Transition from piezoelectric to deformation potential mechanism of hypersound photogeneration in n-doped GaAs semiconductors

Abstract

Both experiments with deeply penetrating femtosecond laser pulses and theoretical analysis demonstrate that at low laser fluences on (111) and (1−1−1−) surfaces of n-doped GaAs semiconductors the hypersound generation mechanism is the inverse piezoelectric effect. The transient electric field causing the inverse piezoelectric effect is due to the spatial separation in the built-in near-surface electric field of the electrons and holes photoexcited directly in the depletion region and also of those photoexcited outside the depletion region and diffusing toward it. However, with increasing laser fluence the amplitude of the acoustic signal generated by laser-induced transient electric fields saturates and the hypersound generation through electron–hole–phonon deformation potential mechanism becomes predominant. The peculiar dependencies of the hypersound amplitude and phase on pump laser fluence reveal the transition between the two physical mechanisms of optoacoustic conversion. The phase of the acoustic signal contains information on the temporal dynamics of the screening of the built-in electric field.