Monolithic graphene−qBIC resonators for electrically tunable terahertz modulation

Terahertz (THz) modulators are essential components for dynamic wavefront control in imaging, sensing, and wireless communication systems. However, practical THz modulators require simultaneous optimization of multiple functional metrics, including modulation depth, resonance quality, insertion loss, switching speed, and device integration, which are often difficult to balance within a single device architecture. Graphene is an attractive active material because of its electrical tunability, high carrier mobility, and compatibility with planar integration, but its intrinsically weak interaction with THz waves limits the modulation efficiency of pristine monolayer devices. Here, we demonstrate a monolithic graphene-quasi-bound-state-in-the-continuum (qBIC) resonator platform that addresses this multidimensional performance trade-off. By integrating patterned graphene within symmetry-broken double-rod metallic resonators, the dissipative loss of a high-Q qBIC resonance can be continuously controlled through electrostatic gating. The fabricated modulator achieves a modulation depth of 35%, a resonance bandwidth of 0.12 THz centered at 1.1 THz, an equivalent Q factor of 9.28, a 3 dB modulation speed of 14 kHz, and a low insertion loss of 2.08 dB. These results demonstrate balanced optimization of key modulation metrics in a compact, planar architecture, providing a practical route toward active THz photonic devices.