Real-time electrochemical tuning of ultrahigh index mid-infrared metamaterials

Resonant mid-infrared (MIR) plasmonic metamaterials strongly enhance light at their surfaces, enabling sensitive analyte detection via surface-enhanced IR absorption (SEIRA). However, exploiting their geometric sensitivity for resonance tuning has so far not yielded large real-time MIR modulation. Here, the MIR resonance of close-packed colloidal metamaterials is tuned in real-time to extremely high values of broadband refractive index n ≥ 6, with resonance spectral shifts exceeding 40% thus enabling controllable SEIRA optimization. By exploiting the unique surface chemistry of chloroauric acid, we develop reproducible wide tuning by manipulating the facets of gold nanoparticles. The resulting modulation of ultrahigh refractive index metamaterials opens advances in MIR waveguiding and sensing.