Quantum Efficiency and Polarization Effects in NbN Superconducting Single Photon Detectors

Superconducting Single Photon Detectors based on niobium nitride (NbN) nanowires have been optimized in regards to the quality of the epitaxial layer grown on M-plane 3-inch Sapphire wafer, leading to Tc ≈ 13 K and Jc ≈ 5 MA/cm for a 5 nm thick layer patterned down to 80 nm stripe width using an e-beam writer. Using those films, 7 % of quantum efficiency at 4.2K for 100 nm linewidth nanowires detectors has been achieved. We measured the kinetic inductance of our SSPD by 2 differents ways. Clear effects of light polarization on Detection Efficiency (DE) dependent has also been observed and quantified. DE varies by a factor 2 to 4 for a great number of tested SSPDs, meander width varying from 100 nm to 300 nm. The SSPD has been modeled as a detector with 2 different linear DEs as incident light can be polarized parallel or normal to the linewidth. This model is in good agreement with experimental data and roughly corresponds to the calculated absorption of the 5 nm thick NbN layer. However polarization effects also observed in multi-photon regime raise new issues.