Infrared Superconducting Nanowire Single Photon Detector

Superconducting nanowire single photon detectors(SNSPD)are a highly promising technology for low noise detection of infrared photons.The basic device consists of a nanowire patterned in an ultrathin niobium nitride superconducting film.The device is cooled to liquid helium temperature(4.2 K)and biased just below the superconducting critical current.When a photon strikes the wire,a resistive hotspot is formed,triggering an output voltage pulse.These detectors outperform conventional photon counting technologies(photomultipliers and avalanche photodiodes)and are an enabling technology for a host of applications at the frontiers of science,including quantum key distribution,development of optical quantum computers,integrated circuit failure analysis,atmospheric remote sensing and fluorescence detection.SNSPD,as a new single photon detector,could simultaneously perform high detection efficiency(DE),broad response bandwidth,low dark count rate,low timing jitter and fast repetition rate.However,DE in infrared drops with the increased wavelength because of the detection mechanism of SNSPD.Researchers have taken great efforts to improve DE,firstly,based on hotspot or hotspot-diffusion mode,the intrinsic detection efficiency have been enhanced by adjusting the dimensions of nanowires or changing the material of superconducting materials.Secondly,by introducing lens fibers or nanopositioners,the coupling efficiency between fibers and nanowires could be improved.Then,by integrating on-chip optical cavity,optical nanoantanne and optical waveguide,the absorbing efficiency of nanowire could be improved.Based on the methods above,the best performance of DE at 1550 nm could exceed 70%.The next target of SNSPD is extending the response bandwidth and improving the performance in infrared.In this thesis,we focus on the point of low DE in infrared and do research on improving DE of SNSPD in infrared.The main results include:(1).We designed and fabricated a special doped niobium(Nb*)superconducting nanowire single-photon detector(SNSPD)on MgO substrate.The superconductivity of this ultra-thin Nb*film was further improved by depositing an ultra-thin aluminum nitride protective layer on top.Compared with traditional Nb films,Nb*films present higher TC and JC.We investigated the dependence of the characteristics of devices,such as cut-off wavelength,response bandwidth,and temperature,on their geometrical dimensions.Results indicate that reduction in both the width and thickness of Nb*nanowires extended the cut-off wavelength and improved the sensitivity.The Nb*-SNSPD(50 nm width and 4.5 nm thickness)exhibited single-photon sensitivities at 1310,1550,and 2010 nm.We also demonstrated an enhancement in the detection efficiency by a factor of 10 in its count rate by lowering the working temperature from 2.26 K to 315 mK.(2).We present a broadband single-photon detector by integrating optical cavities to a meandered superconducting nanowire on thermalized silicon substrate,where the resonate wavelength is designed at 1700 nm.In order to increase the response bandwidth,the resonate effect is optimized by adjusting the thickness of both dielectric layers.Under the environment of 2.3 K,our device obtained a critical current of 6.02 ?A and provided efficient performance ranging from 1350 nm to 2010 nm.The highest detection efficiency exceeded 10%at 2010 nm and 20%at both 1550 nm and 1650 nm.Further more,the detector performed ultra low dark counts.(3).We demonstrate a strong DE enhancement in NbN-SNSPD by coupling the SNSPD with a plasmonic nanostructure made of Au nanoparticles.Based on FDTD simulation,we have observed the enhancement of electric field intensity in nanoparticles.Comparing with the device without Au nanoparticles,DE enhancement and the abnormal compression of critical current are observed,which attributed to the strong local field enhancement of Au nanoparticles on the SNSPD.