Fundamentals of Picosecond and Femtosecond Laser Solid Interactions
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FUNDAMENTALS OF PICOSECOND AND FEMTOSECOND LASER SOLID INTERACTIONS H. Kurz Institute of Semiconductor Electronics, Aachen University of Technology, D-5100 Aachen, F.R. Germany
ABSTRACT The current understanding of the interaction between ultrashort laser pulses and condensed matter is demonstrated on a few selected samples. Hot electronic carrier relaxation in GaAs, plasmon aided recombination in highly excited silicon and ultrafast energy transport in metals are discussed.
Introducti on In the last materials research society meetings the essential role of time resolved optical methods have been demonstrated convincingly as successful techniques interactions
to study
fundamental processes
of laser
material
[1-9]. The power and range of optical methods has been
remarkably enlarged by the rapid development delivering tunable and intense femtosecond pulses.
of new laser sources
In this review most of the attention is paid to details of femtosecond pulse interactions in semiconductors and metals at high excitation levels. Three specific examples of systems driven far out of equilibrium are chosen. After a general survey of physical principles involved the relaxation of hot carriers in III-V compounds is adressed. The influence of collective plasma oscillations on energy relaxation and carrier dynamics is considered in a high excitation study of elemental semiconductors. New relaxation channels are opened as soon as the plasma energy becomes comparable to phonon-
and bandgap energies. Finally the energy transport
in metals by hot electrons is discussed. The high transport velocities observed in very recent experiments are of technical importance for laser processing of metals and alloys.
Mat. Res. Soc. Symp. Proc. Vol. 74. 1987 Materials Research Society
4
II. PHYSICAL PRINCIPLES The primary dissipative interaction step is the absorption photons by electrons. In a secondary step the energy is transferred from the excited state to other electrons or holes and lattice vibrational modes. In metals the conduction electrons are excited by inelastic free-free transitions, well described by a high frequency conductivity. In semiconductors electron-hole pairs are created if the photon energy exceeds the bandgap. Photons below the bandgap may create electron-hole pairs via multiphoton absorption. Once the carrier density is sufficiently high, phonon assisted free carrier absorption and intervalence transitions between light and heavy holes increase the energy content of the electron-hole system without adding new carriers. In semiconductors only narrow regions in the central valley of conduction band and valence band are coupled by the laser field. The lifetime of the optically coupled states is limited principally by inelastic intercarrier and phonon collisions. It requires femtosecond laser pulses to saturate the interband transition noticeable in most of the semiconductors.
At carrier densities above 1019 collective oscillations of electrons and holes have a major impact on the physics of relaxation. With in
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