In this paper, we propose a high-sensitivity optical sensor at terahertz frequencies based on a composite structure containing a one-dimensional photonic crystal (1D PC) coated with a layer of monolayer graphene. Between the 1D PC and the graphene there is a sensing medium. This high-sensitivity phenomenon originates from the excitation of optical resonance between the graphene and the 1D PC. The proposed sensor is highly sensitive to the Fermi energy of graphene, the thickness and refractive index of the sensing medium, and the number of graphene layers. By selecting appropriate parameters, the maximum sensitivity ( ${407.36}^{\circ }/\mathrm{RIU}$ ) is obtained. We believe the proposed configuration is promising for fabricating graphene-based biosensor- or gas-sensor devices and other related applications in the terahertz band.

Group delay of the optical pulse reflected from the structure composed of graphene and one dimensional photonic crystal (1D PC) is studied. It is shown that the large negative group delay in this configuration can be realized due to the excitation of the optical Tamm state (OTS) at a graphene-dielectric interface. The reflected group delay can be enlarged negatively and can be switched from positive to negative or vice versa. We show that the conductivity of graphene has a significant influence on reflected group delay, making the graphene-PC structure be a good candidate for dynamic tunable optical delay device in the THz frequencies. Furthermore, the influences of the relaxation time of graphene, the incident angel, and the dispersion characteristics of top layer on group delay are clarified.