Electrically Tunable Graphene-Metal Hybrid Metasurfaces for High-Efficiency, Broadband Mid-Infrared Transmission Modulation.

Transmissive active metasurfaces are essential for ultra-compact on-chip optical systems and are widely used in sensing, imaging, and spectroscopy. However, these devices generally suffer from lower efficiency and greater design complexity compared to their reflective counterparts. In this work, an electrically tunable transmission filter based on single-layer graphene integrated with a capacitive gold grating on a mid-IR transparent CaF2 substrate is demonstrated. By employing an ionic gel gating scheme to induce a high Fermi level in graphene, the device achieves a high transmission modulation efficiency of 73.3% for TM-polarized light, along with a broad operational bandwidth of 742 cm-1. Furthermore, by modifying the device structure, an ultrawide operational bandwidth of 1949 cm-1 can be achieved at the expense of reducing the modulation efficiency to 44%, outperforming previously reported electrically tunable mid-IR transmissive metasurfaces. Equivalent circuit model analysis reveals that this extraordinary modulation and bandwidth arise from the dynamic formation of a parallel LC resonance between the tunable inductive response of highly doped graphene and the metal grating's capacitive response. Furthermore, leveraging the spectral diversity generated by this Fermi-level modulation, the platform is computationally demonstrated to operate as a single-pixel mid-IR spectrometer. These results provide a platform for ultra-compact mid-IR applications.