Ultrafast generation of acoustic waves in copper

The ultrafast generation of acoustic waves in copper films is investigated with a femtosecond optical pump and probe technique. By studying the generation at times before the electrons and the lattice come into equilibrium, the strength of their interaction can be measured and the dynamics of ultrafast electron diffusion can be studied. The acoustic strain pulses observed are bipolar in shape with exponential tails that are much broader than expected from simple thermoelastic stress generation. This can be explained by the supersonic diffusion of electrons over distances larger than the optical skin depth. The nonequilibrium diffusion equations governing stress generation are nonlinear, and are solved numerically. Using a linearized formulation, we also solve them analytically to a good approximation. The acoustic strain profile provides a 'snapshot' of the initial spatial temperature distribution of the lattice, thus allowing a sensitive probe of the nonequilibrium dynamics of the diffusion. The electron-phonon coupling constant can be estimated directly from the acoustic pulse duration, provided that the sound velocity and thermal conductivity are known. In general, the relaxation and diffusion of carriers is specific to the sample in question, whether metal or semiconductor, suggesting the use of this method for thin film characterization.<
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