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

TL;DR

This study explores the non-linear energy absorption mechanisms in bulk silicon under intense ultrashort laser pulses, revealing regimes dominated by perturbative three-photon ionization and bulk plasmon resonance excitation, with implications for laser-material interactions.

Contribution

It provides a detailed analysis of energy absorption regimes in silicon under ultrashort laser pulses, highlighting the role of bulk plasmon resonance at high intensities and the effects of sequential pulses.

Findings

Enhanced absorption due to bulk plasmon resonance at high intensities

Energy transfer exceeds thermal melting threshold of silicon

Reflectivity changes align with experimental observations

Abstract

We investigate the non-linear response and energy absorption in bulk silicon irradiated by intense 12-fs near-infrared laser pulses. Depending on the laser intensity, we distinguish two regimes of non-linear absorption of the laser energy: for low intensities, energy deposition and photoionization involve perturbative three-photon transition through the direct bandgap of silicon. For laser intensities near and above 10$^{14}$ W/cm$^2$, corresponding to photocarrier density of order 10$^{22}$ cm$^{-3}$, we find that absorption at near-infrared wavelengths is greatly enhanced due to excitation of bulk plasmon resonance. In this regime, the energy transfer to electrons exceeds a few times the thermal melting threshold of Si. The optical reflectivity of the photoexcited solid is found in good qualitative agreement with existing experimental data. In particular, the model predicts that the main features of the reflectivity curve of photoexcited Si as a function of the laser fluence are determined by the competition between state and band filling associated with Pauli exclusion principle and Drude free-carrier response. The non-linear response of the photoexcited solid is also investigated for irradiation of silicon with a sequence of two strong and temporary non-overlapping pulses. The cumulative effect of the two pulses is non-additive in terms of deposited energy. Photoionization and energy absorption on the leading edge of the second pulse is greatly enhanced due to free carrier absorption.