Ultrafast electronic and structural dynamics in solids

Abstract:

This thesis investigates the dynamics of electrons and atoms in solids driven by intense, ultrashort laser pulses. Results of two series of experiments are presented. In the first set, the changes in the electronic properties of the semiconductor GaAs are determined by measuring the changes in its optical properties in response to 70-fs laser pulses. A fluence range of up to, and above, the damage threshold is examined. The experiments differ from previous work in the field, in that they are direct time-resolved measurements of the dielectric function and second- order optical susceptibility fundamental quantities that characterize the optical state of the material. The dielectric function is measured from 1.5 to 3.5 eV, and at 4.4 eV, while the second-order susceptibility is measured at a single frequency of 2.2 eV. The results suggest a new view of the underlying electronic and structural changes. Three regimes of behavior are observed: at low excitation, rapid bandstructure changes are followed by lattice heating for about 10 ps; at medium excitation, stronger bandstructure changes are followed by a loss of long-range order in the crystal within several picoseconds; and at high excitation, an increasingly rapid transition to a metallic state is seen. In the second set of experiments, the effect of ultrafast excitation inside the bulk of a solid is studied. It is shown that submicron-diameter voxels can be produced inside many transparent materials by tightly focusing 100-fs laser pulses. The use of such voxels for high-density 3D optical data storage is demonstrated. Scanning electron microscopy and atomic force microscopy are used to examine 200-nm diameter voxels. The results suggest that extreme temperatures and pressures create a micro-explosion, leading to the formation of a void surrounded by densified material. Permanent structural changes are produced even in such hard materials as quartz and sapphire.