Theory of phonon-assisted luminescence in solids: application to hexagonal boron nitride

TL;DR

This paper develops a theoretical framework using non-equilibrium Green's functions and perturbation theory to explain phonon-assisted luminescence in solids, specifically applied to bulk hexagonal boron nitride, clarifying experimental observations.

Contribution

It introduces a new formula for light emission in solids that incorporates electron-phonon interactions perturbatively, applied here to explain luminescence in hBN.

Findings

Derived a formula for phonon-assisted luminescence in solids.

Applied the theory to explain ultraviolet luminescence in hBN.

Provided a comprehensive explanation aligning with experimental data.

Abstract

In this manuscript we study luminescence of hexagonal boron nitride (hBN) by means of non-equilibrium Green's functions plus time-dependent perturbation theory. We derive a formula for light emission in solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk hBN. This material has attracted interest due to its strong luminescence in the ultraviolet [Watanabe et al., Nature Mat. 3, 404(2004)]. The origin of this luminescence has been widely discussed, but only recently has a clear signature of phonon mediated light emission emerged in the experiments [Cassabois et al., Nature Phot. 10, 262(2016)]. By means of our new theoretical approach we provide a clear and full explanation of light emission in hBN.