Research On Light-absorbing Structures In A Superconducting Nanowire Single Photon Detector

Superconducting nanowire single photon detector (SNSPD) operated at the near-infrared wavelength has demonstrated great advantages, such as high detection efficiency, rapid response, low dark counts, which has been widely applied in the quantum realm and long-distance telecommunication. SNSPD is sensitive to the polarization state of the incident wave due to its meander nanowires, that is, it has a higher absorption for the incident electric field parallel to the NbN nanowires (called TE wave below) than that for the incident electric field perpendicular to the NbN nanowires (called TM wave below). If we need a polarization sensitive device such as in remote sensing and polarization imaging, PER, the ratio of absorption for TE wave and that for TM wave should be increased. But if we need to detect the intensity of light, the polarization sensitivity should be reduced as much as possible.This thesis focuses on the polarization sensitivity of an efficient superconducting single-photon detector. After the theoretical analysis on the factors affecting absorption rate of NbN nanowires, we propose two different designs of optical structures, one to improve the polarization sensitivity of the detector and the other to reduce polarization sensitivity of the detector. The main results are as follows:Firstly, we have polarization selective Coupling Asymmetric Split-ring Resonator loaded onto SNSPD, inhibiting the device’s absorption for TM wave. And the device has a high absorption of 85.5% for TE wave attributed to the optical cavity. Consequently polarization extinction ratio, PER, reaches 585.In addition, the nanowire has a width of 120nm and a gap of 480nm, which gets a small filling factor and is easy to fabricate. By analyzing the working principle and design method of the device in detail, we give strict steps to optimize the performance of the device, which is a good example for devices operated at other wavelengths.Secondly, we have a Bragg mirror with high reflectivity introduced to the design of SNSPD operated at 1550nm, which has achieved a absorption rate of 98% for TE wave but a so low absorption rate for TM wave. To eliminate polarization sensitivity of NbN nanowire itself, we covered 20nm-thick Si film above NbN nanowire while adjusting the thickness of SiO2 (159nm) beneath NbN nanowire. Surprisingly, a high absorption rate of 95% has been discovered in NbN nanowire for both TE wave and TM wave. Besides the simulation results, we have also provided some experimental data as a reference, including reflection spectrum and roughness of a Bragg mirror containing 14 pairs of Nb2O5/SiO2, as well as the sheet resistance and the critical temperature of 6nm-thickNbN deposited on the Bragg mirror.Thirdly, we have deduced the transmission matrix suitable for SNSPD model based on the electromagnetic field theory and the theory of thin-film optics and showed expressions of reflectance, transmittance and absorption rate. After programming with MATLAB, we have verified the absorption spectrums of a Bragg mirror based SNSPD and SNSPD integrated with a double cavity, which indicates that the time required for this method makes up only 1/83 of that required for FDTD simulation software.