Ultrafast dynamics and optical control of coherent phonons in tellurium.

摘要

This dissertation reports the ultrafast dynamics of tellurium after excitation by one or more intense femtosecond laser pulses. Irradiation of tellurium by femtosecond pulses is known to excite coherent phonons, but the nature of the excitation process and the details of the material dynamics under intense excitation are, as of yet, not precisely determined. We investigate these dynamics by monitoring the response of tellurium using an optical pump-probe technique designed to measure the dielectric tensor across the visible spectrum with femtosecond time resolution.; The observed dynamics are similar to the ultrafast dynamics of molecules, where photoexcitation of electrons establishes a new potential surface on which the nuclei move. The time-resolved dielectric tensor measurements provide a “snapshot” of the material in a particular lattice configuration. From the observed changes in the optical properties, we infer the underlying changes in the lattice and thereby develop a picture of the nuclear motion.; We find that the main resonance for interband electronic transitions in tellurium shifts to lower photon energy due to the lattice displacement that results from photoexcitation. Under single pulse excitation, a rapid change in the equilibrium lattice configuration leads to a long-lived shift in the resonance energy along with fast oscillations around this value. Under double pulse excitation, the lattice dynamics can be controlled; we achieve both enhancement and cancellation of coherent phonons for excitation strengths up to the damage threshold.