Intense Tunable Phonon Fluorescence in Superconductors

The "phonon fluorescence" spectrum of superconducting Sn and ${\mathrm{Pb}}_{0.5}$${\mathrm{Tl}}_{0.5}$ films pumped by a heat pulse is studied through the observation of resonance absorption by Sb donor levels in uniaxially compressed Ge. The spectrum consists primarily of a narrow band of phonons centered about the energy gap $2\Delta$ (1.2 meV for Sn, 1.7 meV for ${\mathrm{Pb}}_{0.5}$${\mathrm{Tl}}_{0.5}$). The power generated in this narrow band is at least hundreds of milliwatts. In the case of Sn the phonon energy has been tuned from 1.2 meV down to 0.7 meV by a magnetic field parallel to the plane of the film.