Optical absorption by quasiparticles in superconducting multilayers

The absorption of photons by quasiparticles in the Bloch bands of multilayers, formed by alternating normal ( N ) and superconducting ( S ) regions, is computed for the vector of the oscillating electric field being 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 conventional scattering at the N‐S interfaces. For not too different Fermi energies ϵ FS and ϵ FN in the S and N layers there is one prominent resonance peak. With increasing temperature low frequency absorption goes up. For ϵ FS ≫ ϵ FN , on the other hand, a sharp multipeak structure at low frequencies is found.