Effect of plasma formation on the double pulse laser excitation of cubic silicon carbide.

We calculate the electron excitation in cubic silicon carbide caused by the intense femtosecond laser double pulses using the time-dependent density functional theory (TDDFT). After the first pulse ends, excited electrons should be relaxed by collisional processes. Because TDDFT does not include scattering processes, thermalization is mimicked by following three assumptions. First, we assume no collisions and relaxation processes. Second, we assume the partially thermalized electronic state defined by two quasi-temperatures in the conduction and valence bands individually. Third, we assume the thermalized electron distribution, which is expressed by single electron temperature. Our results indicate that the plasma frequency (ωpl) formed by the first pulse is the key parameter in energy absorption in the second pulse. When the plasma frequency of the plasma formed by the first laser approaches the frequency of the laser, resonant excitation by the second pulse occurs. The lower electron temperature shows higher ωpl and higher efficient energy absorption because the effective mass of the electron becomes smaller. [ABSTRACT FROM AUTHOR]