Coupling of excitons with excitations of the Fermi sea in asymmetric quantum wells

In asymmetric quantum wells, optical transitions couple a hole state with several conduction-band states. The coupling of excitons associated with higher subbands and free electrons in the lowest subband are investigated using a two-band Mahan--Nozi\`eres--De Dominicis Hamiltonian. The emission spectrum for carriers in the lowest subband shows a very weak Fermi edge singularity at the Fermi level. When the Fermi level approaches the second subband, coupling of excitons with excitations of the Fermi sea leads to a large enhancement of the emission spectrum at the Fermi level.