A fundamental strategy is developed to enhance the light-matter interaction of ultra-thin films based on a strong interference effect in planar nanocavities, and overcome the limitation between the optical absorption and film thickness of energy harvesting/conversion materials. This principle is quite general and is applied to explore the spectrally tunable absorption enhancement of various ultra-thin absorptive materials including 2D atomic monolayers.

The recent reported trapped “rainbow” storage of light using metamaterials and plasmonic graded surface gratings has generated considerable interest for on-chip slow light. The potential for controlling the velocity of broadband light in guided photonic structures opens up tremendous opportunities to manipulate light for optical modulation, switching, communication and light-matter interactions. However, previously reported designs for rainbow trapping are generally constrained by inherent difficulties resulting in the limited experimental realization of this intriguing effect. Here we propose a hyperbolic metamaterial structure to realize a highly efficient rainbow trapping effect, which, importantly, is not limited by those severe theoretical constraints required in previously reported insulator-negative-index-insulator, insulator-metal-insulator and metal-insulator-metal waveguide tapers, and therefore representing a significant promise to realize the rainbow trapping structure practically.

Perfect absorbers are important optical/thermal components required by a variety of applications, including photon/thermal-harvesting, thermal energy recycling, and vacuum heat liberation. While there is great interest in achieving highly absorptive materials exhibiting large broadband absorption using optically thick, micro-structured materials, it is still challenging to realize ultra-compact subwavelength absorber for on-chip optical/thermal energy applications. Here we report the experimental realization of an on-chip broadband super absorber structure based on hyperbolic metamaterial waveguide taper array with strong and tunable absorption profile from near-infrared to mid-infrared spectral region. The ability to efficiently produce broadband, highly confined and localized optical fields on a chip is expected to create new regimes of optical/thermal physics, which holds promise for impacting a broad range of energy technologies ranging from photovoltaics, to thin-film thermal absorbers/emitters, to optical-chemical energy harvesting.

A one-step fabrication method has been developed to realize graded holographic photopolymer reflection gratings with gradually varied period in the lateral direction, leading to a rainbow-colored reflection image in the same viewing angle. This low-cost rainbow-colored filter can be integrated with detectors or imaging devices to realize compact and portable spectroscopic analyzers.

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