Physical review. B, Condensed matter
Coherent optics for pumping near the absorption edge
Stephan Glutsch, Ralf Zimmermann
Abstract
The optical properties of a semiconductor pumped near the 1s exciton resonance are studied. The equation of motion for the density matrix in the Hartree-Fock approximation is traced back to the Bloch equation, well known from two-level systems. The essential difference, however, is the coupling of different wave vectors via the Coulomb potential. Principal features of the different stationary solutions are discussed. For a closer inspection of stability, we calculate the response to a smooth switchon of the pump field. For pump frequencies above the 1s exciton, the system follows first adiabatically the pulse shape, showing a blueshift and a slight increase of the oscillator strength as in the off-resonant case. The system jumps later into a second solution characterized by optical gain below the pump frequency, related to complete saturation (excitonic insulator). The major objective of our paper is to provide a numerical solution to the equation of motion in the Hartree-Fock approximation and to compare the results to those obtained by stationary considerations.