Structure Design Of The Superconducting Nanowire Single Photon Detector With High Efficiency

The single photon detector with high performance is a key device for quantum optics and quantum information processing.The superconducting single photon detector based on superconducting nano wires is an important genre in a variety of single photon detectors.It has a large amount of advantages such as high efficiency,low dark count and short time jitter.In order to improve the system efficiency in the superconducting nano wire single photon detector(SNSPD),several coupled structures for SNSPD based on the electromagnetic field theory and the equivalent-circuit-theory have been constructed to achieve a high absorption.These structures have the capacity to adjust the polarization and the multi-band-pass of SNSPD.Some of the structures have been verified by experimental results.The primary results are given in brief as follows:Firstly,due to the polarization sensitivity of superconducting nano wires,the absorption efficiency is low when the polarization of incidence is perpendicular to nanowires.To address the problem,a method with a high index medium covering nanowires has been presented to improve the absorption.Based on this method,SNSPD is designed with a high efficiency and non-sensitive polarization.In this paper,the numerical simulation results show that the absorption efficiency of the device is about 96%at lambda is 1550 nm,and meanwhile the absorption of polarized incidence differs less than 0.5%in the range of the bandwidth of the 300 nm.The device structure is simplified and tested and the difference between experimental results and design results is slight.Secondly,in the applications of optical polarization imaging,the detection efficiency of the photon detector and polarization extinction ratio(PER)are two most important parameters.An efficient and highly polarized sensitive SNSPD is designed in this paper.The simulation results show that the absorption efficiency of NbN nano wires is about 95%@ 1550nm and the PER is about 15×104@1550nm.The PER of this structure is the highest in the case of vertical incidence of light among the current reports.In this paper,the reason of high absorption efficiency is explained by electric field analysis,and the theoretical model is used to analyze the Lorentz-like line of PER curve.The fault tolerance of each parameter of the device is calculated,and the design structure is compared with the method of adding polarizer on the incident port.It is found that this design can make the absorption efficiency and PER reach the optimal value at the same time.Thirdly,because has there is not restriction of transmission bandwidth for positive incidence,a design method of efficient and double bandwidth SNSPD analysis is presented in this paper.Take the lambda =1310 nm and lambda =1550 nm as examples,the device design for Au reflector and the Bragg reflector are done respectively.The simulation result shows that NbN nanowires have absorption peaks in the two bands and absorption peak is over 96%.Then the influence that impedance jx and nanowire's duty ratio on the full width at half maxim of absorption peak discussed.The work provides a reference for designing multi-band-pass SNSPD.Fourthly,the reflection phase and impedance are measured when superconducting nanowires are considered as a load impedance.The reason why the measurement system resonates at long wavelength is analyzed.Then,the difference between the transmission characteristics and the reflection characteristics of the nanowires in the superconducting and non-superconducting states is compared.However,the measurement can not be interpreted using a physical model.It need to further verify with the experiment.