Transient optical properties of semiconductors under femtosecond x-ray irradiation

Abstract

Semiconductors under femtosecond x-ray irradiation are transiently excited to nonequilibrium states. This can lead to observable material modifications. During the excitation and relaxation dynamics, optical properties of the solid are changing, affected by both transient electron excitation as well as the evolution of the atomic structure. In this paper we apply a unified hybrid model to trace these two effects. Transient evolution of the optical properties is calculated within the transferable tight-binding approach. The presented methodology of calculation of the complex dielectric function proves to be capable of describing changes in the optical parameters during the phase transitions, when the solids are driven out of equilibrium by intense laser pulses, in a reasonable agreement with experiments.

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

“…A small discrepancy between predictions and the data during the pulse can be expected due to the reasons presented above. However, the overall agreement of data and predictions is good [10].…”

“…This model does not take into account interband transitions. As it was discussed above, any model that does not account for both structural and electronic changes within irradiated material will fail to describe the material excitation for doses close to the damage threshold, when atomic dynamics starts to be important [10]. …”

“…Optical refractive indices n(ω) and k(ω) represent the real and the imaginary part of the complex refractive index,ñ(ω) = n(ω) + ik(ω) = (ω), where only the diagonal matrix elements of the CDF tensor are used [10]. They can be evaluated for many materials with XTANT.…”

“…The constant γ is assumed to be γ = 1.5 × 10 13 s −1 throughout the calculation [10]. Required parameters can be obtained with the tight-binding scheme, i.e., E nn = E n − E n (transition energy between two energy states); f n and f n (the corresponding transient electron occupation numbers obtained assuming their transient Fermi distribution).…”

“…In this review paper we summarize the results of our theoretical studies on transient optical properties of semiconductors irradiated by ultrafast X-ray pulses [10]. Such pulses can trigger thermal and non-thermal transitions.…”