Fabrication Optimization of van der Waals Metasurfaces: Inverse Patterning Boosts Resonance Quality Factor

Abstract: Van der Waals (vdW) materials have garnered growing interest for use as nanophotonic building blocks that offer precise control over light-matter interaction at the nanoscale, such as optical metasurfaces hosting sharp quasi-bound states in the continuum resonances. However, traditional fabrication strategies often rely on lift-off processes, which inherently introduce imperfections in resonator shape and size distribution, ultimately limiting the resonance performance. Here, an optimized fabrication approach for vdW-metasurfaces is presented that implements inverse patterning of the etching mask, resulting in increased resonator quality solely limited by the resolution of the electron beam lithography resist and etching. Applying this inverse fabrication technique on hexagonal boron nitride (hBN), quality (Q) factors exceeding $10^3$ in the visible spectral range were demonstrated, significantly surpassing previous results shown by lift-off fabricated structures. Additionally, the platforms potential as a biosensor was displayed, achieving competitive sensitivity and figure of merit of 220 in a refractive index sensing experiment. The inverse technique was applied to create chiral metasurfaces from hBN, using a two-height resonator geometry to achieve up to 50 % transmittance selectivity. This inverse lithography technique paves the way towards high-performances vdW-devices with high-Q resonances, establishing hBN as a cornerstone for next-generation nanophotonic and optoelectronic devices.