Modeling And Design Of Broadband Optical Absorption Characteristics Of Superconducting Nanowire Single Photon Detector

Compared with the traditional semiconductor single photon detector,superconducting nanowire single photon detector(SNSPD)has attracted more and more attention due to its high detection efficiency,low dark count and short recovery time.As one of the decisive factors of the detection efficiency of the system,the optimization and design of the optical absorption efficiency of SNSPD has always been the research focus in this field.Although the structure design of SNSPD to improve the optical absorption efficiency in the near-infrared band has become mature,the absorption efficiency of SNSPD,especially the broadband absorption efficiency,still needs to be greatly improved in the medium and long infrared band,which has important application value in astronomical observation and national defense.In this paper,the optical absorption characteristics of SNSPD are modeled,and the structure of the detector is optimized by different methods to achieve broadband high absorption design in near and mid infrared bands.The specific work is as follows:1.Based on the transmission line theory,the optical absorption characteristics of SNSPD with different structures are designed and optimized by traversing the thickness of each dielectric layer.Firstly,by matching the input impedance of the device surface at a single wavelength and the wave impedance in vacuum,the absorption efficiency of at least 50%in the near infrared band of 1100nm?500nm and at least 60%in the mid infrared band of 2600nm?4400nm are achieved respectively.In this process,it is found that the optical absorption bandwidth of SNSPD can be further extended by using another absorption peak near the original matching wavelength through double resonant peak coupling.On this basis,two wavelengths are used as the target wavelengths of impedance matching to realize the dual peak resonance characteristics with good coupling,expand the absorption bandwidth,and realize the ultra wideband optical absorption in the mid infrared band.However,the point matching method based on impedance matching has some problems,such as complex calculation process,long calculation time and so on.2.In order to improve the design efficiency,the single objective optimization of the light absorption characteristics of SNSPD is carried out based on particle swarm optimization algorithm.The device structure that meets the design objective is obtained by searching the optimal thickness of each dielectric layer in the solution space.On the one hand,in the near-infrared band,the SNSPD structures with high optical absorption efficiency at a single target wavelength and higher optical absorption efficiency at 1236nm?1700nm are designed by taking the absorption efficiency at 1400nm and the absorption bandwidth with absorption efficiency greater than 80%in the wavelength range of 1100nm?1700nm as objective functions respectively.On the other hand,in the mid infrared band,the absorption efficiency at 4000 nm and the absorption bandwidth over the whole mid infrared wavelength range are taken as the objective functions respectively,and the light absorption efficiency at a single target wavelength is up to 99%and the light absorption efficiency over the wavelength range of 3628 nm?4954 nm is greater than the design requirements.3.The actual SNSPD design often needs to optimize multiple performance indicators at the same time.For example,in the design of medium and long infrared,because the size of common optical structures such as optical resonator is proportional to the wavelength,when considering the requirements of integration and manufacturing cost,we want to limit the thickness of the device.Therefore,based on the multi-objective particle swarm optimization algorithm,this paper studies the joint optimization problem among multiple performances of SNSPD.On the basis of finding the Pareto optimal solution set in the solution space,the paper optimizes the design objectives,and finally obtains the global optimal solution which can balance different design objectives.In the near-infrared band,the thickness of the device and the absorption efficiency at the single frequency point,the absorption efficiency at the double frequency point,the thickness of the device and the bandwidth with the absorption efficiency greater than 70%in the specific near-infrared band are taken as the optimization objectives respectively.While improving the integration degree,the high-efficiency absorption at the 1400nm frequency point and the ultra wideband absorption of more than 50%in the target frequency band are realized.In the mid infrared band,the thickness of the device and the absorption efficiency at the single frequency point,the absorption efficiency at the double frequency point,the thickness of the device and the bandwidth with the absorption efficiency greater than 70%in the specific mid infrared band are taken as the optimization objectives,respectively.The high-efficiency absorption at the frequency of 4000 nm and the ultra wideband absorption above 65%in the mid infrared band are achieved.