Baolong Zhang

The optical properties of quantum systems are fundamentally governed by their electromagnetic environment through the local density of optical states (LDOS), enabling powerful control over atomic, excitonic, and electronic transitions. However, the radiative properties of phonons have remained largely unexplored within this framework of environmental engineering. Here we demonstrate LDOS control of terahertz (THz) phonon emission in the van der Waals ferroelectric NbOI ₂ using a simple mirror geometry. By modifying the photonic environment, we achieve pronounced and reversible tuning of the phonon Q ‐factor, peak frequency, linewidth, and emission intensity, with modulation approaching an order of magnitude. The observed behavior arises from Purcell‐like enhancement and suppression of the phonon radiative rate, revealing a near‐unity radiative quantum efficiency in NbOI ₂ . Our results establish radiative phonons as tunable quasiparticles governed by photonic boundary conditions, extending LDOS engineering from electronic to lattice excitations. This framework enables deterministic control of phonon lifetimes and emission spectra, opening new opportunities for compact tunable THz sources and phonon‐based quantum and photonic technologies.

Terahertz (THz) phonon polaritons, fundamental quasi‐particles that couple lattice vibrations with electromagnetic fields at THz frequencies, are found in a variety of materials that offer the potential for a wide range of THz optoelectronic devices and on‐chip integrated systems. However, these compact devices and on‐chip systems are hampered by the absence of on‐chip powerful THz phonon polariton sources. In this study, the efficient generation and amplification of THz phonon polaritons are proposed and directly visualized on a lithium niobate (LN) chip via a tilted pulsefront phase matching technique. By combining lateral pumping and phase matching schemes, two orders of magnitude are successfully attained in the interaction distance between the pump light and the LN chip, accompanied by a substantial amplification of generated THz phonon polariton. The results of this study may lead to abundant potential applications in chip scale THz photonic devices and systems based on LN materials and its integrated heterostructures.