Nano-Optical Studies of Superconducting Nanowire Single Photon Detectors

Single-photon detectors based upon superconducting nanowires offer single-photon sensitivity from the visible well into the infrared, encompassing the crucial telecommunication wavelengths. In addition they benefit from low dark counts (Hz) and low timing jitter (sub 100 ps). These detectors have recently been employed in photon-counting applications such as quantum cryptography and characterization of quantum emitters. Key challenges in the development of this emerging single-photon detector technology are improving device uniformity and improving optical coupling efficiency. We report on photoresponse mapping of superconducting nanowire single-photon detectors using nano-optical techniques. The device is mounted on a high resolution translation stage and illuminated with a focal spot significantly smaller than the device area (20 μm x 20 μm). Using a confocal microscope configuration, we achieve a diffraction limited spot size of 800 nm at λ = 950 nm. We have previously shown that uniform (high detection efficiency) and constricted (low efficiency) nanowire devices give contrasting photoresponse signatures - a broad plateau response across the whole device and a single point response respectively. Recent work on purposely-designed low fill-factor devices (100 nm linewidth, 1 mm period) has allowed us to resolve individual 100 nm width wires.