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IEEE Transactions on Applied Superconductivity

A concept for a submillimeter-wave single-photon counter

R. J. Schoelkopf, S. H. Moseley, Carl M. Stahle, Paula Wahlgren, Per Delsing

Published Jun 1, 1999• 69 citations
Publisher

Abstract

We discuss the design for a submillimeter-wave photometer, using a combination of superconducting and single-electron devices, which would have high quantum efficiency, very low noise-equivalent powers, and eventually even submicrosecond timing resolution. The absorption of above-gap photons occurs in a small strip of superconducting Al, whose normal-state resistance can be matched efficiently to an antenna of a higher gap (Nb) superconductor. The quasiparticles produced by photon absorption are then confined via Andreev reflection, and forced to tunnel through a small SIS tunnel junction. The tunneling time is much shorter than the known (>10 /spl mu/s) quasiparticle recombination time, so collection efficiency will be high. The device sensitivity would be limited by the small subgap current in the high-quality Al/AlO/sub x//Al tunnel junction at temperatures (100 mK) well below T/sub c/. Scaling based on the larger junctions used in X-ray detector applications suggests that the total dark current can be <0.1 pA, or of order 10/sup 5/ electrons/second, corresponding to an NEP of less than 10/sup -19/ W//spl radic/Hz at 500 microns (600 GHz). The photocurrent will be measured using a fast single-electron transistor (RF-SET), which allows a shot-noise-limited performance even for the very small currents delivered from this low capacitance and high impedance SIS junction. Results of initial fabrication and dc characterization of an integrated photodetector are also given.

