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.