OPTICAL-ABSORPTION BY QUASI-PARTICLES IN SUPERCONDUCTING MULTILAYERS

The absorption of photons by quasiparticles in the Bloch bands of mult ilayers, formed by alternating normal (N) and superconducting (S) regi ons, is computed for the vector of the oscillating electric field bein g perpendicular to the N-S interfaces. The absorption spectrum extends down to frequencies below the gap of the S-layers, and its structure results from the combined effects of Andreev scattering and convention al scattering at the N-S interfaces. For not too different Fermi energ ies epsilon(FS) and epsilon(FN) in the S and N layers there is one pro minent resonance peak. With increasing temperature low frequency absor ption goes up. For epsilon(FS) much greater than epsilon(FN), On the o ther hand, a sharp multipeak structure at low frequencies is found.