Efficient Mid‐Infrared Light Confinement within Sub‐5‐nm Gaps for Extreme Field Enhancement

Optical field can be concentrated into deep‐subwavelength volumes and realize significant localized‐field enhancement (so called “hot spot”) using metallic nanostructures. It is generally believed that smaller gaps between metallic nanopatterns will result in stronger localized field due to optically driven free electrons coupled across the gap. However, it is challenging to squeeze light into extreme dimensions with high efficiencies mainly due to the conventional optical diffraction limit. Here a metamaterial super absorber structure is reported with sub‐5 nm gaps fabricated using atomic layer deposition processes that can trap light efficiently within these extreme volumes. Light trapping efficiencies up to 81% are experimentally demonstrated at mid‐infrared wavelengths. Importantly, the strong localized field supported in these nanogap super absorbing metamaterial patterns can significantly enhance light–matter interaction at the nanoscale, which will enable the development of novel on‐chip energy harvesting/conversion, and surface enhanced spectroscopy techniques for bio/chemical sensing. By coating these structures with chemical/biological molecules, it is successfully demonstrated that the fingerprints of molecules in the mid‐infrared absorption spectroscopy are enhanced significantly with the enhancement factor up to 10⁶–10⁷, representing a record for surface enhanced infrared absorption spectroscopy.