Study On Optical Properties Of NbN Nanowires In Superconducting Detectors

Superconducting nanowire single photon detectors(SNSPDs)have important applications in quantum communication,satellite laser ranging,deep space laser communication,optical time domain reflection,sea fog measurement,atmospheric detection,laser radar and other fields due to its high detection efficiency,low dark count,low time jitter,wide response spectrum and many other advantages Instruments.Niobium nitride(NbN)is a commonly used photosensitive material for SSNPD,and NbN nanowires are also a core part of SNSPDs.Its photon transfer properties are one of the key factors affecting device efficiency.At present,the research on the electrical properties of superconducting nanowires is more in-depth,but the optical properties are less studiedIn this thesis,four kinds of SNRPD structures were designed and fabricated around the optical properties of NbN nanowires.The light absorption efficiencies of these four structures were measured by spectrometer.The Optical properties of nanowire detector device structures were studied systematically with experimental data and simulation results.We have also built an optical property measurement system for micron-sized SSNPD,which can now be used to measure the light absorption efficiency of effective nanowire detection regions at room temperature.It can also be used to measure the optical properties of SSNPD nanowires at low temperatures with a refrigerator.The main results of this paper are as followsFirst,in order to compare the effects of SSNPD on the light absorption rate of different material structures,we designed four SSNPD structures.(1)a double-sided thermal silicon oxide substrate backside optic structure;(2)a double-sided SiN silicon substrate backside optic structure;(3)a front side of the silicon substrate with a gold layer+ SiN buffer layer as a mirror Structure;(4)a front-facing optical structure with a Bragg mirror(DBR)as a substrate.On the basis of the above four device structures,NbN films with different thicknesses were grown to observe the absorption efficiencies of NbN films with different thicknesses.Second,we measured the reflectivity and transmittance of the four structures using a spectrometer.We found that a double-sided SiN silicon substrate with an anti-reflection layer can substantially eliminate the reflection because the SiN dielectric constant satisfies Si and air.The reflectance is as low as 0.9%.The light absorption efficiencies of the four structures under different NbN thickness conditions were also measured.After analysis,it is found that the relationship between the optimal NbN thickness and the light absorption rate under different device structures is as follows:The NbN layer on the double-sided thermal silic on oxide substrate has a maximum absorption rate of 91.7%at 1606 nm,and the rest of the structure can reach 99%or more under the optimum NbN thickness.The maximum absorption rate of the double-sided SiN silicon substrate structure is 99.3%,the Au+SiN is 99.8%,and the DBR is 99.9%.The difference between the measured results of the DBR device and the simulation results was analyzed.These results have guiding significance for the design and development of high-efficiency SSNSD.Third,the ordinary spectrometer can only measure the detection area of the millimeter level,and the general device is micron level,which is difficult to manufacture.To this end,we designed a micrometer-scale optical property measurement system using a circulator and an optical power meter.The high-efficiency SSNPD device with DBR as the substrate was measured and its absorption rate was over 95%.