Broadband high-efficiency dielectric metasurfaces for the visible spectrum

Abstract

Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Original dielectric metasurfaces are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Thus, it is critical that new materials and nanofabrication techniques be developed to extend dielectric metasurfaces across the visible spectrum and to enable applications such as high numerical aperture lenses, color holograms, and wearable optics. Here, we demonstrate high performance dielectric metasurfaces in the form of holograms for red, green, and blue wavelengths with record absolute efficiency (>78%). We use atomic layer deposition of amorphous titanium dioxide with surface roughness less than 1 nm and negligible optical loss. We use a process for fabricating dielectric metasurfaces that allows us to produce anisotropic, subwavelength-spaced dielectric nanostructures with shape birefringence. This process is capable of realizing any high-efficiency metasurface optical element, e.g., metalenses and axicons.devices composed of subwavelength-spaced units and near-flat profiles compared with refractive optics (7-9)-have allowed unprecedented control over optical wavefronts (5) while circumventing Ohmic losses associated with plasmonic metasurfaces. Due to the scientific and technological importance of visible wavelengths, there has been increasing effort to realize DMs at these wavelengths. For example, silicon nitride DMs have been realized at a red wavelength (λ = 633 nm) (10), scattering properties of coupled TiO 2 resonators (11) have been examined, and TiO 2 binary diffraction gratings have been fabricated via top-down etching (12). However, no current DM implementation has been capable of providing arbitrary phase control of an optical wavefront while maintaining high efficiency across the entire visible spectrum (especially at blue and green wavelengths). Furthermore, the typical top-down techniques used to implement these metasurfaces can introduce significant surface roughness and make it difficult to create subwavelength sampling of a desired optical phase profile.In this paper, we demonstrate amorphous TiO 2 metasurfaces that maintain high efficiency across the entire visible spectrum. Our approach to creating DMs uses a bottom-up nanofabrication via atomic layer deposition providing high-aspect ratio, anisotropic dielectric nanostructures with minimal surface roughness. As proof of the concept that we can provide control of the phase of a wavefront from 0 to 2π, a requirement for many optical components, we produced metasurface holograms based on geometric phase. Efficient metasurfaces with metallic components operating in reflection have been demonstrated at red and nearinfrared wavelengths (13, 14) but have efficiencies of <1% and <10% at blue and green wavelengths, respectively (15, 16). Thus, the TiO 2 metasurfaces we demonstrate here provide substantial improvement with efficiencies fro...

Metasurface Polarization Optics: Independent Phase Control of Arbitrary Orthogonal States of Polarization

“…This waveplate-like behavior could be realized with, for example, plasmonic antennas [1], liquid crystals [3] (which, due to their size cannot truly be considered metasurface elements), or waveguide-like dielectric pillars exhibiting mode-birefringence fabricated from Si [4][5][6], GaAs [7], or TiO 2 [8,9] with, e.g., elliptical or rectangular crosssection.…”

“…4 [16]. Fabricated with a recently reported TiO 2 process on glass [8], the pillars were arranged in a square lattice with 500 nm nearest-neighbor separation (Fig. 2b-c).…”

Coherent control of high efficiency metasurface beam deflectors with a back partial reflector

“…This relationship can be modified by introducing patterned dielectric/metal nanofilms at the boundary. 1 By properly designing these patterns as in a passive phased array antenna (namely, "metasurfaces (MSs)"), the nanofilms can be used to tailor the wavefront of the input light and obtain myriad functions such as beam-deflection/splitting depending on the polarization, 1-3 polarization conversion and generation, [4][5][6] holograms, 4,7,8 achromatic focusing lenses, [9][10][11][12] optical angular momentum beam conversion, 1,13 and pulse shaping. 14 However, in the case of beam deflectors, the beam deflection efficiency was somewhat low for the initial proposed design; only a small part of the input light (∼10%) can couple to the MS if there is only a single layer with a conventional MS structure on a dielectric substrate.…”

“…14 However, in the case of beam deflectors, the beam deflection efficiency was somewhat low for the initial proposed design; only a small part of the input light (∼10%) can couple to the MS if there is only a single layer with a conventional MS structure on a dielectric substrate. 1,16 To overcome this, high efficiency MSs have recently been proposed including MSs with an optically-thick back reflector (BR), 2,3 multiple-layered MSs, 17 high aspect ratio all-dielectric MSs, 4,8,12,13 Huygens' MSs, 18 and their combinations. 19 Of these approaches, introducing a BR is the simplest way to obtain the highest beam deflection efficiency (80%-90%) in the broadband regime.…”

Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays

“…In general, metasurfaces can be divided into two categories: plasmonic metallic metasurfaces56 and all-dielectric metasurfaces78. Recently, low-loss dielectric metasurfaces have been proposed to achieve a transmissive flat lens by using titanium dioxide (TiO 2 ) in the visible spectrum2122. As one of the most fundamental components in the field of optics, these metasurfaces-based planar focusing lens breaks the thickness limit of conventional lens.…”