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Response of a Nb/Al{sub 2}O{sub 3}/Nb Tunnel Junction to Picosecond Electrical Pulses

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

Published Sep 1, 1992• 0 citations
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Abstract

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.

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•Mar 1, 1985•0 citations
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 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.
We present comprehensive experimental and numerical studies of the subpicosecond switching dynamics of YBa2Cu3O7-x (YBCO) grain-boundary Josephson junctions, excited by single-picosecond electrical pulses. The test structures were patterned in 100-nm- thick YBCO films grown by pulsed laser deposition on (100) MgO bicrystal substrates. Each sample consisted of a coplanar strip (CPS) transmission line, a microbridge acting as the electrical pulse generator, and a single Josephson junction positioned between the CPS lines about 100 micrometer away from the bridge. The junctions were characterized by the nonhysteretic current-voltage characteristics with the characteristic voltage approximately equal to 2.0 mV at 20 K (temperature of our experiments). A train of 100- fs-wide optical pulses from a Ti:sapphire laser photo-excited the microbridge and generated 2-ps-wide electrical pulses, which were then applied to switch the junction. In addition to the input pulse, the junction was dc-biased at +0.7 Ic, -0.7 Ic, +1.5 Ic, -1.5 Ic, and zero-Ic, where Ic is junction critical current. Time-resolved dynamics of the junction response was studied with the help of our cryogenic electro-optic sampling system, which can be regarded as a sampling oscilloscope featuring < 200-fs time resolution and < 150-(mu) V voltage sensitivity. We obtained 0.7-ps-wide single-flux-quantum (SFQ) pulses generated as a result of the junction switching process. The measurements were compared to numerical computations based on the equivalent circuit containing a resistively shunted Josephson junction, and we have found a satisfactory agreement between our simulations and experimental data. We believe our findings provide experimental confirmation of the potential of YBCO Josephson gattes as building blocks of ultrafast (sub-THz) digital electronics.
The main goal of this work is to evaluate theoretically and experimentally the properties of ultrafast opto-superconducting interfaces used to trigger superconducting digital RSFQ circuits externally and to read the outcoming signal from the rapid single-flux-quantum (RSFQ) circuits. To do this, optoelectronics sampling techniques based on a Ti-Sa femtosecond pulsed laser are used. In a first step, miscellaneous Josephson junction-based devices (series of junctions, SQUIDS, simple RSFQ circuits) have been measured to evaluate the properties of the superconductive devices. In particular, I-V curves and magnetic diffraction patterns curves have been obtained. Since the picosecond pulses carried by the RSFQ circuits are weak, of the order of 1 mV or less, the interfaces with the external world have to be carefully designed. Therefore, the potentialities of different photoconductive switches for ultrafast and sensitive detection of picosecond pulses emitted by superconducting digital circuits are evaluated. To do so, a study of the influence of different electrical parameters associated to the geometry of photoswitches electrodes deposited on LT-GaAs materials is performed. The electrical response of dc-biased photoswitches illuminated by femtosecond pulses from a pulsed 800 nm Ti-Sa laser has been calculated for different geometries of electrodes. Using electro-optical sampling techniques, the response of some photoswitches are currently being measured.
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 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.