Controlling Radiative Phonons in the van der Waals Ferroelectric NbOI₂

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 NbOI₂ 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 NbOI₂. 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.