Ultrafast phenomena in diamond and silicon in the presence of crystal defects

Femtosecond-laser pulses can induce extreme conditions in solid systems, which causes a variety of ultrafast phenomena, like ultrafast melting or thermal phonon squeezing, a coherent atomic motion. Underlying reason for those laser-induced atomic effects are changes in the interatomic bonding due to the highly excited electron system. However, crystal vacancies already change the local bonding properties in the ground state. In order to analyze ultrafast phenomena in the presence of crystal defects we used finite temperature DFT to perform ab initio MD simulations of silicon and diamond with a defect density of up to 3%. Our results show for intensities well below the laser-melting threshold an increase of the normalized Bragg peak intensity for both materials, an indication for defect annealing. At moderate intensities close to but below the melting threshold, we show that the frequency and the amplitude of thermal phonon squeezing can be manipulated by the defect density. For high intensities well above the melting threshold, the atoms follow different atomic pathways during nonthermal melting than in the perfect crystal.