Fano interference of collective excitations in semiconductor quantum wells and lasing without inversion

Absorption cancellation via tunneling induced Fano interference in semiconductor quantum wells is studied in the presence of the Coulomb interaction between electrons. For a small subband dispersion, gain or loss is determined by single-electron Fano interference. For a large subband dispersion, collective excitations dominate the absorption spectrum and are crucial for the observability of tunneling induced transparency, which exists in spite of subband dispersion. Pumping destroys collective excitations; therefore gain without inversion is possible only for small subband dispersion.