Tunable Gold-Graphene Metasurface for Beamsteering

Michelle Sherrott Materials Science and Applied Physics, California Institute of Technology, Pasadena, California, USA Victor Brar Materials Science and Applied Physics, California Institute of Technology, Pasadena, California, USA Philip Hon Metamaterials and Nanophotonics Lab, Northrop Grumman Aerospace Corportation, Los Angeles, California, USA Luke Sweatlock Materials Science and Applied Physics, California Institute of Technology, Pasadena, California, USA Metamaterials and Nanophotonics Lab, Northrop Grumman Aerospace Corportation, Los Angeles, California, USA Harry A. Atwater Materials Science and Applied Physics, California Institute of Technology, Pasadena, California, USA

Reconfigurable flat optical components in the mid-infrared (mid-IR) have been a longstanding goal for civil and military applications including holographic displays, beam shaping, and spatial light modulation. However, despite progress in the field, existing approaches are typically limited in speed, efficiency, and size. Graphene-modulated metasurfaces offer the opportunity to overcome these limitations.

Here we will experimentally demonstrate an actively modulated beamsteering device using a metasurface comprised of 2um long gold bowtie antennas on a continuous graphene sheet with a gold back-reflector separated by a 170nm thick SiO₂ layer (see schematic below). We treat the graphene as a controllable dielectric environment which we can actively modulate via electrostatic gating. By electrostatically doping the graphene from its charge neutral point (CNP, E_F=0eV) to E_F=0.55eV, a tunability of reflection phase over 240° is demonstrated at a wavelength of 8um. This allows us a calculated range of beamsteering over an angular range of +/-50° from the normal with a signal:noise exceeding 2:1. We explain this large range of tunability by the widely variable optical conductivity of graphene with charge carrier concentration. In addition, our design exploits the gap between the gold antennas to serve as a Fabry-Perot cavity, confining a graphene plasmon and increasing the local field intensity, resulting in a stronger interaction with the antennae. By individually gating each element in an array of these gold antennas, we create a tunable reflectarray and show that the reflection angle of an incident beam can be modified in real time, with the potential for ultrafast switching times.

Schematic illustration of tunable gold/graphene element

msherrot@caltech.edu