Tunable Broadband Terahertz Modulation in a Multi-Stacked Toroidal Metamaterial

Metamaterial-based photonic components are driving significant scientific and technological advancements in the terahertz (THz) frequency range. In this study, we present a multilayered metamaterial based on toroidal excitation for ultra-broadband THz modulation. The toroidal resonance within the stacked resonators enables subwavelength coupling with individual resonators, significantly broadening the transmission bandwidth. Our multi-stacked metamaterial demonstrates a full width at half maximum (FWHM) of 370 GHz, which is approximately 137% of the 270 GHz bandwidth of a single-layer metamaterial. A detailed multipolar analysis reveals that the toroidal dipole dominates the electromagnetic response, confirming the enhanced performance of the stacked configuration over its planar counterpart. Furthermore, by integrating a temperatureresponsive vanadium dioxide (VO₂) layer into the multilayer structure, we enable tunable amplitude transmission through external thermal control. By varying the temperature of the VO₂ layer from 30 °C to 120 °C, we achieve a transition from a band-reject filter to a broadband reflector, as verified by experimentally obtained THz-TDS spectral results. In addition, we investigate the temporal switching dynamics of the modulator using time-resolved transmission measurements under periodic electrical pulsing. The device exhibits efficient and reversible electro-thermal modulation, with rise and fall times in the range of a few seconds, making it suitable for practical applications requiring dynamic THz control. This tunable and switchable THz modulator holds significant promise for broadband communication systems and adaptive photonic platforms.