The stationary electronic state of an electron-hole system driven by a strong monochromatic light source is studied within a mean-field approach. The problem can be formulated in terms of a classical static source dressed by a nonlinear Schrödinger field of N electron-hole pairs. The linearized form reduces to the Coulomb-Schrödinger field describing the internal motion of pairs with zero total momentum. While the respective homogeneous field equation has solutions for discrete energies only—the stationary bound states—the linearized field with source exhibits resonances at these points, but allows for solutions at any energy. These off-resonance states, for which N goes to zero with the source strength, may be characterized as a cloud of virtual excitons dressing the source. However, due to interactions between identical fermions, the solution changes its character evoking real exciton branches for all excitation energies above the exciton ground state. In this range approximations should be taken with care, as they might sensitively deform basic features of the solution. The possibility of a novel hysteresis effect is discussed.

• Received 14 March 1988

DOI:https://doi.org/10.1103/PhysRevB.38.10517