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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Nano-optical studies of superconducting nanowire 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 2, 2008• 2 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 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.

DOI
10.1117/12.819360

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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 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.
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.
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 report efficient single-photon detection (η = 20% at 1550 nm wavelength) with ultranarrow (20 and 30 nm wide) superconducting nanowires, which were shown to be more robust to constrictions and more responsive to 1550 nm wavelength photons than standard superconducting nanowire single-photon detectors, based on 90 nm wide nanowires. We also improved our understanding of the physics of superconducting nanowire avalanche photodetectors, which we used to increase the signal-to-noise ratio of ultranarrow-nanowire detectors by a factor of 4, thus relaxing the requirements on the read-out circuitry and making the devices suitable for a broader range of 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.
We report on superconducting nanowire single photon detectors (SNSPDs) based on 30 nm wide nanowires with detection efficiency η ∼ 2.6-5.5% in the wavelength range λ = 0.5-5 μm. We compared the sensitivity of 30 nm wide SNSPDs with the sensitivity of SNSPDs based on wider (85 and 50 nm wide) nanowires for λ = 0.5-5 μm. The detection efficiency of the detectors based on the wider nanowires became negligible at shorter wavelengths than the 30 nm wide SNSPDs. Our 30 nm wide SNSPDs showed 2 orders of magnitude higher detection efficiency (η ∼ 2%) up to longer wavelength (λ = 5 μm) than previously reported. On the basis of our simulations, we expect that by changing the optical coupling scheme and by integrating the detectors in an optical cavity, the detection efficiency of our detectors could be increased by a factor of ∼6.
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.