Optimization Design And Experimental Study Of Fractal Superconducting Nanowire Single-Photon Detectors

Since the invention of superconducting nanowire single-photon detectors(SNSPDs),they have attracted significant amount of attention and have got many applications with their excellent performances,including high system detection efficiency,high photon-counting rate,low dark-count rate,low timing jitter and other advantages.Although the SNSPDs with traditional meandering structure have high system detection efficiency,the detection efficiency of the device was polarization sensitive.It limits their application spaces.On the other hand,SNSPDs with fractal photo-sensitive regions have the same absorption efficiency for light with different polarization states.In this way,we can decrease the polarization sensitivity of the devices.This thesis contains simulation and optimization of the polarization-insensitive fractal nanowire structures.We increased the internal quantum efficiency of devices and absorption efficiency of devices,by optimizing the line widths and the fill factors.In addition,we realized efficient optical coupling by increasing the photo-sensitive area.The aim is to increase the system detection efficiency of the fractal SNSPDs.We show that nanowire structures with narrower line widths and lower fill factors,and integrated with microcavities,are ideal optical structures.In order to fabricate the nanowires with narrow line widths,we selected HSQ as the electron-beam resist.A fractal nanowire structure of 16.8 ?m×16.8 ?m photosensitive region with line width of 40 nm and period of 200 nm was successfully fabricated.Additionally,in order to realize the efficient optical coupling between the SNSPDs and the incident light,we chose a self-alignment scheme.Meanwhile,keyhole-shaped SNSPD chips were fabricated by processes such as coupled plasma etching,and then the fiber sleeve was used to achieve self-alignment between the device and the optical fiber.To reduce the timing jitter of devices,we designed SNSPDs integrated with impedance-matching structure which can improve the slew rate of the pulse's rising edge.The result of simulation showed that this method can successfully increase the peak value of the output pulse by 3.58 times,and increase the slew rate of the leading edge of the pulse by 4.95 times.But the impedance matching structure increases the falling time of the pulse,this effect can be remedied by designing high impedance output structures.