SUPERCONDUCTING NANOSTRUCTURESFORCOUNTINGOFSINGLE PHOTONSINTHEINFRAREDRANGE

We present ourstudies onultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbNnanostructures. OurSSPDsarepatterned byelectron beamlithography from 4-nmthick NbNfilm into meander-shaped strips covering square areaofl0x1O pM2. Theadvances inthefabrication technology allowed ustoproduce highly uniform 100-120-nm-wide strips withmeander filling factor close to0.6. The detectors exploit acombined detection mechanism, whereuponasingle- photon absorption, anavalanche of excited hotelectrons andthebiasing supercurrent, jointly produce apicosecond voltage transient response across thesuperconducting nanostrip. TheSSPDsaretypically operated at4.2K,butthey haveshownthat their sensitivity intheinfrared radiation range canbesignificantly improved bylowering theoperating temperature from4.2K to2K.Whenoperated at2K,theSSPDquantumefficiency (QE)forvisible light photons reaches 30-40%, which isthesaturation value limited byoptical absorption ofour4-nm-thick NbNfilm. For1.55gm photons, QEwas-20%anddecreases exponentially with theincrease oftheoptical wavelength, butevenatthe wavelength of6gmthedetector remains sensitive tosingle photons andexhibits QEofabout10.2 %.Thedark (false) count rate at2K isaslowas2x10-4 s-', whatmakesourdetector essentially abackground-limited sensor. Theverylowdark-count rate results inthenoise equivalent power(NEP) aslowas10-18 WHz-t12 forthe mid-infrared range (6gm). Further improvement oftheSSPDperformance inthemid-infrared range canbe obtained bysubstituting NbNfortheother, lower-Tc superconductors with thenarrow superconducting gapand lowquasiparticle diffusivity. Theuseofsuch materials will shift thecutoff wavelength towards thevalues even longer than 6gm.