EXCITON GREENS-FUNCTION APPROACH TO OPTICAL-ABSORPTION IN A QUANTUM-WELL WITH AN APPLIED ELECTRIC-FIELD

An exciton Green's function is derived and used to calculate the polarization-dependent optical absorption in a semiconductor quantum well with an applied electric field. With use of the exciton (or Coulomb) Green's-function approach, the optical-absorption coefficient due to the bound and continuum states of excitons can be obtained simultaneously and this approach also takes into account the coupling between different subband pairs. This is in contrast with the conventional approach in which the 1s exciton bound state is calculated variationally and the continuum states are calculated simply using the Sommerfeld enhancement factor from the pure two-dimensional case without the correct quantum size effect. Also, the coupling between different subband pairs is usually neglected. We compare the numerical results of the Green's-function method with those of the commonly used variational method and find that the variational method overestimates the oscillator strength by 20% for the 1s bound state and by 50% for the continuum, although the 1s bound-state energy can be quite accurate. The numerical results using the exciton Green's function are compared with experimental data and found to be in very good agreement.