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Physical Review Letters

Far Infrared Response of Point-Contact Josephson Junctions

C. C. Grimes, P. L. Richards, S. M. Shapiro

Published Aug 22, 1966• 60 citations
Publisher

Abstract

We have measured spectral-response curves of superconducting point contacts suggesting the existence of frequency-dependent Josephson current amplitudes peaking in the vicinity of the energy gap. These point-contact Josephson junctions respond sensitively and with high speed to millimeter and submillimeter radiation. Radiation emitted to free space by one such junction was detected by another junction.

DOI
10.1103/physrevlett.17.431

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Far infrared detection by Josephson junctions

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A group at Bell Telephone Laboratories has reported using superconducting point contacts that show zerovoltage Josephson current as “sensitive, broadband, high-speed detectors of millimeter and submillimeter radiation.” The work was done by C. C. Grimes, P. L. Richards and S. Shapiro and reported in Physical Review Letters 17, 431 (1966).

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•Dec 1, 1976•0 citations

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Journal of Applied Physics•Jul 1, 1972•101 citations

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Although a number of investigators have generated microwave radiation by means of properly biased Josephson junctions, little work has been reported on similar applications to the submillimeter wave region where tunable coherent oscillators are not yet available. We have generated continuously tunable millimeter and submillimeter wave radiation with a point contact Josephson junction in a superconducting transmission line. This transmission line is centered in an oversized multimode brass cavity. With a broad band detector placed in the brass cavity near the transmission line radiation is detected for all bias voltages below 2 mV. The maximum power actually absorbed by the detector is about 2 × 10-10 W. In order to analyze the frequency content of the radiation we have used a lamellar interferometer together with Fourier transform spectroscopic techniques. The amount of power detected in the remote detector cryostat is two orders of magnitude smaller than that detected adjacent to the junction. Only at bias voltages where a particularly large amount of radiation is emitted are spectroscopic measurements possible. For these cases the frequency of the radiation is that predicted from the bias voltage and the Josephson relation. The linewidth is less than the resolution of the interferometer. To see if the radiation is continuously tunable a low temperature spectrometer has been constructed. The instrument consists of a germanium bolometer cooled to 0.3 K by a He3 charcoal adsorption pump, a four position sample rotator at 1.2 K and a Josephson junction transmission line source. By sweeping the bias voltage from zero to 2 mV absorption line spectra have been recorded in three different crystals, they are : 1. the antiferromagnetic resonance at 8.7 cm-1 in MnF2 ; 2. the impurity mode absorption at 14.3 cm-1 in CsI : Tl and 3. the impurity mode absorption at 18.0 cm-1 in KBr : Li. A low temperature double beam spectrometer is now under construction.

Revue de Physique Appliquée•Jan 1, 1974•4 citations
We have measured spectral-response curves of superconducting point contacts suggesting the existence of frequency-dependent Josephson current amplitudes peaking in the vicinity of the energy gap. These point-contact Josephson junctions respond sensitively and with high speed to millimeter and submillimeter radiation. Radiation emitted to free space by one such junction was detected by another junction.
It has been found that point-contact Josephson junctions can be clearly classified into bridge-type and tunnel-type by introducing new parameters characterizing their I-V curves. Nearly-ideal tunnel-type junctions with Nb–SuO x –Sn (Film) point-contacts were obtained. Tunnel-type junctions showed larger gap energies than bridge-type junctions in Nb–SnO x –Sn and Nb–NbO x –NbN point-contacts. The submillimeter-wave responses were measured in Nb–Nb point-contacts, and it was found that the contacts with the best performance were bridge-type junctions with a sharp gap structure; tunnel-type junctions showed rather bad performance. On irradiation with laser light, the center of the Shapiro step moved to the lower-current side in bridge-type junctions but to the higher-current side in tunnel-type junctions. These differences can be explained by differences in the heating effects between bridge-type and tunnel-type junctions.
The response of Josephson junctions to microwave and far infrared radiation was studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junctions. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22GHz to 105GHz. The dependence of the responsivity of these current on film thickness over a range from 50 A to 500 A was studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies.
For a current-driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high-frequency currents is calculated with a second-order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic-frequency regions depending on whether the internal self-generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0< or >ω the response could be expressed by easily observable parameters of the dc voltage-current characteristics. This permits comparison of the predictions with experimental results obtained on point-contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the inverse battery current in terms of the critical current, and (iv) the voltage amplitude of the inherent Josephson oscillations. For ω0≫ω ``classical'' detection proportional to curvature is obtained. For ω0∼ω resonance detection occurs, which depends to a considerable degree on fluctuations of the average contact voltage. These predictions are in reasonable agreement with experiments in which point-contact junctions were illuminated with klystron radiation at 10 and 90 GHz. Evidence is presented that the voltage fluctuations limiting the resonance response are the result of noise generated in the junction. Comparison of the noise-equivalent power expected from the simple model with measured results suggests that parasitic elements, such as shunt capacity, are not negligible at 90 GHz.
Although a number of investigators have generated microwave radiation by means of properly biased Josephson junctions, little work has been reported on similar applications to the submillimeter wave region where tunable coherent oscillators are not yet available. We have generated continuously tunable millimeter and submillimeter wave radiation with a point contact Josephson junction in a superconducting transmission line. This transmission line is centered in an oversized multimode brass cavity. With a broad band detector placed in the brass cavity near the transmission line radiation is detected for all bias voltages below 2 mV. The maximum power actually absorbed by the detector is about 2 × 10-10 W. In order to analyze the frequency content of the radiation we have used a lamellar interferometer together with Fourier transform spectroscopic techniques. The amount of power detected in the remote detector cryostat is two orders of magnitude smaller than that detected adjacent to the junction. Only at bias voltages where a particularly large amount of radiation is emitted are spectroscopic measurements possible. For these cases the frequency of the radiation is that predicted from the bias voltage and the Josephson relation. The linewidth is less than the resolution of the interferometer. To see if the radiation is continuously tunable a low temperature spectrometer has been constructed. The instrument consists of a germanium bolometer cooled to 0.3 K by a He3 charcoal adsorption pump, a four position sample rotator at 1.2 K and a Josephson junction transmission line source. By sweeping the bias voltage from zero to 2 mV absorption line spectra have been recorded in three different crystals, they are : 1. the antiferromagnetic resonance at 8.7 cm-1 in MnF2 ; 2. the impurity mode absorption at 14.3 cm-1 in CsI : Tl and 3. the impurity mode absorption at 18.0 cm-1 in KBr : Li. A low temperature double beam spectrometer is now under construction.