Transient Changes of Optical Properties in Semiconductors in Response to Femtosecond Laser Pulses

Abstract

Abstract:In this paper we present an overview of our theoretical simulations on the interaction of ultrafast laser pulses with matter. Our dedicated simulation tool, X-ray induced Thermal And Non-thermal Transitions (XTANT) can currently treat semiconductors irradiated with soft to hard X-ray femtosecond pulses. During the excitation and relaxation of solids, their optical properties such as reflectivity, transmission and absorption, are changing, affected by transient electron excitation and, at sufficiently high dose, by atomic relocations. In this review we report how the transient optical properties can be used for diagnostics of electronic and structural transitions occurring in irradiated semiconductors. The presented methodology for calculation of the complex dielectric function applied in XTANT proves to be capable of describing changes in the optical parameters, when the solids are driven out of equilibrium by intense laser pulses. Comparison of model predictions with the existing experimental data shows a good agreement. Application of transient optical properties to laser pulse diagnostics is indicated.

Picosecond relaxation of X-ray excited GaAs

“…While performing the simulations, we included the electron-ion coupling in the way described in Refs. [6,40]. According to our earlier finding [40], the dynamical electron-ion coupling scheme starts to converge at the MD time step ∆t ∼ 0.01 − 0.02 fs , whereas the Fermi's Golden Rule scheme is inherently divergent in this regime (i.e., for any time step chosen).…”

“…In what follows we use a more rigorous model, the in-house code XTANT (which stands for 'X-ray induced thermal and nonthermal transitions') [6,41,[43][44][45] This affects the accuracy of complex dielectric function (CDF) calculated at higher photon energies, when a certain number of electrons is excited to high-lying states in the conduction band [6]. Here, we use the calculated CDF and the resulting optical parameters only within low energy regime in which they are reliable.…”

“…An extension of XTANT to enable an on-the-fly description of transient optical properties was proposed in [5] and successfully verified by comparison to the available data on optically excited diamond and silicon. In [6] the code has been for the first time applied to describe time evolution of X-ray excited GaAs. Fig.…”

“…As the band gap width in semiconductors is a function of the lattice temperature, the interband transition rate can reflect the transient changes of the lattice temperature. In this study we report on our theoretical studies of the relaxation of X-ray excited GaAs, utilizing transient optical properties of the sample which are sensitive to the interband transitions [4][5][6].…”

Time-resolved ionization measurements with intense ultrashort XUV and X-ray free-electron laser pulses

“…Furthermore, transient optical properties of the material can be calculated either with the RPA (random phase approximation) or the Drude approach [13]. The tight binding parameterization in the current version of XTANT is restricted to carbon, silicon and gallium arsenide [14,15], although a new version of XTANT being developed now includes a more universal parameterization scheme for numerous materials. A simplified scheme of XTANT modules is shown in Fig.…”