Polariton pulse propagation through GaAs: Excitation-dependent phase shifts - art. no. 235202

We report on amplitude and phase measurements of ultrashort laser pulses af ter propagation through a 3.8 mum thick GaAs platelet at 2 K. The incident center frequency of the 40 fs, 1.525 eV pulses was tuned slightly above the fundamental gap energy. Apart from intensity beats due to polariton propag ation we find characteristic phase shifts in the transmitted optical field and investigate their dependence on excitation density. For low excitation [<10(13) electron-hole (e-h) pairs/cm(3)] a phase jump of +<pi> occurs at e ach individual beat node. With increasing excitation (up to 10(15) e-h pair s/cm(3)) these jumps flip successively from +pi to -pi. A simple Lorentzian oscillator model for the dielectric function fails to describe this effect , which is shown to be caused by density-dependent asymmetries and shifts o f the exciton lines. A thorough theoretical analysis of the edge spectrum b ased on semiconductor Bloch equations (SBE) shows that the asymmetries are due to many-body effects. The standard SBE treatment is extended by includi ng dephasing and renormalization of the interband energies in both wave num ber and frequency. The frequency dependence of these many-body effects is d ue to non-Markovian memory effects in the scattering term of the SEE. We fi nd very good agreement with experiment in line shape and phase shift behavi or for a wide range of pair densities.