Figure 2

(a) Oscillatory components observed in the time reflectance variation of n-type GaAs semiconductors at each carrier density (\(n_{exe}\)). The horizontal axis shows the time of measurement, and the vertical axis shows the signal strength. The signals are plotted vertically as a function of the carrier density. (b) Fourier spectrum of the time-resolved reflectivity change at each carrier density. The horizontal axis indicates the frequency, and the vertical axis indicates the intensity. The Fourier spectra are plotted vertically as a function of the carrier density as in (a). The dashed lines of the LB and UB curves in show the upper branch on the high-frequency side and lower branch on the low-frequency side, respectively, of the LOPC mode as calculated from the phenomenological effective Hamiltonian. The thin vertical dashed lines represent the frequencies of LO phonons at 8.7Â THz.