Nano-optical studies of superconducting nanowire single-photon detectors
uperconducting single-photon detectors based on superconducting nanowires offer broadband single-photon sensitivity, from visible to mid-infrared wavelengths. They have attracted particular attention due to their promising performance at telecommunications wavelengths. The additional benefits of superconducting nanowire single-photon detectors (SNSPDs) include low dark count rates (Hz) and low timing jitter (sub 100 ps). SNSPDs have been employed in practical photon-counting applications such as quantum key distribution (QKD), operation of quantum waveguide circuits and quantum emitter characterisation. Major challenges in the development of SNSPDs are the improvement of device uniformity and achieving efficient optical coupling. Nano-optical techniques such as confocal microscopy can be used to image localised areas of SNSPDs providing a direct measurement of the device uniformity. The work in this thesis describes both initial nano-optical testing at visible wavelengths in liquid helium and the construction of a fibre based miniature confocal microscope configuration operating at telecommunications wavelengths for use in a closed cycle refrigerator. In both cases localised areas of SNSPDs can be studied whilst maintaining efficient optical coupling. The miniature confocal microscope configuration has sub-nanometre position resolution over a 30 μm x 30 μm area by way of a piezoelectric X-Y scanner. A full width at half maximum (FWHM) optical resolution of 1305 nm at a wavelength of 1550 nm is achieved. SNSPDs based upon niobium nitride (NbN) nanowires fabricated on magnesium oxide (MgO) have been studied. The microscope system has allowed us to map the temporal response (timing jitter and output pulse timing delay) of constricted (non-uniform) SNSPDs. By fitting to a theoretical model, the variations in output pulse timing delay have been shown to be caused by variations in hotspot resistances across the device. This observation has provided insights into the underlying physics of SNSPDs and especially the origins of timing jitter in SNSPDs. This provides a pathway to exploitation of this effect in next-generation device designs for applications such as imaging.
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