Design of tunable topological valley photonic crystal filter for arbitrary wavelength notch filtering

Optical filters are essential components in modern high-speed optical communication systems, playing a pivotal role in signal selectivity, channel isolation, and system capacity limits. As demand for high-density wavelength-division multiplexing (WDM) and high-bandwidth signal processing increases, the precision of wavelength screening by filters becomes increasingly critical. However, traditional microwave photonic filters suffer from inherent limitations, including constrained free spectral range, fixed geometric periods, and rigid tuning mechanisms, all of which hinder their ability to meet the flexibility required for advanced WDM systems. In this paper, we present a tunable topological valley photonic crystal notch filter integrated with barium titanate, leveraging cavity-edge coupling for continuous notch frequency tuning. Our simulation results reveal that while the microcavity size determines the filter wavelength as a deterministic design degree of freedom, the intrinsic scattering resistance is sustained by the topological protection of the edge states. By cascading microcavities of different sizes, filters with arbitrary wavelength intervals can be achieved. Furthermore, by utilizing the microcavity as a functional point-defect, the refractive index of the microcavity can be tuned to achieve a sensitivity of 391.7 nm/RIU. The proposed structure features a synergistic wave manipulation mechanism that integrates the robust transport of topological edge states with the highly sensitive modulation of localized point-defects. With its compact footprint, high tuning linearity, and versatile wavelength allocation, this design represents a promising solution for the next generation of reconfigurable, high-density photonic integrated circuits.