device (59.0 µA vs 60.1 µA), while full irradiation reduced it to 22.4 µA. Our results suggest that the irradiation-induced reduction of the thermal conductance significantly enhances SNSPD sensitivity, offering a novel approach to locally engineer substrate properties for improved detector performance.