Anisotropy of plasmon-phonon coupling under intense optical excitation of GaAs.

The dependence of coherent optical processes in time-frequency domain on the polarization of pump and probe beams can shed light on different generation and detection mechanisms. Here, we report the response of (100) oriented n-doped GaAs (n$_{d}$=2x10$^{18 }$cm$^{-3})$ when excited to an e-h pair density n$_{exc}\sim $10$^{18}$-10$^{20}$ cm$^{-3}$ with a 10 fs laser pulse centered at 400 nm. Experiments are performed in the transient reflectivity and reflective electro-optic sampling geometries with various pump and probe orientations. Time domain signal showing plasmon-phonon oscillations has a weak pump polarization dependence indicating isotropic generation mechanism and strong probe polarization dependence revealing symmetry properties of various carrier-phonon interaction mechanisms. Fourier Transform analysis of the time domain signals at different probe orientations show both plasmon and phonon anisotropy. Results are discussed considering different possible carrier-phonon interaction mechanisms with different symmetry dependences. Frequency evolution of plasmon-phonon coupled mode with increasing photoexcited carrier density is consistent with the hole-phonon coupling in the high damping regime.