Polariton pulse propagation through GaAs: Excitation-dependent phase shifts
We report on amplitude and phase measurements of ultrashort laser pulses after propagation through a 3.8 {mu}m 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 propagation we find characteristic phase shifts in the transmitted optical field and investigate their dependence on excitation density. For low excitation [{lt}10{sup 13} electron-hole (e-h) pairs/cm{sup 3}] a phase jump of +{pi} occurs at each individual beat node. With increasing excitation (up to 10{sup 15} e-h pairs/cm{sup 3}) these jumps flip successively from +{pi} to {minus}{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 of the exciton lines. A thorough theoretical analysis of the edge spectrum based on semiconductor Bloch equations (SBE) shows that the asymmetries are due to many-body effects. The standard SBE treatment is extended by including dephasing and renormalization of the interband energies in both wave number and frequency. The frequency dependence of these many-body effects is due to non-Markovian memory effects in the scattering term of the SBE. We find very good agreement with experiment in line shape and phase shift behavior for a wide range of pair densities.
- Sponsoring Organization:
- (US)
- OSTI ID:
- 40203577
- Journal Information:
- Physical Review B, Journal Name: Physical Review B Journal Issue: 23 Vol. 63; ISSN 0163-1829
- Publisher:
- The American Physical Society
- Country of Publication:
- United States
- Language:
- English
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