Enhanced absorptance of infrared single-photon detectors comprising plasmonic structure integrated NbN pattern on silicon substrate

Summary form only given. Integration of plasmonic structures into superconducting nanowire single-photon detectors (SNSPD) is capable of resulting in enhanced absorptance [1], and makes possible to realize single plasmon detection [2], i.e opens novel avenues also in secure communication and quantum-key computing. In our previous studies we have presented single optical-cavity (OC) and nano-cavity-array integrated (NCAI) SNSPDs, covered by noble metal reflector, and horizontal as well as vertical gold segments integrated into the meandered niobium-nitride (NbN) pattern on sapphire substrate, respectively [1]. Patterns with periodicity (~600-710 nm) approximating the plasmon wavelength were also inspected to realize parallel electric optimization [1].Our recent studies revealed that integration of longer vertical gold segments embedded into the substrate, namely nano-cavity-deflector integrated NCDAI-SNSPD designs can result in huge NbN absorptance enhancement. However the patterning of sapphire substrates is a challenge, so here we present the optimal illumination directions of integrated devices based on silica substrate, which can be etched more easily. The periodicity (792.5 nm) of the NbN pattern was set to match the condition of optimal cavity filling in NCAIand NCDAI-SNSPDs (corresponding to m=1, k=4 condition in ref. [1]), as well as to approximate the 0.75*λplasmon wavelength. Perpendicular incidence onto OC-SNSPD in P-orientation results in 27% absorptance, while S-orientation is more advantageous in NCAI-SNSPD and in NCDAI-SNSPD. For practical applications perpendicular incidence onto NCAI-SNSPDs, where 34% absorptance is attainable, is already advantageous, while strongly enhanced 75 % absorptance is achievable in NCDAI-SNSPDs at 19.35° tilting (Fig. 1a, b).The near-field study (Fig. 1c) revealed that in OC-SNSPD the E-field antinode at the silica substrate interface promotes the NbN absorptance. In NCAI-SNSPD the resonant transmission an- the localized resonances on the nano-cavity-array result in enhanced absorptance at small polar angles. Although the polar angle dependent absorptance exhibits a plasmonic-band-gap characteristic [3], only a small local maximum (20.8°, 29 %) can be observed due to the field concentration below the NbN segments. This originates from backward propagating Brewster waves with enhanced wavelength according to ksurface wave=kphoton-kgrating relation. The advantage of the NCDAI-SNSPD is that the gold deflectors prevent the re-radiation of surface plasmon polaritons, as a result large absorptance maximum appears at the orientation where the grating couples into SPPs. In NCDAI-SNSPDs high detection efficiency is attainable in contempt of small fill-factor, i.e. parallel electric optimization is realizable as a result of coupled resonances of local and propagating plasmonic modes.