Observation of stimulated radiative emission from normal to superconducting metal junctions

Approximately 10−7 W of radiation at the energy-gap frequency of Ta (3 mm wavelength at 4.26 °K) has been observed traveling through free space from specially formed Ta–Al point contacts. This radiation appears discontinuously at a threshold current density of about 105 A/cm2, disappears as the sample is warmed above Tc, and appears to be tunable by changing the energy gap with temperature. These results are consistent with our predictions of stimulated emission from normal–superconductor contacts.