Non equilibrium optical properties in semiconductors from first--principles: a combined theoretical and experimental study of bulk silicon

TL;DR

This paper combines theoretical and experimental approaches to study the transient optical properties of bulk silicon, revealing phenomena like bleaching and optical gap renormalization with a parameter-free Bethe-Salpeter equation method.

Contribution

It introduces a parameter-free non-equilibrium Bethe-Salpeter equation approach to accurately describe transient optical phenomena in semiconductors.

Findings

Successful theoretical reproduction of transient reflectivity in silicon

Dissection of phenomena like bleaching and stimulated emission

Introduction of optical gap renormalization concept

Abstract

The calculation of the equilibrium optical properties of bulk silicon by using the Bethe--Salpeter equation solved in the Kohn--Sham basis represents a cornerstone in the development of an ab--initio approach to the optical and electronic properties of materials. Nevertheless calculations of the {\em transient} optical spectrum using the same efficient and successful scheme are scarce. We report, here, a joint theoretical and experimental study of the transient reflectivity spectrum of bulk silicon. Femtosecond transient reflectivity is compared to a parameter--free calculation based on the non--equilibrium Bethe--Salpeter equation. By providing an accurate description of the experimental results we disclose the different phenomena that determine the transient optical response of a semiconductor. We give a parameter--free interpretation of concepts like bleaching, photo--induced absorption and stimulated emission, beyond the Fermi golden rule. We also introduce the concept of optical gap renormalization, as a generalization of the known mechanism of band gap renormalization. The present scheme successfully describes the case of bulk silicon, showing its universality and accuracy.