High-speed optical modulation is crucial for various applications, including optical interconnects, ultrafast molecular spectroscopy, material processing, and optical information processing. All-optical modulation offers the highest modulation bandwidth (up to THz) compared to other techniques. Existing all-optical modulators utilize various materials and designs, such as semiconductor waveguides, plasmonic nanocrystals, silicon nanoantennas, and graphene-based structures. Most operate in the visible and near-infrared (NIR) range, while ultrafast modulation for mid-and far-infrared wavelengths is highly desirable for applications like ultrafast molecular spectroscopy, space communication, remote sensing, biomedical diagnostics, and astronomy. Achieving ultrafast all-optical modulators for wavelengths >6 µm remains challenging due to material limitations. Graphene is a promising material due to its ultrafast carrier relaxation (sub-picosecond) and broad spectral coverage. However, graphene-based modulators face challenges like limited absorption and high pump fluence requirements. Strategies to enhance light-graphene interaction include integration with dielectric waveguides, microfibers, cavities, and plasmonic slot waveguides. This paper addresses the challenge of realizing ultrafast all-optical modulation with low pump fluences, particularly for mid-IR wavelengths, by using subwavelength-thick graphene-plasmonic hybrid metasurface structures for near-IR and mid-IR wavelengths beyond 6 µm. The integration of graphene with plasmonic metasurface absorbers enhances light-graphene interaction at both pump and probe wavelengths. This leads to improved all-optical modulation due to increased photocarrier density in nanoscale hotspots and enhanced sensitivity of the device response to graphene optical properties.

The paper reviews existing all-optical modulators based on various materials and designs, highlighting their limitations, especially at mid- and far-infrared wavelengths. It emphasizes the potential of graphene for ultrafast all-optical modulation due to its unique properties but notes the challenges posed by limited absorption and the resulting need for high pump fluences. Previous attempts to enhance light-graphene interaction using various techniques are discussed, setting the stage for the proposed graphene-metal hybrid metasurface approach.

The proposed device consists of a plasmonic metasurface, a graphene layer, a metallic back reflector, and a dielectric spacer layer. Closely coupled optical antennas in the metasurface maximize light-graphene interaction at both pump and probe wavelengths. A metallic back reflector forms a tunable graphene-metallic metasurface absorber (GMMA) to further enhance modulation. Full-wave simulations demonstrate significant near-field intensity enhancement at both pump and probe wavelengths in the nanogaps between coupled nanoantennas. The simulations show a one-order-of-magnitude improvement in graphene absorption compared to suspended monolayer graphene. A two-temperature model (TTM) simulates photocarrier dynamics and thermal relaxation. The TTM simulates the increase in electron temperature upon pump pulse incidence, highlighting the significantly larger temperature modulation in the GMMA device compared to devices without plasmonic antennas. The real and imaginary parts of graphene surface conductivity are calculated using the random phase approximation (RPA) theory. Full-wave simulations show the reflection spectra and differential reflection at various hot-electron temperatures, demonstrating the blue shift of the resonance wavelength upon pump pulse incidence. The response time is determined by ultrafast photocarrier dynamics in graphene, with simulations showing a modulation depth of ~50 (or 5000%). For experimental validation, GMMA devices were fabricated on silicon substrates using electron beam evaporation, atomic layer deposition (ALD), electron beam lithography (EBL), and a wet transfer process for graphene. Reflection spectra were measured using an FTIR spectrometer coupled with a mid-IR microscope, confirming the designed functionalities.

The study demonstrates high-speed all-optical modulators at near- and mid-infrared wavelengths with significantly reduced pump fluence (1-2 orders of magnitude) compared to previous graphene-based modulators. The use of graphene-metal hybrid plasmonic metasurfaces leads to enhanced light-graphene interaction, resulting in a much larger photocarrier density and stronger modulation effect. Ultrafast pump-probe measurements confirm that the response times are on the picosecond scale, ultimately limited by graphene's ultrafast photocarrier relaxation times. The mid-IR modulator represents, to the authors' knowledge, the first experimental demonstration of a high-speed all-optical modulator for wavelengths beyond 6 µm with a pump fluence of <70 µJ cm². Full-wave simulations support the experimental findings, showing significant near-field intensity enhancement, absorption enhancement, and modulation depth. The fabricated devices exhibit absorption and reflection spectra consistent with the simulations, confirming the effectiveness of the designed structure. The observed bi-exponential decay times in the transient response are consistent with the ultrafast photocarrier dynamics in graphene.

The findings demonstrate the successful realization of ultrafast all-optical modulators with significantly improved performance compared to existing graphene-based devices. The use of graphene-metal hybrid plasmonic metasurfaces effectively addresses the limitations of low absorption and high pump fluence requirements in graphene-based all-optical modulators. The achieved modulation speed and low pump fluence are highly significant for advancing applications requiring high-speed modulation at mid- and far-infrared wavelengths. The picosecond response time validates the potential of graphene for ultrafast optical modulation applications. The results open up possibilities for developing high-performance all-optical devices for various applications.

This paper demonstrates ultrafast all-optical modulators based on graphene-metal hybrid metasurfaces, achieving significantly reduced pump fluence and enhanced modulation at near- and mid-infrared wavelengths. The success validates the design concept and its potential for realizing ultrafast modulators for mid-to far-infrared spectral regions. Future research could explore optimizing the metasurface design for even broader bandwidth and higher modulation depth, as well as investigating other materials for further performance enhancement.

The study focuses on specific designs and wavelengths. Further investigation is needed to explore the scalability and adaptability of the proposed design to different wavelengths and operating conditions. The two-temperature model used in the simulations simplifies the complex photocarrier dynamics in graphene, neglecting certain aspects like non-thermal carrier populations and hot-carrier transport. The fabrication process, while successful, could be further optimized to improve yield and uniformity.