Responsive Visible-Wavelength Metasurfaces from Porous Silicon Based on Fano Guided-Mode Resonance

Subwavelength metasurfaces with responsive optical properties in the visible spectrum are of growing interest for applications in dynamic structural color and sensing. Introducing porosity into these structures is a promising route to enhance environmental and surface sensitivity, yet realizing deeply subwavelength porosity through conventional top-down fabrication remains challenging due to limitations in achieving high aspect ratio nanoscale features. Here, mesoporous silicon metasurfaces (pSi-MSs) supporting Fano guided-mode resonances are reported, realized via a straightforward hybrid top-down patterning and bottom-up porosification approach. The resulting structures exhibit tunable resonances across the visible spectrum, vivid structural coloration, and dynamic color changes in response to environmental stimuli, including vapor and liquid interactions. By integrating laser illumination, colorimetric contrast for real-time detection is further enhanced. Finally, label-free refractive index sensing and real-time small molecule detection are demonstrated using an integrated differential sensing design with microfluidic pSi-MSs. These results establish the pSi-MS as a versatile platform for responsive photonic devices, dynamic color modulation, and surface-based chemical and biological sensing. More broadly, these results highlight the potential of hybrid top-down and bottom-up fabrication strategies for enabling the next generation of dynamic and environmentally responsive metasurfaces.