Spontaneous emission of light by non-equilibrium phonons
Abstract
When a system is brought out of equilibrium by an external excitation, its relaxation to thermodynamic equilibrium generates phonons. These non-equilibrium phonons degrade via cascaded anharmonic decay processes, progressively leading to a thermal population of phonons following a Bose-Einstein distribution at the system temperature. Preceding heat dissipation by convection, conduction and incandescence, this early phase of the relaxation dynamics is commonly assumed to be exclusively non-radiative. Here, we demonstrate that the radiative emission by phonons can be an efficient relaxation pathway competing with the intrinsic anharmonic decay. Optical spectroscopy under femtosecond two-photon excitation in boron nitride unveils a photoluminescence signal in the mid-infrared spectral range, stemming from the spontaneous emission of light by non-equilibrium phonons. This observation of non-thermal radiation from phonons introduces a new paradigm for out-of-equilibrium physics, mid-infrared optics, and thermal management.
Keywords
Cite
@article{arxiv.2607.06399,
title = {Spontaneous emission of light by non-equilibrium phonons},
author = {J. Plo and P. Valvin and M. Moret and T. Taliercio and J. Batista and T. Sohier and A. Vasanelli and C. Sirtori and B. Gil and W. Desrat and G. Cassabois},
journal= {arXiv preprint arXiv:2607.06399},
year = {2026}
}