Picosecond acoustics in polythiophene thin films

Abstract

The picosecond correlation technique has been used to study the generation and detection of ultrasonic strain waves in polythiophene thin films, in the range of 0.3--19 GHz. We show that the strain generation is mainly due to thermal expansion following energy relaxation associated with exciton formation and that the spectral dependence of the strain response is proportional to \ensuremath{\partial}\ensuremath{\alpha}(\ensuremath{\omega})/\ensuremath{\partial}\ensuremath{\omega}, the first partial derivative of \ensuremath{\alpha}(\ensuremath{\omega}). We found a linear dispersion relation for the LA-phonon branch in the measured frequency range, with a room-temperature sound velocity of 28 \AA{}/ps; it stiffens by 10% at 80 K.