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physica status solidi (a)

Responsivity of SIS tunnel junctions to mm-wave radiation

K. H. Gundlach, Hans J. Hartfuss, Kikuo Okuyama

Published Jul 16, 1980• 3 citations
Publisher

Abstract

The theory of photon-assisted tunneling is outlined. The I–U characteristic with and without radiation and the junction conductance are evaluated for the temperature of 4.2 K. From these results the current responsivity ηc and the voltage responsivity ηv are derived. At a frequency of 72 GHz the theory yields the maximum values ηc = 3.3 × 103 A/W and for a 100Ω junction ηv = 2.5 × 108 V/W. Leakage current and other shortcomings of the real junctions reduce these values to ηc ≈︁ 3 × 103 A/W and ηv ≈︁ 106 V/W.

DOI
10.1002/pssa.2210600116

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The theory of photon-assisted tunneling is outlined. The I–U characteristic with and without radiation and the junction conductance are evaluated for the temperature of 4.2 K. From these results the current responsivity ηc and the voltage responsivity ηv are derived. At a frequency of 72 GHz the theory yields the maximum values ηc = 3.3 × 103 A/W and for a 100Ω junction ηv = 2.5 × 108 V/W. Leakage current and other shortcomings of the real junctions reduce these values to ηc ≈︁ 3 × 103 A/W and ηv ≈︁ 106 V/W.
The strong nonlinearity of the quasiparticle tunneling current in superconductor-insulator-superconductor junctions near the full gap voltage 2 Delta/e can be used for direct detection of microwave signals. Measurements at 36 GHz yielded a current responsivity of 3,5000 A/W, which is within a factor of 2 of the quantum limited value e/h-bar omega. The measured NEP of 2.6 + or - 0.8 x 10 to the -16th power W/sq-rt. Hz is the lowest value reported to date and can probably be improved significantly. The experimental results are compared with both the standard classical analysis and photon assisted tunneling theory. (Author)
The rapid onset of quasiparitcle tunneling current in superconductor-insulator-superconductor (Josephson) junctions at voltages above 2Δ/e is being used for millimeter-wave heterodyne mixing. Junctions with a 2-μm diameter and RN=50 Ω have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Double sideband conversion efficiencies of 0.32 and mixer noise temperatures as low as TM⩽7 K=4hν/k have been observed. The results are compared with shot-noise-limited mixer theory. Photon-assisted tunneling effects are seen which indicate the approach to photon-noise-limited operation.
The strong nonlinearity of the quasiparticle tunneling current in superconductor-insulator-superconductor junctions near the full-gap voltage 2Δ/e can be used for direct detection of microwave signals. Measurements at 36 GHz yielded a current responsivity of 3500 A/W, which is within a factor of 2 of the quantum-limited value e/∇ω. The measured NEP of 2.6±0.8×10−16 W/,Hz1/2 is the lowest value reported to date and can probably be improved significantly. The experimental results are compared with both the standard classical analysis and photon-assisted tunneling theory.
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 Josephson radiation emitted at millimeter-wave frequencies by a long Josephson junction dc-current biased on Fiske steps of the current-voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero-field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in-line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon-assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30-Ω normal-state resistance tunnel junction.
We report operation of superconductor-insulator-superconductor quasiparticle tunnel junctions as mixers and detectors at millimeter wavelengths. The voltage scale of the nonlinearity in the I-V curve of such junctions can be small enough that classical device theory breaks down. Calculations based on photon assisted tunneling theory are then necessary to understand the observed performance. The performance predicted in this novel regime is sufficiently good that practical applications appear very promising. The best mixer result observed at 36 GHz is a double side-band mixer noise temperature Tm≤ 7 K which is about 4 times the photon noise limited value of hv/k. When operated as a direct detector, a current responsivity of 3,500 A/W was observed. This is half the quantum limited value e/hv, which corresponds to one tunneling electron for each incident microwave photon. The best observed NEP was 3 × 10-16W√Hz.