Bragg-enhanced time-domain Brillouin scattering from a propagating acoustic grating
Generation and detection of coherent phonons in semiconductors by femtosecond optical pulses is a powerful tool for high-frequency acoustic control of their electronic properties. Here, we demonstrate a propagating one-dimensional acoustic grating in bulk semiconductors using above-band-gap excitation by a train of laser pulses with high repetition rate of 1 GHz. This approach enables shaping of the coherent acoustic phonon spectrum and leads to a significant enhancement of Brillouin light scattering at selected probe wavelengths in pump-probe configuration. We demonstrate this effect at a cryogenic temperature of 5 K in prototypical semiconductor systems, namely bulk crystalline GaAs and (Cd,Zn)Te, which serve as benchmark materials for the proposed method. The spectral dependence of the time-domain Brillouin light scattering amplitude exhibits resonant peaks at discrete probe wavelengths arising from Bragg reflection of the probe light by the propagating acoustic grating. A 10-30-fold resonant enhancement of the signal amplitude is observed for GaAs and (Cd,Zn)Te, determined by the finite spectral width of the probe pulse. With further spectral narrowing, enhancements of the order ~ 100-150 are expected, set by the number N of strain pulses in the acoustic grating within the sample and ultimately limited by the material parameters and sample thickness.