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We investigate the non-linear response and energy absorption in bulk siliconirradiated by intense 12-fs near-infrared laser pulses. Depending on the laserintensity, we distinguish two regimes of non-linear absorption of the laserenergy: for low intensities, energy deposition and photoionization involveperturbative three-photon transition through the direct bandgap of silicon. Forlaser intensities near and above 10$^{14}$ W/cm$^2$, corresponding tophotocarrier density of order 10$^{22}$ cm$^{-3}$, we find that absorption atnear-infrared wavelengths is greatly enhanced due to excitation of bulk plasmonresonance. In this regime, the energy transfer to electrons exceeds a few timesthe thermal melting threshold of Si. The optical reflectivity of thephotoexcited solid is found in good qualitative agreement with existingexperimental data. In particular, the model predicts that the main features ofthe reflectivity curve of photoexcited Si as a function of the laser fluenceare determined by the competition between state and band filling associatedwith Pauli exclusion principle and Drude free-carrier response. The non-linearresponse of the photoexcited solid is also investigated for irradiation ofsilicon with a sequence of two strong and temporary non-overlapping pulses. Thecumulative effect of the two pulses is non-additive in terms of depositedenergy. Photoionization and energy absorption on the leading edge of the secondpulse is greatly enhanced due to free carrier absorption.

Ultrafast energy absorption and photoexcitation of bulk plasmon in crystalline silicon subjected to intense near-infrared ultrashort laser pulses