Enhanced Energy Absorption and Electron Excitation in Crystalline Silicon Induced by Two-Color Intense Femtosecond Laser Pulses

Over the past decades, the interaction between femtosecond intense lasers and semiconductors or dielectrics has been attracting significant attention as for high harmonic generation [1], high-quality laser micromachining without the thermal damage [2]. Several experimental and theoretical studies have reported that the use of two-color laser pulses enables highly efficient laser ablation of transparent materials compared to a single-color pump pulse [3], [4]. In the present work, to elucidate how two-color femtosecond laser pulses deposit energy to electrons in semiconductors and dielectrics, we utilize the time-dependent density functional theory (TDDFT) and examine the energy absorption of crystalline silicon under overlapped two-color [ultraviolet (UV) and infrared (IR)] intense femtosecond laser pulses as a function of relative intensity with the total fluence conserved. The deposited energy is dramatically enhanced by two-color laser field and maximized when they are equally mixed [see Fig. 1(a)]. The interplay between intraband electron motion in the valence band (before excitation) driven by the IR component and resonant valence-to-conduction interband excitation (carrier injection) induced by the UV component is identified as the underlying mechanism. Interestingly, the former plays an influential role, increases the excited electrons [see Fig. 1(b)]. The effect of multiple multiphoton absorption paths, relative phase of carrier waves, or intraband motion of the created carriers in the conduction band play a minor role.