Wavelength Dependent Coherent Phonon Excitation near the E0 + Δ0 Critical Point in GaAs[001]

I present a study of the wavelength dependence coherent phonon response of n and p-doped GaAs around its E0 + Δ0 critical point. When the excitation energy is above the fundamental band gap of GaAs (1.42 eV), excited charge carriers generate e-h plasma in the lattice, that can couple with the coherent longitudinal-optical (LO) phonons resulting in plasmon-phonon coupled modes. This response of the lattice and coupling with the charged carriers is observed using a pump-probe transient electro-optical reflectivity spectroscopic technique. I use a non-collinear optical parametric amplifier (NOPA) as a highly tunable excitation laser source with ~ 20 fs pulses for the transient reflectivity experiments. The femtosecond resolution makes it possible to follow the time evolution of the LO phonon and the plasmon-phonon coupled modes. I focus on the carrier-lattice response when excited across the E0 + Δ0 critical point (1.76 eV) in its band structure. Experiments are done using a wavelength range of 680 - 740nm (1.68eV – 1.83 eV). This wavelength range not only excites across the spilt-off band of GaAs but also allows the possibility of intervalley scattering from Γ valley into the L valley. As the scattering into the L valley becomes possible, a decrease in the phonon response is observed. An increase in phonon amplitude is seen when excitation energy is above split-off band gap. As excitation energy is further increased, a decrease in phonon amplitude can be seen again as scattering into L valley becomes more favorable. To better understand the experimental trends simulations were done using the basic plasmon-phonon coupling equations. The frequency and amplitude of the plasmon-phonon coupled modes depends on the carrier mass and density. With the increase in carrier density the damping rate of the coupled modes initially goes up before eventually decreasing. This point of inflection can shift with change in carrier mass or plasmon frequency, thus being different for carriers in different valleys or bands. From the experimental data and using the insight gained from the simulations I present a qualitative picture for the wavelength dependent results.