The use of capacitive-mesh output couplers for optically pumped far-infrared molecular lasers has been extended throughout the far-infrared spectrum, between 42 μm and 1.2 mm, and the optimum grid constants have been found for several lines. At shorter wavelengths, performance has been improved by the use of a novel hybrid capacitive-mesh hole output coupler.

The high‐frequency behavior of niobium cat‐whisker point contacts has been studied using radiation from an optically pumped far‐infrared laser. When the point contacts are classified on the basis of their high‐frequency performance, their dc I‐V curves fall into recognizable groups. We find that the ac Josephson effect has a strong correlation with the gap‐related structure on the I‐V curve, but none at all with the apparent excess current observed in all the contacts. For high‐performance junctions, these and other features of the I‐V curves are very reproducible from contact to contact, allowing a comparison with the available theories. The experimental evidence seems to suggest that our point contacts are best modeled as extremely small metallic constrictions.

We have measured the far-infrared frequency dependence of the strength of the ac Josephson effect in Nb cat-whisker point contacts, which have consistent and reproducible behavior and minimal extrinsic high-frequency limitations due to capacitance and heating. We monitor the constant-voltage Josephson steps induced on the dc I−V curves by an optically pumped far-infrared laser at fundamental frequencies corresponding to voltages from ∼0.2 to ∼2 times the energy-gap voltage. At all the frequencies studied, we find that the shape of the power dependence of the step amplitudes is fit reasonably well by Werthamer's frequency-dependent theory for tunnel junctions in the voltage-bias approximation. However, the observed magnitude of the steps is considerably less than predicted by the theory. By fitting to the I−V curves of the steps, we find that some of this discrepancy can be accounted for by heating-enhanced noise rounding. The remaining discrepancies (of the order of a factor of 2) are attributed to departures from a voltage bias at low frequencies and, tentatively, to the effects of the Ginzburg-Landau relaxation time at higher voltages. Our data confirm the expected intrinsic roll off of the strength of the ac Josephson effect above the energy gap.

We have studied niobium point contacts which are very consistent and reproducible from junction to junction, in both their dc and high-frequency behavior. We find a strong correlation between the sharpness of the gap structure and the ac Josephson effect, and we present the first quantitative measurements of the far-infrared frequency dependence of the Josephson effect above the energy gap. The measured I−V curves are also compared with available theoretical models.

The far-infrared (FIR) properties of niobium cat-whisker point contacts were studied using radiation from an optically pumped FIR laser. The reproducible behaviour of junctions with excellent high-frequency performance allowed a measurement of the FIR frequency dependence of the strength of the a.c. Josephson effect. The shape of the laser-induced steps was used to measure the effective noise temperatures, which increase with bias voltage in agreement with a heating model of metallic constrictions. The high-quality junctions were tested as frequency-selective, incoherent FIR detectors, with the d.c. bias in the vicinity of the incipient laser step. The response was found to be linear in the laser power, and the best measured responsivity at 604 GHz was 2 × 105 V/W, while the best noise equivalent power was 10p−13 W/Hz, with a 450 Hz chopping frequency. The NEP is limited by the voltage noise in the junction, which was found to have an approximately  frequency dependence. The detector performance is degraded considerably at higher FIR frequencies. Also studied was the low-laser-power behaviour of the I–V curves near the critical current, which may be of importance for mixing applications with external local oscillators.