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

Abstract

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 we report a metamaterial super absorber structure with sub-5-nanometer 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 midinfrared 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, we successfully demonstrated that the fingerprints of molecules in the mid-infrared absorption spectroscopy is enhanced significantly with the enhancement factor up to 10 6~1 0 7 , representing a record for surface enhanced infrared absorption spectroscopy.Due to the diffraction limit of conventional optics, coupling and confinement of light into deepsubwavelength volume is usually very challenging, resulting in difficulties in exploring the lightmatter interaction within these ultra-thin (one-dimensional, 1D) or ultra-small dimensions (two-or three-dimensional, 2D or 3D). The unprecedented ability of metallic nanostructures with nanometric gaps to concentrate light has attracted significant research interest in recent years. [1,2] It has been reported that the optical field can be concentrated into deep-subwavelength volumes and realize This article is protected by copyright. All rights reserved. significant localized-field enhancement using a variety of nanoantenna structures, [3] showing promise for the development of enhanced nonlinear optics, [4] surface photocatalysis [5,6] and

Sub‐5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

“…Cross‐sectional SEM images of a corrugated MDM structure with a 5 nm gap. Reproduced with permission . Copyright 2017, WILEY‐VCH Verlag GmbH & Co. d) Scheme for fabricating graphene‐templated gaps between gold nanowires using ultramicrotomy.…”

“…In addition to ion milling, another low‐cost technique has been employed, using adhesive tape to remove the second layer metal to form nanometer‐sized gaps . As illustrated in the top panel of Figure c, after patterning the vertical sidewall first, nanogaps are exposed by partially peeling off the second metal layer that coated on the ALD layer.…”

“…If the second metal layer is a continuous film, then the three‐layered nanopattern (i.e., the first predesigned nanopattern of the ALD layer and the second metal coating) will be peeled off together from the substrate. A periodic metal–dielectric–metal (MDM) nanostructure in which each pair of nanometer gaps was connected using planar ALD layers was massively fabricated using atomic layer lithography . Furthermore, 3D split‐wedge antennas with gaps as small as 1 nm were also produced over an entire wafer by combining template stripping and atomic layer lithography .…”

“…In addition, interior gapped core–shell nanoparticles have proven to be a good sensing platform for drug delivery . Besides SERS, plasmonic metal nanogaps have also been employed in SERIA spectroscopy . As presented in Figure c, Dong et al produced a bowtie‐shaped Au structure with a sub‐3 nm gap for SERIA spectroscopy to quantitatively analyze chemical components .…”

“…Absorption spectra of three devices with different gap sizes from 3 to 6 nm. Reproduced with permission . Copyright 2017, WILEY‐VCH Verlag GmbH & Co. c) Optical microscopy image of a visible and near‐IR (NIR) device based on rectangular silver nanogaps.…”