Single high-order mode interface states in terahertz quasi-periodic waveguides

Interface states in photonic structures have attracted increasing interest due to their unique localization and wave manipulation capabilities. However, in the terahertz (THz) regime, the realization and control of such states remain challenging, especially when high-order modes are involved in quasi-periodic systems. Here, we demonstrate both experimentally and numerically a single high-order-mode interface state in a quasi-periodic corrugated slab waveguide. By constructing a mirror-symmetric structure based on the fifth-generation Fibonacci sequence, we observe the emergence of a narrow transmission peak located within the Bragg stopband. Finite element simulations and THz time-domain spectroscopy measurements reveal strong field localization at the structural interface. Despite excitation with a first-order mode, modal decomposition shows that the localized resonance is almost entirely governed by a third-order mode (>95%), which confirms the formation of a pure high-order-mode interface state. This work establishes a symmetry-induced mechanism for selective mode excitation in non-periodic THz waveguides and provides a practical route toward compact, narrowband, and high-Q THz photonic devices.