Transient reflectivity response with negative time delay caused by femtosecond pulse propagation in GaAs thin films

Abstract

We have investigated the propagation effects on the transient response of confined excitons in GaAs thin films by a reflection‐type pump‐probe technique. The spectrally integrated signal demonstrated slower rise than the pulse width in the negative time region. Moreover, reflectivity change spectra indicate that the response in the negative time region originates from the effect of the confined excitons and that the signal rise time at each exciton state is different. The propagation velocities estimated from the rise times and the film thickness correspond to the group velocities of the exciton polariton in bulk crystals. Therefore, the origin of the responses in the negative time region is the pulse modulation caused by the propagation effect of exciton polariton. In addition, it was confirmed that this propagation disappears with an increase in temperature due to the spatial decoherence of excitons. These results demonstrate that the propagation effect of exciton polariton on optical response in femtosecond time scale has a considerable influence in the realization of ultrafast optical devices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Propagation velocity of excitonic polaritons confined in GaAs thin films

“…For optical control in the exciton region, the characteristics of excitonic polaritons in the transient signals should be clarified. In the transient-grating and pump-probe signals under exciton resonant conditions, a slow rise much longer than the pulse width was observed in the negative time region [12,13]. The propagation velocity estimated from the film thickness and the rise time τ Rise almost agrees with the group velocity of the exciton-polariton in a bulk GaAs crystal [13].…”

“…In the transient-grating and pump-probe signals under exciton resonant conditions, a slow rise much longer than the pulse width was observed in the negative time region [12,13]. The propagation velocity estimated from the film thickness and the rise time τ Rise almost agrees with the group velocity of the exciton-polariton in a bulk GaAs crystal [13]. In this study, we investigated the dynamics of exciton-polaritons in GaAs thin films at various film thicknesses using a reflection-type pump-probe technique.…”

“…3. Here, though the spectral widths of pulses include the higher excitonic components, the lowest exciton will strongly contribute to the transient signal [13]. For reference, the group velocities calculated from the dispersions of HH-and LH-exciton polaritons in bulk GaAs crystal are depicted as solid and dashed curves, respectively.…”

“…the maximum intensity. All samples have a reflectivity change in the negative time region, which originates from the propagation effects of the exciton-polaritons [13]. τ rise in each signal estimated by a single exponential function is plotted as a function of d in the inset of Fig.…”

Dephasing of Excitonic Polaritons Confined in GaAs Thin Films

“…In the transientgrating and pump-probe signals under exciton resonant conditions, a slow rise of the signals much longer than the pulse width was observed in the negative-time region. [17][18][19] The propagation velocity estimated from the film thickness and the rise time Rise almost agrees with the group velocity of excitonic polaritons in a bulk GaAs crystal. 18,19) In this study, we investigated the dynamics of excitonpolaritons in GaAs thin films at various temperatures using reflection-type pump-probe techniques.…”