Polarization Characteristics Of Superconducting Nanowire Single Photon Detectors

In light of its superior roles played in many applications,superconducting nanowire single photon detector(SNSPD)has experienced a rapid development over the past decade and entered a new era in term of technical performances.Compared with other types of single-photon detectors,SNSPD has various advantages such as high efficiencies,low dark counts,fast response speeds and low timing jitters.Owing to these advantages,SNSPDs have found various interesting applications,including the quantum key distribution,quantum optics,time flight depth imaging,two-dimensional array imager,and very recently the single photon quantum imager.It is well-known that due to the periodical meander structures of the superconducting nanowires of the SNSPDs,there is a strong dependence between the absorption efficiencies and the polarization states of the incident light waves.Polarization sensitivity is an intrinsic characteristic of the SNSPD and may severely limit the scope of the SNSPD’s applications.This thesis focus on the research of polarization characteristics of SNSPD.We designed and demonstrated several structures for both polarization-insensitive and high polarization extinction ratio SNSPDs for different applications.The primary results are given in brief as follows1.We have designed and fabricated a SNSPD with much reduced polarization sensitivity,based on a previously published theoretical result that involves high refractive index compensation materials.It is suggested that by using high refractive index materials(such as silicon)to compensate the dielectric mismatch between the NbN nanowire and the cavity layer material,the polarization sensitivity issue of the SNSPD can be completely removed,rendering a result that incident photons with parallel and perpendicular polarizations will have the same absorption efficiencies.The measurement results show that the polarization sensitivity of the fabricated device is greatly reduced,with the device detection efficiencies at 1550 nm being 61%and 56%for cases of parallel and perpendicular polarizations,respectively.The measurement results are in good agreement with the numerical simulations.2.In the polarimetric imaging system it is sometime preferred that the photodetector by itself is highly polarization sensitive,since such a polarization diversified technique enables the construction of polarimetric imaging systems with improved frame acquisition rates.We report the experimental demonstration of a SNSPD that is ultra-sensitive to the polarization state of the incident photons.The design framework follows the theory of the polarization dependent interferometric effect of a composite mirror.Guided by such a framework,a polarization ultra-sensitive SNSPD operating at 1550 nm is designed,fabricated and measured,with a device efficiency of～48%and a polarization extinction ratio(PER)of～420 being achieved.3.In some particular application scenarios,high detection efficiencies are required for both parallel and perpendicular polarization light and polarization extinction ratio should reach a very high value at the same time.We proposed and designed a double-layer-nanowire SNSPD with high detection efficiency for parallel and perpendicular polarization,respectively,and high extinction ratio for each nanowire layer.We proposed a double-layer-perpendicular-nanowire SNSPD.The new structure SNSPD achieved high detection efficiency for both parallel and perpendicular polarization light and polarization extinction ratio.The absorptance efficiency of the upper layer nanowire is 68.6%for parallel polarization,and the polarization extinction ratio(PER)is over 8500.The absorptance efficiency of the under layer nanowire is 74.6%for perpendicular polarization,and the polarization extinction ratio(PER)is about 29.8...