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Applied Physics Letters

Broadband response of the quasiparticle current in a superconducting tunnel junction

S. Verghese, Chandu A. Karadi, C. A. Mears, J. Orenstein, P. L. Richards, Andrew T. Barfknecht

Published Feb 14, 1994• 4 citations
Publisher

Abstract

We have measured the linear response of the quasiparticle current in a Nb/AlOx/Nb junction from 75–200 GHz. Picosecond pulses of millimeter wave radiation were generated by illuminating a photoconductive switch with a mode-locked laser. The pulses were coupled to the junction through an impedance-mismatched, planar lithographed antenna. The broadband response of the superconductor-insulator-superconductor junction was measured by monitoring changes in the dc current induced by interfering two electrical pulses at the junction as a function of the time delay between them. The data are in agreement with the linear theory of photon-assisted tunneling.

DOI
10.1063/1.110994

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We have measured the linear response of the quasiparticle current in a Nb/AlOx/Nb junction from 75–200 GHz. Picosecond pulses of millimeter wave radiation were generated by illuminating a photoconductive switch with a mode-locked laser. The pulses were coupled to the junction through an impedance-mismatched, planar lithographed antenna. The broadband response of the superconductor-insulator-superconductor junction was measured by monitoring changes in the dc current induced by interfering two electrical pulses at the junction as a function of the time delay between them. The data are in agreement with the linear theory of photon-assisted tunneling.
Picosecond electrical pulses were used to measure the broadband quasiparticle response of a Nb trilayer SIS (superconductor-insulator-superconductor) junction in both the linear and nonlinear regimes. The electrical pulses were generated by illuminating a Si photoconducting switch using a Ti-sapphire laser. The SIS response is inferred by measuring the dc current induced by interfering two electrical pulses at the junction, as a function of the time delay between them. Results are obtained for bias voltages of 1.0 and 2.0 mV; at the latter, two-photon absorption was detected.
We have made the first direct measurement of the quantum susceptance that arises from the nondissipative part of quasiparticle tunneling in a superconductor-insulator-superconductor tunnel junction. The junction is coupled to an antenna and a superconducting microstrip stub to form a resonator; the resonant frequency is determined from the response of the junction to broadband radiation from a Fourier-transform spectrometer. A 19% shift of the resonant frequency, from 73 to 87 GHz, is observed, which arises from the change of the quantum susceptance of the junction with dc bias voltage. This shift is in excellent agreement with calculations based on the Werthamer-Tucker theory, which includes the quantum susceptance. We also demonstrate that it is essential to include the quantum susceptance in our theoretical computation to explain the photon-assisted-tunneling steps, which have negative dynamic conductance. Such steps are observed when the junction is pumped at slightly below the resonant frequency of the capacitor and the stub. The quantum susceptance should exist in all tunnel devices whose nonlinear I-V characteristics are due to elastic tunneling.
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.
The submillimeter wave frequency responses of NbN/AlN/NbN tunnel junctions have been investigated using quasioptical coupling techniques and an optically pumped far-infrared laser. A two junction array was integrated at the feeds of a bowtie antenna. Measurements on the junctions were made by monitoring photon-assisted tunneling steps induced in the current–voltage characteristics. These n=±1 photon steps were clearly observed with irradiation at 303, 594, 693, and 762 GHz. The results suggest it will be possible to realize submillimeter wave mixers by using these junctions.
A Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) grain boundary junction formed on a MgO bicrystal substrate showed a superconductor-insulator-superconductor (SIS) behavior with a sharp gap structure and a Josephson current. A 270 GHz millimeter wave mixing in a BKBO junction was performed by using the quasi optical technique. A log-periodic antenna, a hyperhemispherical MgO lens and a Teflon lens were used in order to improve coupling for millimeter wave. The IF outputs have some peaks depending on the photon assisted tunneling steps. This is the first time that quasi-particle mixing could be done in an oxide superconductor.
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.