Polarization-Insensitive Fractal Superconducting Nanowire Single-Photon Detectors

Superconducting nanowire single-photon detectors(SNSPDs)exhibit high detection efficiency(>90%),low timing jitter(<15 ps),and high counting rate(>100 MHz),which make SNSPDs attract a significant amount of research attention and applications.Although SNSPDs have excellent performances,the traditional device structure in meanders make the detection efficiency polarization sensitive,i.e.,the detection efficiency depends on the polarization state of the incident photons.To solve this problem,the polarization sensitiveness of SNSPDs,this thesis proposes and demonstrates SNSPDs based on fractal nanowires that almost completely eliminate the polarization sensitiveness.Specifically,the author designed and fabricated the SNSPDs based on Peano curves with the line width of 100 nm and the fill factor of 1:2.The active area was 10 ?m×10 ?m and the devices were integrated with optical micro-cavities.Tested in a Gifford-McMahon closed-cycle cryocooler with a base temperature of 2.6 K,the maximum device efficiency was 41.7% at the wavelength of 1550 nm and the polarization sensitivity,defined as the ratio of polarization-maximum device efficiency over polarization-minimum device efficiency,was reduced to 1.04.As a comparison,the polarization sensitivity of a SNSPD in the meander structure with the same fill factor was measured to be 1.74.In addition to almost completely having removed the polarization sensitiveness and make SNSPDs detect photons in different polarization states with almost identical efficiency,the fractal SNSPDs that this thesis proposes and demonstrates have the following additional advantages:(1)they are not only polarization insensitive to the photons in the fundamental spatial mode of the optical fibers,but also to the photons in the higher order spatial modes;(2)the polarization insensitiveness does not depend on the high-precision alignment between the optical mode and the active area of the detector;(3)this device structure applies to both polycrystalline(such as NbN,NbTiN)and amorphous(such as WSi)superconducting materials to realize polarization-insensitive SNSPDs.