On-chip spectral manipulation of broadband terahertz polaritons through collinear wavevector beating.

Phonon polaritons-hybrid quasiparticles arising from strong coupling between lattice vibrations and electromagnetic waves-exhibit exceptional capabilities for enhancing terahertz (THz) confinement and nonlinear light-matter interactions. On-chip spectral control of THz phonon polaritons remains challenging due to the limited THz power density available for nonlinear approaches. Here, we propose a collinear wavevector beating scheme implemented in an on-chip lithium niobate subwavelength waveguide that enables flexible spectral manipulation of broadband THz phonon polaritons simply through the selection of propagation distance. By designing a two-mode waveguide and implementing a collinear wavevector beating-a spatial analog of Young's double-slit experiment-we generate longitudinal interference patterns that allow frequency separation of broadband components through propagation distance selection. This approach avoids the power limitations of conventional nonlinear methods and establishes a new paradigm for flexible on-chip spectral manipulation, advancing the development of functional THz polaritonic devices.