Optical generation of high-frequency acoustic waves in GaAs/Al<i>x</i>Ga1−<i>x</i>As periodic multilayer structures

Abstract

A picosecond study of ultrahigh-frequency acoustic phonons in specifically engineered GaAs/AlxGa1−xAs periodic multilayer structures is presented. The lattice-matched boundary conditions for photothermal acoustic generation and optical properties of these materials make these structures ideal for sound-wave generation in the 100 GHz to THz range. The acoustics are generated using ultrashort-laser-pulse excitation and detected in real time by measuring the strain-induced change in reflectivity with the pump-probe technique. By using 12 nJ, 90 fs pulses from a Ti:sapphire laser source, the generation and detection of ∼50 GHz acoustics in a 6-bilayer, [001]-oriented GaAs/Al0.4Ga0.6As structure, 500 Å thickness per layer, on a GaAs substrate, are successfully demonstrated. The structure was specifically designed to give the maximum sensitivity to the acoustics through étalon-induced modulations in the reflectivity spectrum. With similarly designed multilayer structures, the upper frequency limit can be achieved for the thermoelastic generation of coherent acoustic phonons, that is, ∼300 GHz in GaAs for ∼1 eV above band-gap-energy photons.