Ultrafast generation of acoustic waves in copper

摘要

The ultrafast generation of acoustic waves in copper films isninvestigated with a femtosecond optical pump and probe technique. Bynstudying the generation at times before the electrons and the latticencome into equilibrium, the strength of their interaction can be measurednand the dynamics of ultrafast electron diffusion can be studied. Thenacoustic strain pulses observed are bipolar in shape with exponentialntails that are much broader than expected from simple thermoelasticnstress generation. This can be explained by the supersonic diffusion ofnelectrons over distances larger than the optical skin depth. Thennonequilibrium diffusion equations governing stress generation arennonlinear, and are solved numerically. Using a linearized formulation,nwe also solve them analytically to a good approximation. The acousticnstrain profile provides a `snapshot' of the initial spatial temperaturendistribution of the lattice, thus allowing a sensitive probe of thennonequilibrium dynamics of the diffusion. The electron-phonon couplingnconstant can be estimated directly from the acoustic pulse duration,nprovided that the sound velocity and thermal conductivity are known. Inngeneral, the relaxation and diffusion of carriers is specific to thensample in question, whether metal or semiconductor, suggesting the usenof this method for thin film characterization