Light–matter interaction beyond Born–Oppenheimer approximation mediated by stimulated phonon polaritons

Abstract

AbstractFor decades, the light-matter interaction (LMI) based on Born-Oppenheimer (BO) approximation has dominated the fields of photonics, materials, and condensed-matter physics. However, in polar crystals, the BO approximation is not applicable when stimulated phonon polaritons are excited. Unlike the studies on topological physics and physical chemistry that go beyond BO approximation, here we unravel a stimulated phonon polariton-mediated LMI mechanism in polar crystals, which is radically different from the traditional LMI interpreted by BO approximation. In the regime of stimulated phonon polariton-mediated LMI, two exemplary experiments were conducted at different wavelengths: Q-factor increase of a LiNbO3 terahertz microcavity, and a five orders-of-magnitude enhancement of second-harmonic generation of infrared laser pulses in a LiNbO3 slab. Our study enhances the understanding of the LMI mechanism and shows potential for applications in the fields of optics/photonics and condensed-matter physics.