OPTICAL-GENERATION OF HIGH-FREQUENCY ACOUSTIC-WAVES IN GAAS/ALXGA1-XAS PERIODIC MULTILAYER STRUCTURES

A picosecond study of ultrahigh-frequency acoustic phonons in sperifically 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 approximately 50 GHz acoustics in a 6-bilayer, [001]-oriented GaAs/Al0.4Ga0.6As structure, 500 angstrom thickness per layer, on a GaAs substrate, are successfully demonstrated. The structure was specifically designed to give the maximum sensitivity to the acoustics through etalon-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, approximately 300 GHz in GaAs for approximately 1 eV above band-gap-energy photons.