| Since the superconducting nanowire single photon detector(SNSPD)came into being,it has come into the attention of researchers all over the world with its high system detection efficiency,low dark count and fast recovery time.After years of efforts,the comprehensive performance of SNSPD in the near infrared band has been significantly improved,but for the important midinfrared and other longer wavelength bands such as spectral analysis,astronomical observation and other fields,the current system detection efficiency of SNSPD still needs to be further improved.Among the many factors affecting the detection efficiency of the system,the light absorption rate of the nanowires has always been one of the key points in the structural design of SNSPD devices.Therefore,taking the mid-infrared band as the main entry point,it is of great practical significance for the future development of SNSPD to study the optimization design method of the high light absorption factor SNSPD,especially the design method of the wide band high light absorption factor within the range of 3000-5000nm large window.Based on the structure of non-symmetric Fabry-Perot(F-P)cavity,the optical absorption rate of SNSPD was optimized in near and middle infrared bands.Specific work is as follows:1.A SNSPD device structure with asymmetric F-P cavity structure consisting of niobium nitride nanowires(NbN),silicon dioxide(SiO2)resonant cavities,and distributed bragg reflectors(DBR)was designed,and its optical response characteristics in the near and mid infrared bands were analyzed in detail.First,the optical response characteristics of the front facing SNSPD in the near-infrared and mid infrared bands are simulated by using the finite difference time-domain method.The results show that for the basic device structure composed of only NbN nanowires and SiO2 substrate,the high transmittance is the main reason for the low absorption efficiency of the device.Secondly,theoretical research was conducted on the broadband reflectivity characteristics of DBR,and a high refractive index film was identified as yttria stabilized zirconia(YSZ),a low refractive index film was identified as SiO2,and the number of cycles was determined to be 4.A reflection effect of no less than 99%was achieved in the near infrared band of 1100-1700nm and the mid infrared band of 3000-5000nm,respectively.The thickness of the YSZ layer and the SiO2 layer are both one-fourth of the central wavelength of the working band.Finally,an asymmetric F-P cavity structure was constructed by setting a DBR with broadband and high reflectivity on the other side of the substrate opposite to the nanowire.While significantly reducing the device transmittance,SNSPD device structures with broadband and high light absorption were designed in the near infrared band of 1100-1700nm and the mid infrared band of 3000-5000nm,respectively.Further analysis shows that for the case of singlelayer nanowires,compared to existing mid infrared broadband design schemes with dual resonant wavelength coupling,the design based on asymmetric F-P cavity structure can further improve the flatness of in band absorption.On this basis,if a double-layer nanowire structure is used,not only can the minimum value of optical absorption be further improved,Moreover,the maximum absorption rate can be increased to over 97%without pursuing a high absorption rate at a specific wavelength,reaching a level comparable to the dual resonant wavelength coupling method.2.With the thickness of DBR’s high and low refractive index films and SiO2 resonators as optimization variables,the optical absorption efficiency of SNSPD with asymmetric F-P cavity structure is optimized based on particle swarm optimization algorithm,including the following three groups of examples:1100-1700nm near-infrared wideband highlight absorption,1310nm and 1550nm dual-wavelength near-infrared highlight absorption,and 3000-5000nm mid-infrared wideband highlight absorption.The results show that the optimization does not increase the peak value of in-band absorption,but further increases the minimum value of in-band absorption and improves the flatness of in-band absorption.For dual-frequency absorption,the optical absorption efficiency of SNSPD with single-layer nanowire structure at the target frequency is not less than 74%,while that of SNSPD with double-layer nanowire structure at the target frequency is not less than 95%.The SNSPD design of highlight absorption efficiency was realized.3.On the basis of the single objective optimization of the high light absorption rate,the thickness of the device was introduced,and the dual objective joint optimization of SNSPD with asymmetric F-P cavity structure was carried out based on the multi-objective particle swarm optimization algorithm.The comparison of single objective optimization results shows that for the 1100-1700nm near-infrared wideband example,the maximum in-band absorption of SNSPD with single-layer and double-layer nanowires remains unchanged,and the total thickness of dielectric layer decreases by 15.9%and 28.2%when the minimum in-band absorption decreases by 1%and 6.7%,respectively.For the example of 1310nm and 1550nm near-infrared dualwavelength,the total thickness of the dielectric layer is reduced by 52%and 40.4%,respectively,while the absorption efficiency of SNSPD with single-layer nanowires and double-layer nanowires is basically unchanged.For mid-infrared wideband at 3000-5000nm,the minimum inband absorption of SNSPD with single-layer nanowires and double-layer nanowires decreases by 2.4%and 3.2%,respectively,while the maximum in-band absorption remains unchanged,and the total thickness of dielectric layer decreases by 12.1%and 8.3%,respectively. |
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