Photoexcited elastic waves in free-standing GaAs films

Results of a theoretical and experimental investigation of the spatiotemporal extent of optically generated elastic waves in thin, free-standing, semiconductor films are presented. Our study, focused on symmetric S1 modes in the 10 + GHz frequency range, indicates that diffraction-limited pulsed laser sources can produce narrow-band elastic excitations with in-plane group velocities sufficient for multi-micrometer-scale lateral propagation at room temperature in GaAs. The theory predicts that pumping GaAs with sub-ps, near-diffraction-limited, optical pulses excites cylindrically spreading S1 vibration because of the Dirac conelike dispersion of the S1 mode at small wave numbers. Theoretical predictions of Lamb wave propagation for optically excited 190-nm-thick GaAs suspended plates are found to agree with experimental data acquired using a near-infrared, ultrafast, pump-probe technique.