For decades, the light-matter interaction (LMI) mechanism 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 (SPhPs) are excited. Unlike the studies on topological physics and physical chemistry that beyond BO approximation, here we unravel an SPhP-mediated LMI (SPhP-LMI) mechanism in polar crystals, which is radically different from the traditional LMI interpreted by BO approximation. In the regime of SPhP-LMI, two exemplary experiments were conducted at different wavelengths: Q-factor increase of a lithium niobate (LiNbO3) terahertz microcavity, and a five orders-of-magnitude enhancement of second-harmonic generation of infrared laser pulses in an LiNbO3 slab. Our study brings about a new fundamental understanding of the LMI mechanism and shows great potential in the fields of optics/photonics and condensed-matter physics.