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A Far-Infrared Laser Study of Small-Area Superconducting Tunnel Junctions.

W. C. Danchi

Published Oct 1, 1983• 0 citations
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Abstract

Abstract : This document reports the first observations of photon-assisted quasiparticle tunneling and AC Josephson effect in superconducting tunnel junctions irradiated with far-infrared radiation.

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ac Josephson effect in small-area superconducting tunnel junctions at 604 GHz

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We have measured the ac Josephson effect in small-area superconducting tunnel junctions at 604 GHz (λ=496 μm) using a resonant planar dipole antenna to couple the radiation into the junction. In a 176-Ω junction, we have observed up to the 7th Josephson step (at 8.75 mV), a performance comparable to that seen in point contacts. We also compare our data with the results of the RSJ model and of the Werthamer theory.

Applied Physics Letters•Nov 1, 1982•28 citations
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.