In bulk direct-band-gap semiconductors the ground state of a (photoexcited) electron-hole system is normally an exciton gas, instead of an electron-hole plasma, as long as the carrier density is not too large. We demonstrate theoretically that, in the presence of an electric-field modulation, the energy of the plasma is decreased due to the induced charge-density fluctuation. As a results, the plasma becomes the ground state at much lower carrier densities, implying that the critical carrier density for the transition between these two states can be reduced. This transition provides a mechanism of nonlinear optics for bulk direct-band-gap semiconductors with picosecond relaxation time, large nonlinearity, and low pump intensity.

• Received 24 November 1997

DOI:https://doi.org/10.1103/PhysRevB.57.12890