Spectrally resolved nonlinear optical response of weakly confined excitons under femtosecond laser pulse excitation in GaAs thin films

Abstract

We have investigated the dynamical properties of the weakly confined excitons in GaAs thin films by a transient grating method (three-beam four-wave mixing) with the use of femtosecond-laser pulses. The time-domain signals under the exciton-excitation conditions show a nonlinear optical response with a rise time of 4 ps and a long decay time of 120 ps. Moreover, in the case of that the probe pulse is parallel polarized to the two excitation pulses, the ultrafast optical response comparable to the pulse width appears. These characteristics of the nonlinear optical response is different from the experimental results by the picosecond-laser-pulse excitation. The spectra of the transient grating signals at the various time delays show several peaks at the energies, which agree with the energies of excitons confined in the thin films. This fact demonstrates that the characteristics of the nonlinear optical response of the weakly confined excitons are drastically changed by the spectral width of the excitation laser-pulse.1 Introduction Dynamical properties and optical nonlinearity of excitons in semiconductor bulk crystals, multiple quantum wells, and quantum dots have been extensively studied from the viewpoints of physics of ultrafast phenomena and application to ultrafast optical devices [1,2]. Recently, in the weak confinement regime, it was reported that optical nonlinearity and fast response of excitons due to the interaction with the radiation field are remarkably enhanced by the double resonance in energy and size [3][4][5]. However, to our knowledge, many experiments for the nonlinear optical response of the weakly confined excitons have been performed by using laser pulses with a width of a few picoseconds and narrow spectrum width which resonantly excites a single exciton state.If femtosecond-laser pulses with a broader spectral width are used to generate nonlinear optical response of weakly confined excitons, multiple exciton states which are energetically close each other are excited. Thus, we may reveal the effect of the spectral width of the excitation pulse on the nonlinear optical response of the weakly confined excitons by comparing with the results of picosecond-laser pulse-excitations. In the present work, we have investigated the nonlinear optical response of the weakly confined excitons in GaAs thin films by using a transient grating method with 160-fs pulse-duration for the first time. We have found that the transient grating signals of the GaAs thin films under the femtosecond-pulse-excitation conditions indicate a rise time of 4 ps and a decay time of 120 ps. In addition, the ultrafast optical response comparable to the pulse width is observed when all of the pulses of the excitation and probe are copolarized. The origin of the nonlinear optical response is discussed from an aspect of the spectra of the transient grating signals.

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].…”

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

“…The transient signal lasts over 100 ps. The PL Intensity (arb.units) [15,16]. As shown in the inset of Fig.…”

Spectral width dependence of residual carrier effect on nonlinear optical response of weakly confined excitons

“…Figure 2 shows the effect of the additional pulse on the DFWM signal of the weakly confined excitons at ∆E=16 meV. The DFWM signals originate from the optical nonlinearity of the excitons confined in the thin films [5,7]. The dotted curve is the standard signal without the additional pulse.…”

“…The Al 0.3 Ga 0.7 As barrier layer has enough thickness to confine the excitons in the GaAs thin film [5,7]. The nonlinear optical response was measured using a DFWM technique at 3.5 K in the backward direction of 2k 1 − k 2 .…”