Effect of virtual screening on the absorption spectrum of a coherently pumped semiconductor

The dielectric function of a nonresonantly pumped semiconductor is calculated in the generalized random-phase approximation to second order in the pump field. The effect of this ‘‘virtual screening’’ on the exciton ground-state energy E₁ and on its oscillator strength f₁ is then numerically computed and compared with the Stark shift of E₁ and f₁ derived before without screening in the Hartree-Fock approximation. It is found that for a detuning Δ=(E_g-ω_p)/E⁰ of the order of 10 (E_g is the semiconductor band-gap energy, ω_p is the angular frequency of the pump field, and E⁰ is the exciton binding energy) the modifications due to screening are small in a three-dimensional (3D) system, but are of the same order of magnitude as the Hartree-Fock corrections in a 2D system and cause the oscillator strength to decrease slightly with increasing pump intensities.