Spatial dependence of output pulse delay in a niobium nitride nanowire superconducting single-photon detector
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Miniature confocal microscope configuration. This allows high resolution scanning over a area, allowing detection efficiency and instrument response histograms (inset) to be recorded. TCSPC: time-correlated single photon counting card; SSPD: superconducting single-photon detector; SMF: single-mode fiber. Femtosecond fiber laser: with 2 mW output power and 50 MHz repetition rate. The SSPD is mounted upside down on the microscope housing, to provide a thermal link for cooling to . The trigger detector was a second (fiber-coupled) SSPD for the maps (Fig. 2), and an InGaAs PIN diode for the pulse shapes (Fig. 3). The amplifier chain consists of 580 MHz and 1 GHz bandwidth room-temperature amplifiers. Inset: a typical TCSPC instrument response histogram. A Gaussian fit (solid line) gives the relative timing delay, (from the peak position) and the full-width at half maximum timing jitter, .
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Photoresponse maps with timing information across the SSPD obtained via a TCSPC measurement, using a second fiber-coupled SSPD (140 ps FWHM) as trigger. The high and low sensitivity regions of the device are marked. (a) Detection efficiency (DE) map. The maximum DE (0.01%) is observed at the center of the device. As indicated by the insets (taken along a single line across the device through the center) the maximum DE corresponds to the lowest timing delay in (c). (b) Map of FWHM timing jitter (varying between 190 and 205 ps across the device), determined from a Gaussian fit to the histogram at each position [examples of raw data shown in Fig. 3(b)]. (c) Map of output pulse arrival delay, : determined from a Gaussian fit to the instrument response function at each point. The maximum variation in across the device is 50 ps. (d) SSPD scale schematic, (100 nm width, 200 nm period, 4 nm thickness, 2 mm length, area), with close-up of nanowire structure.
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(a) Leading edge of output pulses (averaged over 256 shots on an 8 GHz bandwidth oscilloscope) at both high (red solid line) and low (black solid line) sensitivity spots on the device [identified from Fig. 2(a)]. The dashed lines are the simulated pulses (PSPICE model). Inset: full pulse shapes. (b) Instrument response function histogram data (using a TCSPC card and InGaAs PIN diode with 16 ps FWHM jitter as trigger) at the high efficiency spot (, red line) and low efficiency spot (, black line). The relative timing delay between the peak positions is 30 ps.
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