Refractive index sensing in the mid-infrared regime with ice-lithography enabled 2.5-dimensional metasurfaces

Mid-infrared (MIR) refractive index (RI) sensing holds significant potential for applications in chemical detection, environmental monitoring, and biomedical diagnostics due to the strong molecular vibrational fingerprints in this spectral range.However, conventional metasurface-based sensors face challenges in fabrication complexity, toxic solvents, and performance optimization.Here, we introduce ice-lithographed 2.5-dimensional (2.5D) plasmonic metasurfaces featuring vertically asymmetric gold cross-pillar resonators to overcome these challenges.The solvent-free ice lithography enables in situ scanning electron microscopy (SEM) alignment with high precision, residue-free surfaces, and multilayer stacking in a single vacuum process.Simulations reveal that vertically graded pillars (height 0-800 nm) linearly redshift resonance wavelengths while concentrating electric fields at analyte-binding sites, boosting experimentally measured sensitivity from 735 nanometers per refractive index unit (nm/RIU) to 2266 nm/RIU.This work demonstrates a three-dimensional (3D) architectural strategy for enhancing sensing performance, while simultaneously unveiling the potential of ice lithography in fabricating low-toxicity and flexible 2.5D sensing devices.