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.
Highlights
Modern experiments with free-electron-lasers (FELs) using pump-probe schemes achieve a few-femtosecond time resolution in monitoring transient material properties [1,2,3]
We observe in the simulations that thermal and non-thermal phase transitions can be decoupled if atoms are irradiated at a low dose, and at a very high dose
In this paper we discussed the application of transient optical properties for diagnostics of electronic and structural transitions within semiconductors irradiated with femtosecond laser pulses
Summary
Modern experiments with free-electron-lasers (FELs) using pump-probe schemes achieve a few-femtosecond time resolution in monitoring transient material properties [1,2,3]. We will show examples of thermal and nonthermal phase transformations in diamond, silicon and gallium arsenide These three semiconductors differ by the electronic properties, such as their band gap. The band structure of the material and corresponding potential energy surface are calculated with the transferable tight-binding (TB) method. Comparison with the available experimental [22] and DFT data [23] shows discrepancies, e.g., the main peak is missing in the XTANT simulation This is due to the limited accuracy of the (nearest-neighbour) tight-binding method which we apply here. The various TB parametrizations arise from the underlying density functional theory (DFT) or Hartree-Fock (HF) calculations, to which the TB parameters are fitted They may produce slightly different cohesive energies or band structures. We are unable to quantify this statement at present; it deserves a separate investigation
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