Semiconductor under Ultrafast Laser Excitation: Optical Studies of the Dynamics

Abstract:

This thesis presents studies of semiconductors under intense femtosecond laser irradiation. In order to investigate the nature of the electronic and structural changes induced by laser pulses, a novel broadband technique is developed to measure the linear optical property of semiconductors dielectric function over the entire visible spectrum (1.53.5 eV) with femtosecond time resolution. By employing this broadband spectroscopic technique, the response of the dielectric function of GaAs following an intense 70-fs, 1.9 eV pump pulse is measured. The results provide the most detailed information thus far on the electron and lattice dynamics both above and below the fluence threshold for permanent damage. It is shown that electronic effects, manifested in changes in the band structure, dominate during the first few hundred fs following the excitation. After a few picoseconds, three distinct structural changes are observed depending upon the excitation strength: At low pump fluences, the dielectric function shows heating of the lattice caused by carrier relaxation. At intermediate fluences, the dielectric function reveals a temporary disordering of the lattice. At even higher fluences, a semiconductor-to-metal transition occurs even below the damage threshold. The latter two effects are attributed to the lattice instability caused by the destabilization of the covalent bonds. The time-integrated photoluminescence is also measured to investigate the dynamics of GaAs following fs laser excitation. The luminescence images reveal a reduction of emission due to the structural changes in GaAs. The spectral measurements provide new insight in the carrier dynamics. In addition, a series of II-VI semiconductors are also studied using similar techniques. The response of crystalline Si following fs laser excitation is also explored using the broadband spectroscopic technique. The dielectric function measurements show that lattice heating and semiconductor-to-metal transitions take place within a few picoseconds. The long time (up to 400 ps) behavior is investigated with both reflectivity and dielectric function measurements, providing detailed information on the relaxation of both electronic and structural changes following the excitation.