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arXiv · 2607.06399

Spontaneous emission of light by non-equilibrium phonons

J. Plo·P. Valvin·M. Moret·T. Taliercio·J. Batista·T. Sohier·A. Vasanelli·C. Sirtori·B. Gil·W. Desrat·G. Cassabois
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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.

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J. Plo, P. Valvin, M. Moret, T. Taliercio, J. Batista, T. Sohier, A. Vasanelli, C. Sirtori, B. Gil, W. Desrat, G. Cassabois. 2026-07-07. Spontaneous emission of light by non-equilibrium phonons. https://arxiv.org/abs/2607.06399

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