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

Energy resolution of a superconductor quantum detector

Alexei Semenov, Andreas Engel, Heinz‐Wilhelm Hübers, K. Ilin, M. Siegel

Published Sep 29, 2004• 1 citation
Publisher

Abstract

We report on the energy resolution of a recently developed superconductor single-photon quantum detector. In a superconducting strip quasiparticles created by a single absorbed photon and a bias current jointly cause a normal domain and, subsequently, a voltage transient that manifests absorption of the photon. Given a constant optical coupling, the mechanism suggests a moderate to good energy resolution in the wavelength range from near-infrared to X-rays. We implemented a meander line from a 80-nm wide and 5-nm thick NbN strip to detect single near-infrared photons with the repetition rate 5•107 sec-1 and quantum efficiency of few per cent. Although with this detector operated at 2 K we have indeed observed photon-energy dependent detector response, the energy resolving capability appeared smaller than the detector model predicted. We suggest that the inconsistency owes to the influence of the bias current.

DOI
10.1117/12.552578

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We report on the energy resolution of a recently developed superconductor single-photon quantum detector. In a superconducting strip quasiparticles created by a single absorbed photon and a bias current jointly cause a normal domain and, subsequently, a voltage transient that manifests absorption of the photon. Given a constant optical coupling, the mechanism suggests a moderate to good energy resolution in the wavelength range from near-infrared to X-rays. We implemented a meander line from a 80-nm wide and 5-nm thick NbN strip to detect single near-infrared photons with the repetition rate 5•107 sec-1 and quantum efficiency of few per cent. Although with this detector operated at 2 K we have indeed observed photon-energy dependent detector response, the energy resolving capability appeared smaller than the detector model predicted. We suggest that the inconsistency owes to the influence of the bias current.
Thin film cryogenic bolometers are widely used as integrating detectors for infrared and millimeter wave radiation. A superconducting quantum detector based on a nanometer-scaled structure may be an alternative for future applications where energy resolved single-photon detection and fast response times are simultaneously required. The mechanism responsible for the single-photon detection capability is due to the combined action of a hot-spot formation after photon absorption and a subsequent normal domain caused by an applied bias current [1]. We present recent results on energy resolution and spectral sensitivity of infrared quantum detectors made of different materials (Nb and NbN). The spectral response of the detector exhibits a pronounced threshold at a particular photon energy E0 that depends on the material and detector geometry. According to the detector model, information about the photons energy is contained in the amplitude and duration of the voltage pulse. A reasonable energy resolution is expected only for photon energies E >> E0. Preliminary experiments performed near the threshold energy have shown that the time integral of the response pulses indeed varied with the photon energy. The observed energy resolution of about 20% at 0.6 eV was not sufficient for practical applications. However, these preliminary data validate the response mechanism and support the envisaged performance at higher photon energies. [1] Semenov A D, Gol'tsman G N and Korneev A A 2001 Physica C 351 349.
Single photon detectors based on meanders from 5-nm thin B1 niobium nitride nanostrips show moderate spectral resolution along with the spectral cut-off of the quantum efficiency occurring in the near-infrared spectral range. Beyond the cut-off there is a gradual decrease of the quantum efficiency. To describe the response beyond the cut-off we combine the previously developed hot-spot detection mechanism with the photon-induced unbinding of the vortex-antivortex pairs, which appear in the superconducting film below the Kosterlitz-Thouless phase transition. The detector responds to the absorption of a single photon with a voltage pulse whose amplitude depends on the photon energy. We report the observation of a photon-energy dependent statistical distribution of the pulse amplitude that defines the energy resolution of the detector. We analyzed the detector response to continuous and femtosecond-pulse radiation using broadband microwave amplifiers and a fast single-shot oscilloscope as well as integrating SQUID amplifiers. Our analysis suggests that both the amplitude and the duration of the response pulses are jointly controlled via the kinetic inductance of the meander line and the Joule power dissipated in the meander by the bias current.
