Enhanced Spectral Tunability by Sub-10 nm Nanogaps in Graphene-Metal Hybrid Metasurfaces

Electrically tunable graphene-metal metasurfaces have emerged as a promising platform for precise control of free-space light propagation. However, their resonance tuning range is limited by fabrication constraints, particularly by the achievable gap size between coupled antennas, which is the parameter that influences the device's performance most. In this work, this challenge is addressed by introducing a novel fabrication approach that combines traditional e-beam lithography with physical vapor deposition of an additional thin metal layer and subsequent ion milling. Incorporating an Al₂O₃ etch-stop layer allows to overcome the ≈20 nm gap size limitation of conventional methods. Using this approach sub-10 nm gaps can be fabricated reliably and the tuning range of metasurfaces operating in the mid-infrared is increased from 0.50 to 0.77 µm, together with an enhancement in the maximum modulation depth from 45% to 59%. The better performance is attributed to stronger field enhancement in the reduced nanogap. This work is a critical step toward widely tunable mid-infrared metasurfaces, with potential applications in spatial light modulators, surface-enhanced Raman spectroscopy, and quantum photonics.