An Innovative method for adjustable broadband THz to Mid-IR optical modulator using Graphene Gratings surface plasmon Fabry–Perot resonances with low insertion loss, high speed and modulation depth

Abstract

Through numerical modeling, we present an ultrasensitive tunable Terahertz (THz) to Mid-IR modulator composed of single-layer graphene-based gratings combined with a Fabry–Perot (FP) cavity. The perpendicular-polarized incident light on gratings causes the grating edges to act as an electrical dipole, resulting in an FP interferometer of induced plasmonic waves on the graphene surface. The electric field in the cavity is substantially limited by the activation of standing-waves (SWs) graphene surface plasmon polaritons. The multi-mode SWs formation inside the gratings provides an excellent platform for THz to Mid-IR light modulation with promising features. We have demonstrated a modulation depth of 100% in desired frequencies with near zero insertion loss via full-wave electromagnetic simulations at the normal incident in the THz to Mid-IR range. The light reflection and absorption of the proposed structure have a multi-band profile due to the FP behavior. Therefore, it can be easily tuned by electrically directing the chemical potential of graphene. Due to multi-resonance-based reflected light, each resonance represents different behavior with different modulation parameters. In other words, not only any desired frequency can be chosen, but also at the constant frequency of interest; by choosing the appropriate mode, the desired linewidth, FWHM, and modulation depth can be easily obtained. Finally, we investigated the device bandwidth. The THz modulator requires a large device area for an average number of gratings, and the device area at the 3 dB frequency reaches as high as 1.5 THz, which depicts the high modulation speed in the proposed modulator. The proposed graphene plasmonic grating scheme is a promising candidate for developing the on-chip integrated THz to Mid-IR optical telecommunications. Moreover, the perfect absorption of graphene opens an opportunity to design photodetectors with high responsivity and quantum efficiency, which gives the proposed structure a multi-function capability.