Fano interference of collective excitations in semiconductor quantum wellsand lasing without inversion

Absorption cancellation via tunneling induced Fano interference in semicond uctor quantum wells is studied in the presence of the Coulomb interaction b etween electrons. For a small subband dispersion, gain or loss is determine d by single-electron Fano interference. For a large subband dispersion, col lective excitations dominate the absorption spectrum and are crucial for th e observability of tunneling induced transparency, which exists in spite of subband dispersion. Pumping destroys collective excitations; therefore gai n without inversion is possible only for small subband dispersion. [S0163-1 829(99)04820-1].