Tunable far infrared radiation from Josephson junctions

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