Saturable absorption in highly excited silicon and its suppression at the surface

TL;DR

This study uses first-principles simulations to explore how saturable absorption occurs in highly excited silicon and how surface effects suppress this phenomenon, revealing the roles of electron temperature, laser intensity, and surface symmetry.

Contribution

It provides a detailed first-principles analysis of nonlinear excitation and saturable absorption in silicon, highlighting surface suppression effects at finite electron temperatures.

Findings

Saturable absorption occurs at certain laser intensities in bulk silicon.

Pauli blocking inhibits further one-photon transitions at high intensities.

Surface symmetry breaking suppresses Pauli blocking, enhancing absorption.

Abstract

Nonlinear electronic excitation in laser-irradiated silicon at finite electron temperatures is numerically investigated by first-principles calculations based on the time-dependent density functional theory. In bulk silicon at finite temperatures under near-infrared laser irradiation, we found that the absorbed energy is saturated when using a certain laser intensity even with a few-cycle pulse. Although one-photon processes of conduction-to-conduction and valence-to-valence transitions are dominant at such a laser intensity, the Pauli blocking inhibits further one-photon transition. With higher intensities, multi-photon excitation across the bandgap overwhelms the one-photon excitation and the saturable absorption disappears. At the surface of finite-temperature silicon, the Pauli blocking is suppressed by the symmetry breaking and the absorbed energy is relatively enhanced from the energy of the saturable absorption in the bulk region.