Ultrafast Energy Absorption in Silicon Controlled by Two-Color Double Pulses
We theoretically show that energy absorption in crystalline silicon can be controlled by two-color femtosecond double-pulse irradiation, in which two temporally separated pulses with different wavelengths interact sequentially with the system. Using time-dependent density functional theory, we systematically examine the wavelength and intensity dependence of the absorbed energy over peak intensities of $2\times10^{11}$-$10^{13}$ W/cm$^2$ and wavelengths of 515, 1030, and 2060 nm. We find that the mechanism governing energy absorption and the optimal wavelength combination strongly depend on the intensity regime. In the low-intensity regime, multiphoton interband absorption is dominant, and energy absorption is enhanced for pulse pairs composed of shorter wavelengths. In contrast, in the high-intensity regime, the contributions of tunneling ionization and intraband acceleration become significant, leading to enhanced absorption for longer-wavelength combinations. In the intermediate-intensity regime, a pronounced enhancement is observed when a short-wavelength pulse precedes a long-wavelength pulse. Our analysis reveals that the nonequilibrium electronic state prepared by the first pulse modifies the excitation process induced by the second pulse, thereby enhancing the absorbed energy through an increased energy gain per excited electron. In this regime, the energy absorption is governed not only by the number of excited carriers but also by the energy gain per excited electron, which can be strongly modified by the pulse sequence. These results indicate that ultrafast energy transfer in semiconductors is tunable by appropriately designing the wavelength and intensity combination of the two pulses, and provide microscopic insight into two-color strong-field excitation.