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Applied Physics Letters

Submicrometer photoresponse mapping of nanowire superconducting single-photon detectors

Robert H. Hadfield, Paul A. Dalgarno, John A. O’Connor, E. Ramsay, Richard John Warburton, Eric J. Gansen, Burm Baek, Martin J. Stevens, Richard P. Mirin, Sae Woo Nam

Published Dec 10, 2007• 34 citations
Publisher

Abstract

We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10×10μm2). Using a confocal microscope configuration and solid immersion lens, we achieve a spot size of 320nm full width at half maximum onto the device at 470nm wavelength. We compare the response maps of two devices: The higher detection efficiency device gives a uniform response, whereas the lower detection efficiency device is limited by a single defect or constriction.

DOI
10.1063/1.2824384

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We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10×10μm2). Using a confocal microscope configuration and solid immersion lens, we achieve a spot size of 320nm full width at half maximum onto the device at 470nm wavelength. We compare the response maps of two devices: The higher detection efficiency device gives a uniform response, whereas the lower detection efficiency device is limited by a single defect or constriction.
We report on the photoresponse mapping of nanowire superconducting single-photon detectors using a focal spot significantly smaller than the device area (10 μm x 10 μm). Using a solid immersion lens we achieve a spot size of 320 nm full-width half maximum onto the device at 470 nm wavelength. We compare the response maps of two devices: the higher detection efficiency device gives a uniform response whereas the lower detection efficiency device is limited by a single defect or constriction. A second optical setup is used to simultaneously image and measure the photoresponse of the lower detection efficiency device, allowing the constriction location to be pinpointed.
Superconducting nanowire single-photon detectors (SNSPDs) play an important role in emerging optical quantum technologies. We report on advanced nanometric characterization of a high efficiency near infrared SNSPD design based on a low roughness tantalum pentoxide (Ta 2 O 5 )/silicon dioxide (SiO 2 ) distributed Bragg reflector (DBR) cavity structure. We have performed high resolution transmission electron microscopy analysis to verify the smoothness of the DBR. Optical reflectance measurements show excellent correspondence with DBR simulations. We have carried out precision nano-optical photoresponse mapping studies at 940 nm wavelength at T = 3.5 K, indicating excellent large area device uniformity (peak efficiency 55% at 100 Hz dark count rate (DCR)) with a full width half maximum timing jitter of 60 ps. With manual fibre coupling with single mode fibre, we achieve a system detection efficiency (SDE) of 57.5% at 940 nm wavelength (100 Hz DCR) at T = 2.3 K and a low polarization dependence of 1.20 ± 0.03. For coupling with multimode fibre, we achieve SDE of 90% at 940 nm (200 Hz DCR) at T = 2.3 K. These SNSPD devices are promising candidates for use in quantum dot photoluminescence studies and optical quantum technology applications.
We investigate the source of the large variations in the observed detection efficiencies of superconducting nanowire single-photon detectors between many nominally identical devices. Through both electrical and optical measurements, we infer that these variations arise from “constrictions:” highly localized regions of the nanowires where the effective cross-sectional area for superconducting current is reduced. These constrictions limit the bias-current density to well below its critical value over the remainder of the wire, and thus prevent the detection efficiency from reaching the high values that occur in these devices when they are biased near the critical current density.
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We report on the position-dependent variation in output pulse timing across a superconducting single-photon detector. Our device consists of a single niobium nitride nanowire meander (100 nm width, 4 nm film thickness, 2 mm length). We use a confocal microscope configuration (full width at half maximum-spot size 1.3 μm at 1550 nm wavelength) and a femtosecond laser to study local variations in detection efficiency and output pulse timing. Pulse delays of up to 50 ps across the device correlate to local detection efficiency and resistance variations. This study indicates an underlying mechanism for timing jitter in superconducting nanowire devices.
Niobium nitride superconducting nanowire single-photon detectors were fabricated on thermally oxidized silicon substrates with large active areas of 30 μm× 30 μm.To achieve non-constricted detectors,we improved the film growth and electron beam lithography process to fabricate uniform 100-nm wide Nb N nanowires with a fill factor of 50%.The devices showed 72.4% system detection efficiency(SDE) at 100-Hz dark count rate(DCR) and 74-ps timing jitter,measured at the fiber communication wavelength of 1550 nm.The highest SDE which is 81.2% when the DCR is ～ 700 c/s appears at the wavelength of 1650 nm.