We analyze the spectral performance of recently developed single-photon quantum detector that consists of a narrow, nanometer sized meander-line made from ultra-thin superconducting film. The detector exploits a combined detection mechanism, in which avalanche multiplication of quasiparticles after absorption of a single photon and the bias current jointly produce a normal domain that results in a voltage pulse developing between the meander ends. With either the wavelength increase or the bias current decrease, the single-photon detection regime exhibits a cut-off. The wavelength, at which the cut-off occurs, varies from infrared waves to visible light depending on the superconducting material and operation conditions. Structural and geometrical non-uniformities of the meander line smooth out the otherwise expected sharp drop of the detection efficiency beyond the cut-off. We refine the early detector model and propose a tentative explanation of how superconducting fluctuations may additionally extend the detection efficiency beyond the cut-off wavelength.
We analyze the ultimate performance of a superconducting quantum detector in order to meet requirements for applications in near-infrared astronomy and X-ray spectroscopy. The detector exploits a combined detection mechanism, in which avalanche quasiparticle multiplication and the supercurrent jointly produce a voltage response to a single absorbed photon via successive formation of a photon-induced and a current-induced normal hotspot in a narrow superconducting strip. The response time of the detector should increase with the photon energy providing energy resolution. Depending on the superconducting material and operation conditions, the cut-off wavelength for the single-photon detection regime varies from infrared waves to visible light. We simulated the performance of the background-limited infrared direct detector and X-ray photon counter utilizing the above mechanism. Low dark count rate and intrinsic low-frequency cut-off allow for realizing a background limited noise equivalent power of 10−20 W Hz−1/2 for a far-infrared direct detector exposed to 4-K background radiation. At low temperatures, the intrinsic response time of the counter is rather determined by diffusion of nonequilibrium electrons than by the rate of energy transfer to phonons. Therefore, thermal fluctuations do not hamper energy resolution of the X-ray photon counter that should be better than 10−3 for 6-keV photons. Comparison of new data obtained with a Nb based detector and previously reported results on NbN quantum detectors support our estimates of ultimate detector performance.
Superconducting single-photon detector (SSPD) is patterned from 4-nm-thick NbN film deposited on sapphire substrate as a 100-nm-wide strip. Due to its high detection efficiency, low dark counts, and picosecond timing jitter SSPD has become a competitor to the InGaAs avalanche photodiodes at 1550 nm and longer wavelengths. Although the SSPD is operated at liquid helium temperature its efficient single-mode fibre coupling enabled its usage in many applications ranging from single-photon sources research to quantum cryptography. In our strive to increase the detection efficiency at 1550 nm and longer wavelengths we developed and fabricated SSPD with the strip almost twice narrower compared to the standard 100 nm. To increase the voltage response of the device we utilized cascade switching mechanism: we connected 50-nm-wide and 10-μm-long strips in parallel covering the area of 10 μmx10 μm. Absorption of a photon breaks the superconductivity in a strip leading to the bias current redistribution between other strips followed their cascade switching. As the total current of all the strips about is 1 mA by the order of magnitude the response voltage of such an SSPD is several times higher compared to the traditional meander-shaped SSPDs. In middle infrared (about 3 μm wavelength) these devices have the detection efficiency several times higher compared to the traditional SSPDs.
Superconducting single-photon detectors (SSPD) is typically 100 nm-wide supercondiucting strip in a shape of meander made of 4-nm-thick film. To reduce response time and increase voltage response a parallel connection of the strips was proposed. Recently we demonstrated that reduction of the strip width improves the quantum effciency of such a detector at wavelengths longer than 1.5 μm. Being encourage by this progress in quantum effciency we improved the fabrication process and made parallel-wire SSPD with 40-nm-wide strips covering total area of 10 μm x 10 μm. In this paper we present the results of the characterization of such a parallel-wire SSPD at 10.6 μm wavelength and demonstrate linear dependence of the count rate on the light power as it should be in case of single-photon response.