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Applied Physics Letters

Generation of coherent tunable Josephson radiation at microwave frequencies with narrowed linewidth

C. Varmazis, Robert D. Sandell, A. K. Jain, James E. Lukens

Published Aug 15, 1978• 41 citations
Publisher

Abstract

Coherent tunable microwave radiation (2–18 GHz) has been observed from two microbridge Josephson junctions coupled by a shunt resistor. The coherent power (∼4×10−12 W) is nearly four times the power emitted by a single junction. The linewidth of the radiation in the coherent state is one-half that from the individual microbridges.
DOI
10.1063/1.90336

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Revue de Physique Appliquée•Jan 1, 1974•4 citations
Josephson radiation emitted from superconducting microbridges and coupled two-bridge junctions has been investigated at microwave frequencies. The radiation linewidth Ɗf and total radiation power P are found to vary with the output coupling controlled by a tuning stub in the cavity, indicating feedback-induced effects due to the radiation fields. We have found (Ɗf)min ~ 900 kHz, Pmax ~ 0.9 x 10-12 W for the single microbridge and (Ɗf)min ~ 60 kHz, Pmax ~ 5 x 10-12 W for the coupled bridges. The enhancement of radiated power and line-narrowing in the coupled bridges are attributed to coherent behaviour of the junctions.
A major factor in the practical utilization of any Josephson device for microwave and higher frequency applications (such as the generation and detection of electromagnetic radiation) is the success with which the junction device can be coupled to an external transmission line. As part of an experimental investigation of the electrodynamics of small arrays of Josephson tunnel junctions, the properties of the coherent radiation emitted both by individual junctions and by coupled junction systems have been studied at frequencies between 2 and 12 GHz. Near 9 GHz, more than 10-9 W of coherent radiation has been detected from a single Pb-Pb oxide-Pb junction coupled via waveguide. The development of thin film devices in which the tunnel junctions form part of a microstrip line have made it feasible to observe directly (in a single device) the radiation emitted both at the fundamental geometrical frequency and at the low order harmonics. The observed temperature dependence of the junction cavity Q indicate the low temperature losses are primarily due to cavity loading of the external transmission line.
We report the strong microwave Josephson radiation from an array of high-T/sub c/ junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUID's geometry which guaranteed the oscillation of all junctions at the same frequency. The phase differences between adjacent junctions with different I/sub c/R/sub n/ products could be controlled by an external small magnetic field. For a five-junctions array, we observed the coherent output power of about 13 pW without external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the self-radiation in the coherent state to be less than 1 GHz by adjustment of the external flux.
Although a number of investigators have generated microwave radiation by means of properly biased Josephson junctions, little work has been reported on similar applications to the submillimeter wave region where tunable coherent oscillators are not yet available. We have generated continuously tunable millimeter and submillimeter wave radiation with a point contact Josephson junction in a superconducting transmission line. This transmission line is centered in an oversized multimode brass cavity. With a broad band detector placed in the brass cavity near the transmission line radiation is detected for all bias voltages below 2 mV. The maximum power actually absorbed by the detector is about 2 × 10-10 W. In order to analyze the frequency content of the radiation we have used a lamellar interferometer together with Fourier transform spectroscopic techniques. The amount of power detected in the remote detector cryostat is two orders of magnitude smaller than that detected adjacent to the junction. Only at bias voltages where a particularly large amount of radiation is emitted are spectroscopic measurements possible. For these cases the frequency of the radiation is that predicted from the bias voltage and the Josephson relation. The linewidth is less than the resolution of the interferometer. To see if the radiation is continuously tunable a low temperature spectrometer has been constructed. The instrument consists of a germanium bolometer cooled to 0.3 K by a He3 charcoal adsorption pump, a four position sample rotator at 1.2 K and a Josephson junction transmission line source. By sweeping the bias voltage from zero to 2 mV absorption line spectra have been recorded in three different crystals, they are : 1. the antiferromagnetic resonance at 8.7 cm-1 in MnF2 ; 2. the impurity mode absorption at 14.3 cm-1 in CsI : Tl and 3. the impurity mode absorption at 18.0 cm-1 in KBr : Li. A low temperature double beam spectrometer is now under construction.