Ultrafast dynamics in solids

A pump-probe technique measuring the dielectric function is presented and applied in a series of experiments. An ultrafast semiconductor-to-metal phase transition is observed in highly excited GaAs. High amplitude coherent phonon dynamics in Te are resolved suggesting a THz-driven semiconductor-semimetal transition. Coherent control including both enhancement and cancellation of the lattice vibrations in Te is demonstrated revealing a departure from the low-excitation behavior of similar materials.

We measured the dielectric constant and second-harmonic generation in GaAs following femtosecond laser-pulse excitation. From these measurements, we extract the behavior of the second-order optical susceptibility, which monitors crystal symmetry, during a laser-induced phase transformation. This is the first time, to our knowledge, that the effect of large changes in the dielectric constant has been taken into account explicitly in determining the second-order susceptibility.

1. Electronic and optical properties of solids : interaction of matter with light, dielectric function, phonons, direct and indirect excitation processes, cohesion in solids, covalent and metallic bonding, band structure, interaction of light with matter:

2. Energy transfer and relaxation in solids: single and multiphoton excitation, avalance photoionization, carrier-carrier scattering, carrier-phonon scattering, radiative and nonradiative carrier recombination, carrier diffusion, time scales:

3. Optical probing techniques: Generation of femtosecond laser pulses, pump-probe...

Read more about The behavior of solids under excitation with intense femtosecond laser pulses

Large bandgap semiconductors like ZnO are currently of interest as light emitters in the blue-UV spectral range. Advances in these technologies rely on a fundamental understanding of carrier dynamics and excitonic effects at high excitation densities. Researchers have described highly photo-excited ZnO using the polariton model. However, at high excitation, one would expect screening effects to come into play, such that the polariton model would no longer hold. With high carrier densities induced by intense, ultrashort laser pulses, we are able to monitor exciton dynamics through our micro-... Read more about Ultrafast exciton dynamics in highly excited bulk ZnO

The use of a streak camera makes it possible to improve the study of the melting of silicon during picosecond laser annealing. Its time resolution of 1.8 ps enables us to confirm that the molten silicon is heated above the melting temperature. It also provides spatial information on the melting process.