Fabry–Pérot quasi-bound states in the continuum-enabled active terahertz absorption in bulk Dirac semimetal metasurface

Actively tunable terahertz absorbers featuring both high quality factors and spectral stability are highly desirable for narrowband modulation and sensing, yet achieving these two properties simultaneously remains challenging. Here, we propose a bulk Dirac semimetal metasurface that supports a Fabry–Pérot bound state in the continuum (F-P BIC) arising from doubly degenerate magnetic-dipole modes at the Γ point. By tuning the spacer thickness, the ideal F-P BIC evolves into a quasi-BIC with finite radiative leakage, thereby enabling ultra-narrowband perfect absorption at critical coupling, with the narrowest bandwidth reduced to 1.45 × 10−4 THz. Coupled-mode-theory analysis and full-wave simulations reveal two critical-coupling branches with a total quality factor as high as 1.8 × 104. Electrical tuning of the Fermi energy of the bulk Dirac semimetal modulates the non-radiative loss channel, while a geometric compensation strategy maintains perfect absorption at an almost fixed operating frequency, thereby enabling bandwidth tunability. In addition, the absorber exhibits good angular robustness up to 30°. These results provide a practical route toward high-Q, electrically tunable, and frequency-stabilized terahertz absorbers.