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A Far-Infrared Laser Study of Josephson Point Contacts.

David A. Weitz

Published May 1, 1978• 0 citations
OpenAlex

Abstract

The high-frequency behavior of niobium cat-whisker point contacts is studied using radiation from an optically pumped far-infrared (FIR) laser. The FIR molecular laser had a dielectric-waveguide cavity and was pumped by a CO2 laser. The use of capacitive-mesh output couplers is extended throughout the FIR spectrum, between 42 micrometers and 1.2 mm, and have found the optimum grid constants for several lines. At shorter wavelengths, the laser performance was improved by the use of a novel hybrid capacitive-mesh hole output coupler. The coupling of the FIR radiation to the point contact is found to depend in a simple manner on the resistance of the contact. The FIR laser radiation induces constant-voltage steps on the dc I-V curves of our junctions, allowing us to study the high-frequency ac Josephson effect.

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Journal of Applied Physics•Dec 1, 1983•4 citations
The high-frequency behavior of niobium cat-whisker point contacts is studied using radiation from an optically pumped far-infrared (FIR) laser. The FIR molecular laser had a dielectric-waveguide cavity and was pumped by a CO2 laser. The use of capacitive-mesh output couplers is extended throughout the FIR spectrum, between 42 micrometers and 1.2 mm, and have found the optimum grid constants for several lines. At shorter wavelengths, the laser performance was improved by the use of a novel hybrid capacitive-mesh hole output coupler. The coupling of the FIR radiation to the point contact is found to depend in a simple manner on the resistance of the contact. The FIR laser radiation induces constant-voltage steps on the dc I-V curves of our junctions, allowing us to study the high-frequency ac Josephson effect.
Point-contact-type Josephson junctions have been developed as a harmonic mixer to measure far-infrared (FIR) laser frequencies in a frequency chain up to the infrared region. The junctions were formed by mechanically pressing a niobium wire of 70 μm in diameter with a sharply etched tip against a niobium post of 2 mm in diameter. The voltage-current characteristics of the mixers were investigated under the irradiation of FIR lights from an optically pumped methyl-alcohol laser. The Shapiro steps for FIR lights up to 3.1 THz were observed. The mixing properties of the mixers were studied through harmonic mixing experiments of 50-GHz microwaves and the FIR lights up to the frequency of 3.1 THz. The conversion efficiency of the beat signals to the incident FIR power was found to vary as the — 4th power of the FIR frequency. From the mixing properties, the combination of the critical current Icof the junction and the product IcRnwhere Rnis the normal resistance of the junction, was found important to optimize the mixing performance.
We have measured the response of Nb-aSi-Nb junctions to 604 GHz radiation from an optically pumped far-infrared (FIR) laser source. These rugged and thermally cyclable junctions situated at the midpoint of 180 μm long dipole antennas, were fabricated on oxidized silicon wafers using the Selective Niobium Anodization Process (SNAP). Josephson current densities were\sim 9,000A/cm2, the McCumber parameters βcwere ∼3.5, and the Josephson plasma frequencies\omega_{J}\sim2.5\times10^{12}. On a junction with a normal state resistance of\sim7\Omega, we observed three Josephson steps and one photon-assisted tunneling step. The widths of the Josephson steps were studied as a function of the laser power. An RSJ model computer simulation with a nonlinear quasiparticle conductance and an rf current bias (assumed because of the low junction resistance) is able to account reasonably well for the laser-power dependence of the critical current (zeroth step) and the second step. However, the temperature dependence is more complex, and a discrepancy still exists between the RSJ model calculation and the data for the hysteretic first step.