▎ 摘　　要

We propose an ultrasensitive tunable terahertz (THz) sensor which consists of single-layer graphene-based gratings integrated with a Fabry-Perot cavity through numerical simulation. The excitation of standing-wave graphene surface plasmon polaritons can make the probing field strongly confined in a gap cavity. This provides an excellent platform for ultrasensitive and high figure-of-merit (FOM) refractive index sensing at the THz band. Our full-wave electromagnetic simulations show a frequency sensitivity as high as 4.2 THz per refractive index unit and an FOM beyond 12.5 at normal incidence in the THz regime. At oblique incidence, the operation angle for effective THz sensing can be maintained as wide as 75 degrees without inducing degeneration of the sensitivity and FOM. The sensing performance can be readily tuned by electrically controlling the chemical potential of graphene. The proposed graphene plasmonic grating scheme is a promising candidate for the development of on-chip integrated THz biochemical sensors.