Optical processes of 2D electron plasma in GaAs-(AlGa)As heterostructures.

We observe the characteristic behavior of a 2D electron plasma in optical emission and absorption across the energy gaps in modulation doped GaAs-(AlGa)As quantum-well heterostructures. A renormalization of the energy gap with magnitude comparable to the Hartree energy is found in emission spectra. Large breakdown of the parity selection rule of the optical matrix element in quantum-wells is also observed. In absorption there is evidence of final-state electron-hole interactions even at plasma densities n>~4x10(11)cm(-2).