Far-infrared laser study of small-area superconducting tunnel junctions
Thesis/Dissertation
·
OSTI ID:6023565
The first observations of photon-assisted quasiparticle tunneling and AC Josephson effect in superconducting tunnel junctions irradiated with far-infrared (FIR) radiation are reported. Radiation at 246 GH/sub z/ (lambda = 1.22 mm) and 604 GHz (496 ..mu..m) from an optically-pumped FIR laser source was used. Tin-tin oxide-lead junctions of approx. 1 (..mu..m)/sup 2/ area were fabricated on crystal quartz substrates with integral planar dipole antennas of resonant length at the frequency of the incident radiation. The observed photo-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. 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. A computer simulation has been developed which models the effects of the voltage-dependent (shot) noise and the non-linear quasiparticle resistance of the junctions, assuming T = 2K. Reasonably good agreement is obtained between the simulated I-V curves for un-irradiated but noise affected junctions, reproducing the observed fall-off in the I/sub c/R/sub n/ product with increasing junction resistance. For irradiated junctions, we also obtain good agreement with the observed noise rounding of the ac Josephson steps. Finally we have coupled FIR radiation into a 7..cap omega.. niobium-silicon-niobium junction and have observed three Josephson steps and one photon-assisted tunneling step.
- OSTI ID:
- 6023565
- Country of Publication:
- United States
- Language:
- English
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