Terahertz multiband notch filter based on a dielectric topological valley photonic crystal

This study presents the design of a terahertz multiband notch filter based on a dielectric topological valley photonic crystal (VPC) using a cavity-integrated Mach–Zehnder interferometer geometry. The filter exploits valley-protected edge states created via inversion symmetry breaking in a 2D dielectric-air VPC structure, resulting in a bandgap spanning 310–340 GHz. By integrating a triangular resonator within the VPC, the number of stopbands increased from 1 to 4, considering a reference attenuation depth of −3 dB. The attenuation depth significantly improved from −1.2 to −3.29 dB at 312.7 GHz and from −0.4 to −13.09 dB at 330.6 GHz. The maximum depth of attenuation achieved is −18.51 dB at 327.7 GHz, with a very sharp attenuation profile. Rigorous simulations reveal that the formation and enhancement of the notches arise from the coupling of eigenmodes. Additionally, the impact of the resonator's position on filtering characteristics is quantitatively analyzed, showing that increased cavity distance reduces coupling strength and diminishes filtering performance. The proposed VPC-based filter could be crucial in compact, high-performance THz photonic applications in 6G and beyond communication and sensing technologies.