A mid-IR tunable graphene metasurface for ultrasensitive molecular fingerprint retrieval and refractive index sensing

The enhancement of light-matter interactions based on the resonance of metasurfaces provides a promising opportunity for the identification of various molecules. However, there are still challenges in device miniaturization and trace detection owing to the lack of tunability and deficient sensitivity. Herein, we propose a tunable metamaterial sensor based on graphene nanodisk array operating in the mid-infrared (mid-IR) region with a broadband response for ultrasensitive fingerprint retrieval and refractive index sensing. The designed device has outstanding polarization insensitivity and incident angle stability. The resonance position can be dynamically modulated by tuning the graphene Fermi level, resulting in a series of sharp resonances provided by the dipolar mode of graphene nanodisks. The configuration assigns the metasurface as a metapixel to the corresponding responsive region, establishing a one-to-many mapping to visualize the response of the metamaterial sensor. This technique allows molecular detection without using complex spectral scanners and greatly reduces the device footprints. We illustrate the trace molecular detection capability by overlaying a recombinant protein A/G monolayer, and the designed sensor provides 10⁹ times higher absorbance signal. The sensor has excellent refractive index sensing performance with a sensitivity up to 326 cm⁻¹ per RIU (refractive index unit, RIU), quality factor (Q) of >160, and figure of merit (FOM) of >33. Our proposed technique provides new ideas for fast-designed and miniaturized devices in the function multiplexing of molecular fingerprint detection and refractive index sensing.