Optical properties of semiconductors via pump-probe experiments: statistical-thermodynamical approach, hot plasma and coherent phonon states

Abstract

We present an analysis of the nonequilibrium thermodynamics and, mainly, a response function theory for the study of optical properties in ultrafast-spectroscopy pump-probe experiments. These experiments give rise to the formation of a photoinjected plasma in semiconductors in far-from-equilibrium conditions. The dissipative processes that evolve in this medium greatly influence optical and transport properties. The theory is centred on the application to the study of the phenomenon of modulated changes in the time-resolved reflectivity spectrum. In particular, we show that this phenomenon consists in the coupled effect of coherent-LO-phonon and carrier-charge-density motions, which are driven through the action of the coherent photons of the laser electromagnetic radiation. In the given conditions the modulation effect decays in time and has associated a frequency close to the zone-centre upper LO phonon-optical plasma hybrid mode, as experimentally observed.

Topics in Present-day Science Technology and Innovation: Ultrafast Relaxation Processes in Semiconductors

“…Such phenomenon has been attributed to the generation of coherent lattice vibrations, and several theoretical approaches have been reported [87][88][89][90] . A clear description, on phenomenological bases, which captures the essential physics of the problem, is reported in Zeiger et al 90 , and in Vasconcellos et al 91 is presented an analysis based on NESEF, where it is evidenced that the oscillatory effect is provided by the displacive excitation of the polar lattice vibrations, arising out of the coupling of the carrier-charge density and polar modes, and its decay is mainly governed by the cooling down of the carriers, and where the different physical aspects of the problem are discussed. We briefly describe next the full use of NESEF for dealing with pump-probe experiments for studying the optical properties of semiconductors in nonequilibrium conditions.…”

“…Equation 10 can be calculated. This is an arduous task, and in the process it is necessary to evaluate the occupation functions † ( ) Tr{ ( )} ε = k k k f t c c t  (28) which for weak inhomogeneities, as it is the present case, is presented in Vasconcellos et al 91 . The (nonequilibrium) carrier quasitemperature T c * is obtained, and its evolution in time shown in Figure 15.…”

The role of nonequilibrium thermo-mechanical statistics in modern technologies and industrial processes: an overview

“…• Emission of LO phonons in so called coherent states by electromagnetic excitation, of interest in semiconductor devices [164].…”

“…In Refs. [158][159][160][161][162][163][164][168][169][170][171][172][173] is given an overview of the role of the irreversible thermodynamics based on NESEF in dealing with biosystems. On the question of the relation of Biology and Physics we may say that the sometimes mentioned gap between both disciplines has been slowly shrinking along recent decades.…”

“…Refs. [158][159][160][168][169][170][171][172][181][182][183][184]) phonon systems [162][163][164], and magnon systems [161]. It can also be noticed that the NESEF-based nonlinear quantum kinetic theory provides, as particular limiting cases, farreaching generalizations of Boltzmann and Mori equations [67][68][69]71] and a Higher-Order Hydrodynamics, linear and nonlinear [85,133,134].…”