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Le Journal de Physique Colloques

FEMTOVOLTMETER AND VOLTAGE TUNABLE HETERODYNE RECEIVER BASED ON THE dc RESISTIVE SQUID

B. T. Ulrich, Rob W. van der Heijden, J. M. V. Verschueren

Published Aug 1, 1978• 0 citations
Publisher

Abstract

A voltmeter with femtovolt sensitivity, δVrms = 1 x 10-l5 V/(Hz)1/2, (i.e., 1 fV/ (Hz)1/2) and a voltage tunable far infrared heterodyne receiver, with intrinsic frequency resolution of 0.5 Hz at signal frequency 0.7 x 1012 Hz (0.7 THz) and a second power conversion efficiency of 0.2, were constructed on the basis of a dc resistive SQUID.

DOI
10.1051/jphyscol:19786540

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A voltmeter with femtovolt sensitivity, δVrms = 1 x 10-l5 V/(Hz)1/2, (i.e., 1 fV/ (Hz)1/2) and a voltage tunable far infrared heterodyne receiver, with intrinsic frequency resolution of 0.5 Hz at signal frequency 0.7 x 1012 Hz (0.7 THz) and a second power conversion efficiency of 0.2, were constructed on the basis of a dc resistive SQUID.
The output voltage of a dc superconducting quantum interference device (SQUID) containing a resistive section Rs oscillates at a frequency νq =Vs/ φ0 (φ0=flux quantum) when a current controlled voltage Vs is maintained across Rs. These oscillations have been observed from ∼10−1 to 1011 Hz, using Rs values ranging from ∼10−10 to ∼10−3 Ω in low inductance (∼10−10 H) point contact SQUID loops. At high frequencies, they manifest themselves in the dc current-voltage characteristics by inducing extra current steps.
A Josephson‐effect differential heterodyne receiver converts a variable signal frequency fS, near a fixed local oscillator fL, to a fixed intermediate frequency fI=fS−fL−fQ, by mixing between fD=fS−fL and a voltage‐tunable variable quantum oscillation fQ=2eV/h provided by a double‐junction resistive interferometer. At fL−0.7 THz provided by a far infrared formic acid laser, with fS=fL+fD provided by a Doppler shift, power conversion efficienciesηQ from fDγfI up to 0.2 were observed. Linewidths of the quantum oscillation δfQ∠ (2kBT/πL)1/2 (2eR/h) down to ∠0.5 Hz were observed for R=3.3×10−9Ω interferometer resistance, and L∠10−9 H interferometer inductance, demonstrating an intrinsic frequency resolution of 1 in 1012 at high conversion efficiency for the technique.
Extremely sensitive far-infrared detectors suitable for both direct detection and heterodyne applications, based on mu m-sized thin films with thickness less than a superconducting penetration depth are possible. The penetration depth of such a film, and therefore its inductance, varies with temperature and with quasiparticle population (described by an effective temperature T*), resulting in both bolometric and nonequilibrium photoinductive responses. Incident radiation is coupled into the small-area kinetic inductor by a lithographic antenna, and the resulting inductance changes are amplified and converted to a voltage signal by an integrated microstrip DC SQUID. The device is sensitive because, unlike junction-based devices with large capacitive reactances, the kinetic inductor is naturally well matched to the antenna impedance at the far-IR frequency ( nu >2 Delta /h) and to the preamplifier (SQUID) impedance at microwave or video frequencies ( nu <<2 Delta /h). The best kinetic inductor materials are those with low electronic mean free path, large penetration depth, and high critical current density. Thus, common magnet alloys such as NbTi are the natural choice for liquid-He temperature operation. A detailed analysis predicts a (phonon-limited) NEP of 4*10/sup -17/ W/ square root Hz for a bolometer with an iridium kinetic inductor operated at 0.1 K. A heterodyne noise temperature of 2250 K (single-sideband) at 3 THz, with a 200-MHz bandwidth, is predicted for a Nb-Ti mixer operated at 4 K.
The performances of a heterodyne amplifier involving a DC SQUID have been modelised and measured. This type of amplification is derived from frequency translation methods using flux modulation. The pump frequency used depends on the signal to be analysed and the translation to a fixed frequency f0 is carried out by the SQUID which is a nonlinear device. Such a system simplifies the impedance matching between the SQUID and the semiconductor preamplifier and can be operated as a spectrum analyser for signals of relatively high and variable frequency. Flux sensitivities of 5 to 10 μΦ0/Hz\mu\Phi_{0}/\sqrt{\rm Hz}μΦ0​/Hz​ have been measured in this configuration for frequencies ranging from 200 kHz to 2 MHz. In order to validate our model, the flux sensitivity of the SQUID in the heterodyne configuration has been compared to that given by direct measurements at the intermediate frequency f0. Our measurements are in good agreement with a theoretical model based on a simplified voltage to flux characteristic of the DC SQUID.
We report continuing investigations into practical applications of the AC Josephson effect as the basis for a voltage-tunable radio-frequency oscillator. We have previously demonstrated experimentally that useful power levels (10 s of nW) and linewidths of a few kHz can be achieved in the heterodyne output from a high-temperature-superconducting resistive SQUID (HTS-RSQUID) operating in the frequency range 1-50 MHz. Those results were achieved with 2-junction R-SQUIDs incorporating current-biased shunt resistors of a few micro-ohms. We have now modified the fabrication procedures, and adjusted the shunt resistors and bias current values so that higher frequencies can be achieved. The Josephson junctions are of step-edge type, rather than the bi-crystal type used in our earlier work. The step-edge technique permits much more flexibility in the geometrical lay-out and utilizes the more cost-effective single-crystal MgO substrates. In the present paper, we report numerical simulations and experimental measurements on these devices in the frequency range up to 2 GHz.
An all thin-film resistive SQUID with two Josephson tunnel junctions has been designed, fabricated and studied. As well as the two junctions, this device consists of a low ohmic (1.9*10 -6 Omega ) resistive part connected by superconducting leads to both junctions in a loop with very low ( approximately 10 -11 H) inductance. A direct current flowing through the resistive part fixes the voltage difference between the two Josephson junctions that are additionally fed by a bias current. The voltage difference is converted into a signal frequency that can be picked up from the SQUID with a conversion factor of ( phi o ) -1 =483.6 MHz mu V -1 , defined only by the electron charge and Planck's constant. An output signal level (peak-to-peak) of about 50 mu V has been obtained. The low frequency range (below 100 kHz) of the output signal frequency has been investigated. The possibilities of considerably extending this frequency range are also discussed.