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Accueil du site > Séminaires > Séminaires 2010 > Semiconductors under Ultrafast Laser Excitation : Studies of the Temporal and Spatio-temporal Carrier Dynamics by Quantum Kinetic Approaches

Lundi 22 novembre — 15:00, attention, horaire exceptionnel

Semiconductors under Ultrafast Laser Excitation : Studies of the Temporal and Spatio-temporal Carrier Dynamics by Quantum Kinetic Approaches

Tzveta Apostolova (Institute of Nuclear Research and Nuclear Energy, Sofia)

par Pierre Pujol - 22 novembre 2010

Compared to continuous lasers, ultra-fast pulsed laser systems damage materials by localized effects before the heated electrons produced during the interaction have time to diffuse away. Optical and transport properties of semiconductors determine their applicability to laser optics and devices such as photodetectors, transistors, and light emitters. Therefore, it is of great importance to describe the microscopic processes taking place when these materials interact with an intense laser irradiation. The governing microscopic processes occur within the time scale of the laser pulse so that the appropriate dynamical models have to explicitly account for them. A theoretical investigation is conducted by deriving a quantum kinetic theory for laser-induced breakdown in semiconductor materials based on a generalized Boltzmann-type equation, that includes energy-drift and free-carrier absorption, anti-diffusion effects, interband excitation, Coulomb scattering, Auger recombination, thermal exchange with the lattice, etc. Higher intensities effects such as enhancement of ionization by transient bandgap renormalization and multiphoton and tunneling ionization are also considered. The created model numerically predicts semiconductor material breakdown using computational codes parameterized by the parameters of the incident laser pulse, including pulse width and peak intensity, and by the properties of the target materials, including band-gap, mobility, initial lattice temperature, dielectric constant, effective mass of electrons, etc.. The energy spectra of the electron distribution function and the time dependence of the electron density are calculated to illustrate how various laser and material parameters influence the conduction electron dynamics. The electron distribution is used to evaluate average electron energy and temperature. In addition to the model described above, a microscopic quantum-kinetic theory based on density matrix approach (using Wigner function representation) is formulated to describe the processes of short pulse laser interaction with semiconductors accounting for arbitrary spatial inhomogeneities in the excitation conditions and other spatial phenomena such as filamentation of tightly focused femtosecond laser pulses, structural modification and catastrophic optical damage. A system of Boltzmann-Bloch transport equations is established that includes both space and momentum dependence of the electron and hole distribution functions and the polarization. Microscopic electron-phonon and electron-electron scattering terms as well as scattering terms that lead to transitions between valence and conduction bands, i.e. impact ionization and recombination terms, are included explicitly in the equations. The formulated theory describes the spatio-temporal carrier dynamics in inhomogeneously excited materials including the coherent interactions of carriers and the laser light field as well as transport due to spatial gradients and electrostatic forces.