DOI
10.1109/77.783645

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We discuss the design for a submillimeter-wave photometer, using a combination of superconducting and single-electron devices, which would have high quantum efficiency, very low noise-equivalent powers, and eventually even submicrosecond timing resolution. The absorption of above-gap photons occurs in a small strip of superconducting Al, whose normal-state resistance can be matched efficiently to an antenna of a higher gap (Nb) superconductor. The quasiparticles produced by photon absorption are then confined via Andreev reflection, and forced to tunnel through a small SIS tunnel junction. The tunneling time is much shorter than the known (>10 /spl mu/s) quasiparticle recombination time, so collection efficiency will be high. The device sensitivity would be limited by the small subgap current in the high-quality Al/AlO/sub x//Al tunnel junction at temperatures (100 mK) well below T/sub c/. Scaling based on the larger junctions used in X-ray detector applications suggests that the total dark current can be <0.1 pA, or of order 10/sup 5/ electrons/second, corresponding to an NEP of less than 10/sup -19/ W//spl radic/Hz at 500 microns (600 GHz). The photocurrent will be measured using a fast single-electron transistor (RF-SET), which allows a shot-noise-limited performance even for the very small currents delivered from this low capacitance and high impedance SIS junction. Results of initial fabrication and dc characterization of an integrated photodetector are also given.
We have measured single-electron tunneling transistors with superconducting islands and conclude that they may be used as ultrasensitive detectors of microwave radiation for frequencies /spl ges/80 GHz. These devices contain a small superconducting Al island that is weakly coupled to a bias circuit through two small-capacitance tunnel junctions and a capacitive gate. At low bias voltages and temperatures, a single quasiparticle may only be introduced to the island through photon-assisted tunneling. Once this occurs, the quasiparticle is trapped on the island for /spl sim/1 /spl mu/s because it takes a relatively long time for this specific quasiparticle to tunnel off. While it is trapped, charge is transported through the system two electrons at a time. Since the photon-assisted transition merely switches the detector current on, this device is not limited to one electron tunneled through the system per absorbed photon. Measurements indicate that at least 100 electrons can tunnel for every absorbed photon, which corresponds to a noise-equivalent power of 3/spl times/10/sup -20/ W//spl radic/Hz at 80 GHz if the current is measured with a commercial current amplifier.>
We are developing superconducting direct detectors for submillimeter astronomy that can in principle detect individual photons. These devices, Single Quasiparticle Photon Counter (SQPC), operate by measuring the quasiparticles generated when single Cooper-pairs are broken by absorption of a submillimeter photon. This photoconductive type of device could yield high quantum efficiency, large responsivity, microsecond response times, and sensitivities in the range of 10-20 Watts per root Hertz. The use of antenna coupling to a small absorber also suggests the potential for novel instrument designs and scalability to imaging or spectroscopic arrays. We will describe the device concept, recent results on fabrication and electrical characterization of these detectors, issues related to saturation and optimization of the device parameters. Finally, we have developed practical readout amplifiers for these high-impedance cryogenic detectors based on the Radio-Frequency Single-Electron Transistor (RF-SET). We will describe results of a demonstration of a transimpedance amplifier based on closed-loop operation of an RF-SET, and a demonstration of a wavelength-division multiplexing scheme for the RF-SET. These developments will be a key ingredient in scaling to large arrays of high-sensitivity detectors.
We have developed submillimeter-wave direct detectors employing niobium-based superconducting tunnel junctions (STJs), with broadband spectral response, high sensitivity and imaging capability. Spectral response peaked at 650 GHz with a fractional bandwidth of 14 percent, which fits one of the important atmospheric windows for an astronomical project. We also confirmed the linearity of the detector response in the dynamic range greater than 10/sup 6/. The measured coupling efficiency was about 0.2, which was lower than the expected value of 0.5. The measured noise current of the detector was 10 fA//spl radic/Hz, which agreed with the shot noise from the residual leakage current of 100 pA at 0.3 K. Resultant noise equivalent power (NEP) is 1.6/spl times/10/sup -16/ W//spl radic/Hz, that is less than the background photon fluctuation limit for ground-based submillimeter-wave observations.
We present our studies on ultrafast superconducting single-photon detectors (SSPDs) based on ultrathin NbN nanostructures. Our SSPDs are patterned by electron beam lithography from 4-nm thick NbN film into meander-shaped strips covering square area of 10/spl times/10 /spl mu/m/sup 2/. The advances in the fabrication technology allowed us to produce highly uniform 100-120-nm-wide strips with meander filling factor close to 0.6. The detectors exploit a combined detection mechanism, where upon a single-photon absorption, an avalanche of excited hot electrons and the biasing supercurrent, jointly produce a picosecond voltage transient response across the superconducting nanostrip. The SSPDs are typically operated at 4.2 K, but they have shown that their sensitivity in the infrared radiation range can be significantly improved by lowering the operating temperature from 4.2 K to 2 K. When operated at 2 K, the SSPD quantum efficiency (QE) for visible light photons reaches 30-40%, which is the saturation value limited by optical absorption of our 4-nm-thick NbN film. For 1.55 /spl mu/m photons, QE was /spl sim/20% and decreases exponentially with the increase of the optical wavelength, but even at the wavelength of 6 /spl mu/m the detector remains sensitive to single photons and exhibits QE of about 10/sup -2/%. The dark (false) count rate at 2 K is as low as 2 /spl times/ 10/sup -4/ s/sup -1/, what makes our detector essentially a background-limited sensor. The very low dark-count rate results in the noise equivalent power (NEP) as low as 10/sup -18/ WHz/sup -1/2/ for the mid-infrared range (6 /spl mu/m). Further improvement of the SSPD performance in the mid-infrared range can be obtained by substituting NbN for the other, lower-T/sub c/ superconductors with the narrow superconducting gap and low quasiparticle diffusivity. The use of such materials will shift the cutoff wavelength towards the values even longer than 6 /spl mu/m.
For high resolution imaging and spectroscopy in the FIR and submillimeter, space observatories will demand sensitive, fast, compact, low-power detector arrays with 104 pixels and sensitivity less than 10(exp -20) W/Hz(sup 0.5). Antenna-coupled superconducting tunnel junctions with integrated rf single-electron transistor readout amplifiers have the potential for achieving this high level of sensitivity, and can take advantage of an rf multiplexing technique. The device consists of an antenna to couple radiation into a small superconducting volume and cause quasiparticle excitations, and a single-electron transistor to measure current through junctions contacting the absorber. We describe optimization of device parameters, and results on fabrication techniques for producing devices with high yield for detector arrays. We also present modeling of expected saturation power levels, antenna coupling, and rf multiplexing schemes.
The science drivers for the SPIRIT/SPECS missions demand sensitive, fast, compact, low-power, large-format detector arrays for high resolution imaging and spectroscopy in the far infrared and submillimeter. Detector arrays with 10,000 pixels and sensitivity less than 10(exp 20)-20 W/Hz(exp 20)0.5 are needed. Antenna-coupled superconducting tunnel junction detectors with integrated rf single-electron transistor readout amplifiers have the potential for achieving this high level of sensitivity, and can take advantage of an rf multiplexing technique when forming arrays. The device consists of an antenna structure to couple radiation into a small superconducting volume and cause quasiparticle excitations, and a single-electron transistor to measure currents through tunnel junction contacts to the absorber volume. We will describe optimization of device parameters, and recent results on fabrication techniques for producing devices with high yield for detector arrays. We will also present modeling of expected saturation power levels, antenna coupling, and rf multiplexing schemes.