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IEEE Transactions on Applied Superconductivity

Direct detection at FIR frequencies with niobium tunnel junctions

J. Prince, Bascom S. Deaver, S. Withington

Published Mar 1, 1993• 1 citation
Publisher

Abstract

Measurements have been made on an array of six niobium/aluminum-aluminum oxide/niobium junctions with an integrated bow-tie antenna at 584 GHz. These measurements include examining the response of current-voltage curves at different incident radiation powers, both with and without an applied magnetic field. The measurements also include the responsivity and the noise of the devices as direct detectors. A planar superconducting quantum interference device (SQUID) was used as the low-frequency amplifier, and a chopped far infrared (FIR) laser generated the radiation. Comparisons with theoretical predictions show good fits to the current-voltage curves, except at the highest powers where there is evidence of heating and possibly nonequilibrium processes. Current responsivities of greater than one-half of the quantum limit and a noise equivalent power of 2*10/sup 1-4/ W/ square root Hz have been obtained.>

DOI
10.1109/77.233554

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•Apr 1, 2004•0 citations
Measurements have been made on an array of six niobium/aluminum-aluminum oxide/niobium junctions with an integrated bow-tie antenna at 584 GHz. These measurements include examining the response of current-voltage curves at different incident radiation powers, both with and without an applied magnetic field. The measurements also include the responsivity and the noise of the devices as direct detectors. A planar superconducting quantum interference device (SQUID) was used as the low-frequency amplifier, and a chopped far infrared (FIR) laser generated the radiation. Comparisons with theoretical predictions show good fits to the current-voltage curves, except at the highest powers where there is evidence of heating and possibly nonequilibrium processes. Current responsivities of greater than one-half of the quantum limit and a noise equivalent power of 2*10/sup 1-4/ W/ square root Hz have been obtained.>
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.
Niobium nitride (NbN) superconducting tunnel junctions (STJs) have an energy gap nearly double that of niobium STJs, making them potential sensitive detectors that operate at higher frequencies and temperatures. In this paper, we investigate the direct-detection behavior (i.e. the spectral response, current responsivity, noise characteristics, and noise equivalent power (NEP)) of a 500 GHz NbN STJ detector at temperatures from approximately 5 to 9 K. The detector shows an uncorrected NEP of 3.8 x 10(-13) W/root Hz around 5 K and 1.5 x 10(-12) W/root Hz at 9 K. Its performance can be further improved by adopting a cryogenically cooled readout circuit and fabricating the device with a wiring layer of higher critical-transition temperatures.
The capability of an all-niobium superconducting quantum interference device (SQUID) array to operate as an electrically small antenna capable of detecting radio frequency from distant sources was demonstrated. The measurements were performed in three different arrays, with each array consisting of 2400 identical cells. The intrinsic broadband characteristics of the device were confirmed by the flat frequency response of the ratio of the output and input voltages (the S21 parameter) of the array between 300 kHz and 1 GHz. Setting the device to its optimal operating parameters allowed the detection of signals from local frequency-modulation stations.
We report the first observations of photon-assisted quasiparticle tunneling and AC Josephson effect in superconducting tunnel junctions irradiated with far-infrared (FIR) radiation. Radiation at 246 GHz (λ=1.22 mm) and 604 GHz (496 μm) from an optically-pumped FIR laser source was used. Tin-tin oxide-lead junctions of ∼1 (μm)2area were fabricated on crystal quartz substrates with integral planar dipole antennas of resonant length at the frequency of the incident radiation. The observed photon-assisted tunneling features are in excellent agreement with the Tien-Gordon theory, and the inferred responsivity approaches the quantum limit at low temperatures for photon energies less than the gap. At 604 GHz, with a 176 ohm junction, we have seen 7 Josephson steps, comparable to point contact performance. The variation of the step widths with laser power is found to agree quite well with both the RSJ model and the Werthamer theory. For low resistance junctions (e.g. 16 ohms), we find the Josephson steps to be flat and to agree well with the shape predicted by the RSJ model without noise rounding, while noise rounding is very evident with the higher resistance junctions. In all cases, the step shape is in reasonable agreement with the theory of P. A. Lee, using a noise temperature of 10-20K with an appropriate small junction capacitance.
We have measured the response of Nb-aSi-Nb junctions to 604 GHz radiation from an optically pumped far-infrared (FIR) laser source. These rugged and thermally cyclable junctions situated at the midpoint of 180 μm long dipole antennas, were fabricated on oxidized silicon wafers using the Selective Niobium Anodization Process (SNAP). Josephson current densities were\sim 9,000A/cm2, the McCumber parameters βcwere ∼3.5, and the Josephson plasma frequencies\omega_{J}\sim2.5\times10^{12}. On a junction with a normal state resistance of\sim7\Omega, we observed three Josephson steps and one photon-assisted tunneling step. The widths of the Josephson steps were studied as a function of the laser power. An RSJ model computer simulation with a nonlinear quasiparticle conductance and an rf current bias (assumed because of the low junction resistance) is able to account reasonably well for the laser-power dependence of the critical current (zeroth step) and the second step. However, the temperature dependence is more complex, and a discrepancy still exists between the RSJ model calculation and the data for the hysteretic first step.
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.