Graphene-assisted hybrid photonic crystal with a VO₂ defect for switchable reflection and tunable nonreciprocal...

We propose a thermally and electrically tunable terahertz (THz) device based on a graphene-VO₂ hybrid one-dimensional photonic crystal (1DPC) of the form G(AB)^NVO₂(BA)^N. Owing to the insulator-metal phase transition of VO₂, the structure behaves as a high-reflectivity mirror at the insulating phase (T = 300 K), while at the metallic phase (T = 350 K) it transforms into a reconfigurable multichannel absorber. The number of absorption channels is deterministically governed by the periodicity: for N ≤ 4, each period introduces one resonance; at N = 5, five resonances emerge with four showing absorption above 0.88; for 5 ≤ N ≤ 10, the highest-frequency mode gradually vanishes, yielding N - 1 channels; and for 11 ≤ N ≤ 13, suppression of the lowest-frequency mode results in N - 2 channels, reaching up to eleven modes at N = 13. Removing graphene collapses the response to a single absorption peak, confirming its essential role in multichannel formation through plasmonic loss. The device also exhibits robust angular and polarization stability, preserving high absorption for all channels up to $45^\circ$, while TE-polarized modes maintain near-unity absorption for higher-order peaks. In addition to thermal and geometric tunability, electrical gating of graphene enables continuous frequency shifts of 1.6-9.6% per eV and controllable redistribution of modal absorption strength. Furthermore, due to the asymmetric placement of graphene, the structure displays pronounced unidirectional nonreciprocity: forward incidence supports multichannel absorption, whereas backward incidence yields only a single resonance for N = 4-7 and no resonances for other N, demonstrating unidirectional THz absorption without magnetic bias or temporal modulation. By synergistically combining VO₂ phase switching, graphene electro-optic control, and periodicity engineering, the proposed platform enables multifunctional THz operation, ranging from single-mode and multimode absorption to direction-dependent nonreciprocal absorption, within a compact and reconfigurable 1DPC architecture.