Temperature-tunable unidirectional non-reciprocal graphene-VO₂ sequential defect-based quasi-photonic terahertz...

This study details a tunable and multifunctional terahertz (THz) absorber leveraging the synergistic interaction between phase-transition vanadium dioxide (VO₂) and adjustable graphene defect layers integrated within aperiodic sequential quasi-photonic structures. Exploiting the phase transition property of VO₂ (insulating to metallic states) and the tunable chemical potential of graphene endows the absorber with dynamically controllable resonance characteristics. The structure shows several high-quality absorption peaks in the 4-6 THz range and a most distinguished one at 4.89 THz in the meantime of VO₂ in the insulating phase. This work reveals strong spectrum tunability of the absorption properties, acted upon separately using heat modulation and alterations in the chemical potential of graphene. Moreover, careful tuning of the Fibonacci defect layer configuration helps to get a very high-quality factor (Q-factor) near 900. Concurrently, the designed structure shows a two-fold increase compared to traditional mono-defect photonics by displaying a large lateral Goos-Hänchen (GH) displacement measured at 389λ. The absorber shows a non-reciprocal feature, achieving unidirectional perfect absorption under forward propagation and no backward absorption, which is an essential requirement for unidirectional energy harvesting. Significantly, the design circumvents the drawbacks of the rigid THz absorbers by incorporating dual tunability (thermal and electrical) and defect engineering. Our optical impedance matched structure ensures near-perfect absorbance (>99%) while demonstrating a clear blue-shifting resonance with defect layer thickness variations. Substantial angular stability (up to 60° of incidence) and polarization insensitivity also add to practicality. Such progress renders these piers an adaptable framework for THz thermal sensing, infrared detection, and energy harvesting, for which tunable spectral regulation and high-Q resonance are beneficial. By uniting the phase-change material functionality, the tunability of graphene, and quasi-periodic defect engineering, the work sets a rubric for a new class of multifunctional nanophotonic devices.