Time-resolved gigahertz acoustic wave imaging at arbitrary frequencies

We describe a way to generate and detect arbitrary frequency components in time-resolved surface acoustic wave imaging based on optical pumping and probing with a periodic light source. The detailed theory of the technique, based on beam modulation and Fourier analysis, for a variety of possible experimental configurations is presented, followed by experimental data for a glass substrate covered with a thin gold film. We show how the acoustic dispersion relation can be obtained to arbitrary frequency resolution, not limited by the laser pulse repetition rate.