Omnidirectional terahertz-wave detection by phonon-polariton coupling in periodically poled lithium niobate

The interaction between terahertz (THz) electromagnetic fields and collective lattice excitations offers a fundamental route to access nonlinear and nonequilibrium light-matter dynamics in solids. However, how THz perturbations induce coherent oscillations in a bulk nonlinear optical medium remains unclear. Here we show that THz irradiation induces a counter-propagating parametric oscillation in a periodically poled lithium niobate crystal through phonon-polariton coupling. The resulting dynamics reveal that the collective vibrational mode bridges a widely separated THz-optical frequency range. This process establishes an unprecedented interaction regime free from the conventional requirements of resonant cavities or strict phase matching. As a proof-of-principle demonstration, we achieved omnidirectional THz detection enabled by this interaction, including counter injection from the phase-matching and excitation along the crystal optical axis. These results establish a platform for flexible THz-optical photon bridging across disparate frequency regimes.