Tunable Optical-to-THz Frequency Conversion Driven by Localized Surface Phonon Polaritons in LiNbO3 Metasurfaces

Abstract We demonstrate tunable THz generation by optical rectification in LiNbO3 rod metasurfaces supporting localized surface phonon polaritons (LSPhPs). By varying the rod length, the LSPhP resonance is tuned across the 4–5 THz range, producing a systematic red-shift of the generated THz spectral peak. THz time-domain spectroscopy and numerical calculations show that the emitted spectrum is governed by the geometry-dependent LSPhP response. Although the total far-field emission remains lower than that of the unpatterned film, numerical calculations show that the metasurfaces enhance the volume-averaged mean-square nonlinear current density and strongly localize the generated THz field near the rods. These results establish phonon-polaritonic metasurfaces as compact platforms for spectrally tailoring optical-to-THz conversion and enhancing localized THz fields.