Superconducting tunnel junctions as quantitative detector of intense thermal phonon radiation

Thermal phonons are emitted from a pulse-heated constantan film into an a-cut sapphire (kept at 1.2 K) and are detected quantitatively by recording the current-voltage characteristic of the tunnel junction during the relatively strong phonon irradiation. These measurements are made with the maximum of the Planck distribution of the emitted phonons varied in the frequency range from 0.3 to 1.5 THz. These momentary diode characteristics coincide within experimental error withd c-characteristics at higher substrate temperatures. Assuming thermal equilibrium between electrons and phonons in the tin film and assuming the validity of the acoustic-mismatch model, a comparison can be made between observed phonon power and theoretically expected phonon power. Good agreement is achieved if the influence of the acoustic cut-off frequencies of the tin detector material is taken into account.Supported by Deutsche Forschungsgemeinschaft