Subwavelength resolution imaging requires high numerical aperture (NA) lenses, which are bulky and expensive. Metasurfaces allow the miniaturization of conventional refractive optics into planar structures. We show that high-aspect-ratio titanium dioxide metasurfaces can be fabricated and designed as metalenses with NA = 0.8. Diffraction-limited focusing is demonstrated at wavelengths of 405, 532, and 660 nm with corresponding efficiencies of 86, 73, and 66%. The metalenses can resolve nanoscale features separated by subwavelength distances and provide magnification as high as 170×, with image qualities comparable to a state-of-the-art commercial objective. Our results firmly establish that metalenses can have widespread applications in laser-based microscopy, imaging, and spectroscopy.

Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging.

Using a metasurface comprising an array of nanorods with different orientations and a backreflector, a hologram image can be obtained in the visible and near-infrared with limited loss of light intensity.

/pdf/metasurface-holograms-reaching-80-efficiency-19jvjiaray.pdf

Metasurface holograms reaching 80% efficiency

/pdf/a-toll-like-receptor-recognizes-bacterial-dna-2lif1icvoq.pdf

A Toll-like receptor recognizes bacterial DNA

This article reviews recent progress leading to the realization of planar optical components made of a single layer of phase shifting nanostructures. After introducing the principles of planar optics and discussing earlier works on subwavelength diffractive optics, we introduce a classification of metasurfaces based on their different phase mechanisms and profiles and a comparison between plasmonic and dielectric metasurfaces. We place particular emphasis on the recent developments on electric and magnetic field control of light with dielectric nanostructures and highlight the physical mechanisms and designs required for efficient all-dielectric metasurfaces. Practical devices of general interest such as metalenses, beam deflectors, holograms, and polarizing interfaces are discussed, including high-performance metalenses at visible wavelengths. Successful strategies to achieve achromatic response at selected wavelengths and near unity transmission/reflection efficiency are discussed. Dielectric metasurfaces and dispersion management at interfaces open up technology opportunities for applications including wavefront control, lightweight imaging systems, displays, electronic consumer products, and conformable and wearable optics.

Recent advances in planar optics: from plasmonic to dielectric metasurfaces

Metamaterials enable the tailoring of properties like dielectric permittivity and magnetic permeability. Electromagnetic excitations of metamaterial constituents and their interactions are reviewed, as well as promising future directions. Despite the extraordinary degree of interest in optical metamaterials in recent years, the hoped-for devices and applications have, in large part, yet to emerge. It is becoming clear that the first generation of metamaterial-based devices will most probably arise from their two-dimensional equivalents — metasurfaces. In this Review, we describe recent progress in the area of metasurfaces formed from plasmonic meta-atoms. In particular, we approach the subject from the perspective of the fundamental excitations supported by the meta-atoms and the interactions between them. We also identify some areas ripe for future research and indicate likely avenues for future device development.

/pdf/plasmonic-meta-atoms-and-metasurfaces-1jqu72kpik.pdf

Plasmonic meta-atoms and metasurfaces

Holograms, the optical devices to reconstruct predesigned images, show many applications in our daily life. However, applications of hologram are still limited by the constituent materials and therefore their working range is trapped at a particular electromagnetic region. In recent years, the metasurfaces, an array of subwavelength antenna with varying sizes, show the abilities to manipulate the phase of incident electromagnetic wave from visible to microwave frequencies. Here, we present a reflective-type and high-efficiency meta- hologram fabricated by metasurface for visible wavelength. Using gold cross nanoantennas as building blocks to construct our meta-hologram devices with thickness ∼ λ/4, the reconstructed images of meta-hologram show polarization- controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. The flexibility demonstrated here for our meta-hologram paves the road to a wide range of applications related to holographic images at arbitrary electromagnetic wave region.

https://nocweba.ntu.edu.sg/laq_mems/journal%20papers/2014/High-Efficiency%20Broadband%20Meta-Hologram%20with%20Polarization-Controlled%20Dual%20Images.pdf

High-Efficiency Broadband Meta-Hologram with Polarization-Controlled Dual Images

Toroidal dipole moments, the third kind of fundamental dipole moment, have unusual electromagnetic properties
different from the electric and magnetic multipoles. We fabricate a new type of 3D plasmonic toroidal metamaterial by
using mutual coupling between dumbbell-shaped gold apertures with vertical split-ring resonators (VSRRs) at optical
frequency. The radiated power of multipole moments are calculated and analyzed to improve the meta-system is
dominated by the toroidal dipole moment. This result paves a way for practical application on metamaterial based
devices, such as biosensor and lasing spaser.

Optical toroidal response in three-dimensional plasmonic metamaterial

Holograms are the optical devices to reconstruct images by recovering amplitude and phase of light, which show many applications in our daily life. Recently, the metasurfaces, an array of sub-wavelength antenna, show the abilities to manipulate both the amplitude and phase of incident electromagnetic wave in a wide electromagnetic region from visible to microwave frequencies. Here, we realized a high-efficiency broadband plasmonic metasurface and metahologram by reflected metasurface made of subwavelength gold nanoantennas which is designed and fabricated in optical frequencies. The reconstructed images of our metahologram show polarization-controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. Aluminum plasmonics based metaholograms with multi-images, multi-channels, or multiplexing at different wavelengths, on the other hand, prefer narrower plasmonic resonances and less phase levels to realize multi-color metahologram. By combining with the techniques of tunable metasurfaces, metahologram can potentially be used to realize active hologram that works at arbitrary electromagnetic wave region.

Hao Tsun Lin

Papers

High-Efficiency Broadband Meta-Hologram with Polarization-Controlled Dual Images

Optical toroidal response in three-dimensional plasmonic metamaterial

Reflective plasmonic metasurface and metahologram