Prof. Chris Stanton

Femtosecond Spectroscopy in Semiconductors: Carrier Dynamics, Coherent Phonons and THz Radiation

Christopher J. Stanton Department of Physics University of Florida

Ultrafast laser spectroscopy has proven to be a powerful technique for studying dynamical processes in semiconductor nanostructures. Since femtosecond experiments probe on the same time scale as scattering, they provide detailed information beyond standard D.C. mobility measurements on electron and hole dynamics. We discuss how comparison between experiments and theoretical calculations can provide: i) information on the many different types of scattering mechanisms in semiconductor heterostructures, ii) information about electronic bandstructure, and iii) information on time dependent many body interactions. In addition to probing the carrier dynamics, the ultrafast optical photoexcitation of hot electrons and holes in semiconductors can trigger coherent phonon oscillations that can be probed in reflection or transmission. In bulk systems, ultrafast lasers have been used to generate coherent optical phonons. In semiconductor superlattices, multiple quantum wells, and epilayers, huge coherent acoustic phonons can be generated. We will discuss the microscopic theory of the generation of coherent phonons in bulk semiconductors and hetrostructures. In epilayers, the coherent phonons are localized wavepackets that can be used to probe nano-scale structures below the sample surface a technique we call Nanoseismology. We also show how multiple laser pulses can be used to coherently control the dynamics of the phonons and to manipulate the large piezoelectric fields in these systems. Finally, we discuss the emission of THz radiation from both the photoexcited carriers as well as the coherent phonons.