| Superconducting nanowire single photon detector(SNSPD)has become the most important single-photon detection technology at near-infrared band for their excellent properties such as high detection efficiency,low dark count,low timing jitter and wide response spectrum since it was put forward by Gol'tsman et al in 2001.However,SNSPDs are required not only for a high detection efficiency,but also for resolving photon number and a high detection speed in a range of applications.Developing SNSPD arrays is one of the most effective methods to realize photon-number-resolving capability and achieve a high photon counting rate.But limited by many factors such as low temperature and low noise readout technology,and refrigeration power of the refrigerating machines,there is not efficient readout solutions for large-scale SNSPD arrays.In order to improve the multiplexing efficiency of the readout,we focus on the design and fabrication of the series structure nanowire array(SND),and the achievement of the SNSPD with high saturation detection efficiency.By increasing the number of serial nano wires and reading them out in a standard SNSPD readout scheme,a high-performance series superconducting nanowire single-photon detector with a maximum detection efficiency of 72.1%,a saturation counting rate of 380 MHz and the capability of resolving 6 photons is achieved,which is the best reported so far.And it provides a technical path for the further development of large-scale SNSPD arrays.The primary results are given in brief as follows:1.A 6-pixel NbN SND with high saturation efficiency was designed and fabricated aiming at the 1550 nm telecommunication wavelength applications.In order to improve the system detection efficiency,a large area of 20×20?m2 was designed to enhance the photon coupling to the detector and an optical cavity was adopted to improve the absorption efficiency of the NbN nanowires.And an optimized nanowire structure was used to eliminate the nanowire current crowding effect,which improved the intrinsic efficiency of the detector to unity.The 6-pixel SND we fabricated achieved a maximum saturation efficiency of 72.1%at 1550 nm,a detection efficiency of more than 10%from 1250 nm to 1650 nm and a wide photon response range from 1100 nm to 2000 nm wavelengths.By measuring the counting-rate distributions of different response pulses as a function of the incident light power,we proved that the 6-pixel SND was capable of resolving 6 photons.2.Aiming at the visible wavelength applications,a SND with the wavelength of 660 nm was designed and measured.By sputtering an Au reflector and depositing a SiOx/Si3N4 optical cavity on the silicon substrate,a frontside-illuminated SND structure was realized.The 6-pixel SND we fabricated had a saturation detection efficiency of up to 58%at the wavelength of 660 nm and the dark count rate of less than 1 Hz,and it could also resolve 6 photons.3.The current recovery mechanism for the SND after a photon detection event was simulated.The total dynamic inductance of the 6-pixel SND nanowires was 1.25 ?H and the simulation results showed that the inductance-dependent output pulse recovery time was about 50 ns,but the recovery time of each nanowire was less than 10 ns.That indicated that each nanowire of the SND detector could reset fast and worked in parallel,although the whole device took a much longer time for recovering.By the weak dependence of detector efficiency on bias current,the SND could realize a high counting rate.Therefore,a method to characterize the detection speed of SND was proposed in this paper.The detected photon counting rate of an SND with a saturation efficiency of 68%as a function of the incident light power was measured by a high-frequency light source system.The results showed that when the detection efficiency was at the 3 dB compression point,the detected photon rate was 380 MHz.Compared with a traditional SNSPD with the same inductance,the photon counting rate was increased by 7.5 times.That indicated the SND could be used for high-speed single photon counting in addition to resolving photon numbers.Recently the 6-pixel SND was used in the laboratory's experimental system of high-speed single-photon communication,and it achieved a communication speed of 800 Mbit/s with an average photon number of 2.01 photons per pulse in a PPM modulation mode4.For high-speed single-photon detection applications,a 64-pixel SND and a 6-pixel SND array with 2×2 elements were designed and fabricated Simulation results indicated that the maximum counting rate of the 64-pixel SND could reach 3.3 GHz theoretically. |
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