Ultrafast dynamics in solids

This dissertation reports the ultrafast dynamics of aluminum during the solid-toliquid phase transition of melting after excitation by an intense femtosecond laser pulse. Photoexcitation with intense femtosecond laser pulses is known to create a novel melting mechanism called non-thermal melting. This mechanism has been observed repeatedly in semiconductors, but not yet in metals. We investigate the melting mechanism of aluminum by monitoring the reflectivity response following excitation by an intense laser pulse. We employ an optical pumpprobe technique designed to measure broadband reflectivity across the visible spectrum with femtosecond time resolution. A non-thermal melting mechanism was proposed for aluminum by optical experiments that demonstrated transition of the optical properties from solid to liquid values within 500 fs after phototexcitation. This result was later challenged by electron diffraction experiments, which showed that the lattice loses long range order within 3.5 ps during photoinduced melting. This time scale implies conventional thermal melting. We find that the broadband optical properties during the solid-to-liquid phase transition in aluminum agree with the results obtained by the electron diffraction experiments. The transition of the broadband reflectivity from solid to liquid values is complete within 1.5 2 ps in our experiments, which is compatible with thermal melting. We dont observe time scales on the order of 500 fs. All the experimental evidence in this dissertation lead to the conclusion that the laser-induced, solid-to-liquid phase transition in aluminum is a thermal process.

We present femtosecond time-resolved measurements of the dielectric tensor of tellurium under single and double pulse excitation. We demonstrate the ability to both enhance and cancel coherent lattice vibrations for large lattice shifts under near-damage threshold excitation. The excitation conditions for which cancellation is achieved in tellurium reveal a departure from the low-excitation strength behavior of similar materials.