Abstract

We report a new ultrafast technique to investigate exciton dephasing in semiconductors based on reflective probing of pump-induced transient linear birefringence. The samples for this demonstration are 001-grown GaAs/AlGaAs quantum wells excited along the growth axis (z) at the n=1 heavy-hole exciton by linearly- (x-) polarised 2ps pump pulses. This generates equal coherent populations of the two degenerate exciton spin states (J_z=±1) which are σ⁺- and σ⁻-allowed, and so gixes a linear dielectric polarisation. P_x and associated linear birefringence n_x-n_y ∝ :P_x². The latter decays in lime T₂/2, as the excitons dephase, and is detected through elliptisation, on reflection, of a weak delayed probe pulse, linearly polarised at 45° to the pump polarisation. The pulses are derived from a mode-locked Ti-sapphire laser with pump excitation density ~7.10⁸ cm⁻² and polarisation analysis of the reflected probe. Rotating the pump polarisation by 90° (see fig.1) interchanges x and y and inverts the signal confirming that the signals are associated with exciton coherence. Temperature-, wavelength- and excitation density-dependence of the decays are consistent with studies by other methods. A magnetic field removes the exciton spin degeneracy causing the pump-induced optic axis to rotate and beating in the probe elliptisation al the Zeeman frequency (fig.2). This gives the exciton g-factor and reveals a dramatic slow-down of dephasing with field due to supression of the spin-relaxation.

© 1996 IEEE