Ultrashort shear acoustic pulse excitation via laser-induced electrostrictive effect (abstract)

Abstract

In principle there are several opportunities to achieve excitation of ultrashort shear pulses by laser radiation. One of them is to use the anisotropy of material properties such as elasticity and thermal expansion (in the case of the thermoelastic mechanism of the optoacoustic transformation) or the electron–phonon deformation potential (in the case of sound excitation via electron–hole plasma photogeneration). Another is related to application of the laser-induced inverse (converse) piezoelectric effect. Laser-generated electron–hole plasma creates an electric field due to the electron–hole spatial separation (Dember field) or can screen an electric field (built-in or induced externally) pre-exsisting near the crystal surface. In piezoelectric materials the induced ultrafast transients in electric field distribution in the proximity of the crystal surface should cause excitation of ultrashort acoustic pulses of various modes. Experimentally achieved excitation of plane TA pulses in piezoelectric crystals by nanosecond laser pulses, and also excitation of both optical phonons and ultrashort electromagnetic pulses (THz radiation) by femtosecond laser pulses all support the possibility of exciting ultrashort TA, as well as LA, pulses. The idea of using the inverse piezoelectric effect is also supported by recent observations of extremely high efficiency of quasi-monochromatic LA coherent oscillations generation by laser-induced screening of built-in electric fields in strained multiple quantum wells of piezoelectric semiconductors. We analyze one more opportunity for ultrashort TA pulse excitation in optoacoustic transformation via the action of laser-induced electrostrictive stress on surface of optically transparent crystals. A simple theory is developed and numerical estimates are presented to demonstrate the possibility of exciting ultrashort acoustic pulses in optically transparent materials by laser-induced electrostrictive surface mechanical stresses. Both longitudinal and shear coherent acoustic pulses can be excited. The conditions for which the polarization of ultrashort shear pulses is controlled by the polarization of an incident laser pulse are established. The profiles and duration of longitudinal and shear acoustic strain pulses are shown to coincide with laser pulse intensity envelope. Picosecond shear pulses might be very useful in diverse applications related to measurements of shear rigidity or shear viscosity, including ultrafast solid–liquid phase transitions, ultrafast tribology, and diagnostics of liquids in confined geometry.