Free-Space Mid-Infrared Complex Modulation by Dual-Channel Graphene Nanoribbon Metasurface

The mid-infrared (MIR) region offers numerous advantages, such as the atmospheric window (8~13 μm), unique molecular fingerprinting, and thermal emission. However, the development of the devices operating in the MIR is hindered by the unexplored optical characteristics of many materials, with much research efforts focusing instead on the visible and near-infrared regions contributing to displays and telecommunications technologies. Recently, numerous papers regarding molecular sensing techniques and thermal emitters have been presented, harnessing molecular vibrations and quasiparticle resonances [1]. Despite their impressive demonstrations and functionalities, a lack of high-degree light modulation capabilities still limits the level of light-matter interaction, resulting in medium efficiencies. Free-space MIR wavefront shaping, with a high amplitude modulation and a wide coverage of phase, could address these challenges.<br/>Graphene, the most renowned 2D material, possesses remarkable thermal, electrical, and mechanical properties. Graphene nanoribbons (GNRs), created by patterning graphene into nanoscale strips, enable enhanced light confinement with optimized configurations [2]. Furthermore, electrostatic doping can modulate the carrier density in GNRs, tailoring the Fermi level (<i>E_F</i>) and the intensity of graphene plasmons. While GNRs offer versatility in nanophotonic fields, the complexity of sophisticated fabrication requirements stunts the widespread application of GNRs-based optical modulators.<br/>Here, we present an active metasurface using dual-channel GNRs for MIR complex modulation. The graphene plasmon (~<i>λ₀</i>/100) in GNRs is much smaller than that of MIR wavelengths, leading us to adopt a multiscale coupling architecture to maximize optical confinement. The optimized structural parameters of metallic scatters on the GNR monolayer improve the optical coupling efficiency between graphene plasmons and free-space photons via intermediate quasiparticle resonances. Owing to their subwavelength structure, we were able to design the nanophotonic structure with near-uniform amplitude and wide phase coverage using a surface admittance model. When the effective surface admittance (<i>Y_sur</i>) matches the substrate admittance (<i>Y_sub</i>), the center of the trajectory of the reconstructed reflection coefficient (<i><u>r</u></i>) with regards to <i>E_F</i> becomes the origin of <i>r</i>-space. Additionally, we employed a genetic algorithm to optimize six structural parameters—<i>w_Au</i>, <i>w_Gr</i>, <i>t_Au</i>, <i>t_Ti</i>, <i>t_SiO2</i>, and <i>gap</i>—achieving phase modulation over 2π with independent amplitude modulation. The separated voltage control of the dual-channel GNRs below the ion-gel modulates the <i>E_F</i> in each channel, providing high-speed tunability in <i>r</i>-space.<br/>In this study, we have presented optimization of the free-space MIR complex modulation with dual-channel GNRs with the surface admittance model and genetic algorithms and our attempt to experimentally demonstrate it. The independently poled array of graphene nanostrips and gold top electrodes will offer wide coverage and enhanced functionality for next-generation optical modulators. The fabrication process, involving focused ion beam (FIB) and e-beam lithography (EBL), facilitates creating of the nanoscale structure with high reproducibility. The showcased MIR complex modulation platform also can be expanded to MIR beam splitters and directive thermal emitters, potentially paving the way for real-world applications including medical imaging and quantum communications.<br/><br/><b>References</b><br/>[1] Chikkaraddy, R., Arul, R., Jakob, L.A. et al. Single-molecule mid-infrared spectroscopy and detection through vibrationally assisted luminescence. <i>Nat. Photon.</i> <b>17</b>, 865–871 (2023).<br/>[2] Brar, V. W., Jang, M. S., Sherrott, M., Lopez, J. J. & Atwater, H. A. Highly confined tunable mid-infrared plasmonics in graphene nanoresonators. <i>Nano Lett.</i> <b>13</b>, 2541–2547 (2013).