High‐Efficiency, Extreme‐Numerical‐Aperture Metasurfaces Based on Partial Control of the Phase of Light

Abstract

add phase shifts ranging from 0-2π to the electric field at the interface via nanoscale scatterers. [4] However, refracting light to large angles requires the ability to introduce large phase differences over a short distance. In single-layer metasurfaces, these phase gradients are introduced by varying the resonance phase or the Pancharatnam-Berry (PB) phase. While the former method uses V-shaped plasmonic antenna modes [4,5,11] or Mie-type scattering of high-index dielectric structures, [12,13] the latter employs simple (rod-shaped) plasmonic antennas [14,15] or propagation through dielectric structures [8,16] (see Figure 1a,b). Recently, metasurfaces based on low-loss dielectric structures have demonstrated high anomalous transmission efficiencies. [12,16,17] However, in plasmonic metasurfaces, the required polarization conversion intrinsically limits the corresponding transmission efficiency of these surfaces to below 25%, [18] with the largest measured efficiency of 15% for focusing light. [19] Moreover, in both plasmonic and dielectric surfaces, it is challenging to efficiently introduce large phase gradients for large angle refraction, since the scatterer size and near-field interactions restrict the distance between phase-shifting elements. [20] Diffractive flat optical elements, while functionally similar, have their own working principle and limitations for large angle deflection or focusing (multiple real and virtual focal points, limited efficiency). [21][22][23] In this work, we show how a high degree of wavefront manipulation can be achieved with only a partial control of the phase (i.e., less than 0-2π, see Figure 1c,d). With this relaxed phase requirement, the choice of scatterers is less restrictive. As a result, more compact scatterers, for example, simple nanorods (Figure 1c), with reduced near-field interactions can be used to introduce large phase gradients by placing them at smaller distances from each other. This allows, in principle, accessing phase gradients for anomalous refraction at extreme angles, up to 90° in air or in dielectrics, otherwise not feasible with full phase coverage surfaces. [24] Furthermore, these metasurfaces do not rely on polarization conversion and can generate scattered light with the same polarization as the impinging light (i.e., the co-polarized beam). In contrast to single-layer plasmonic phase gradient surfaces, there is thus no limitation in efficiency due to a polarization conversion. [18] One period of a phase gradient metasurface with partial phase High-quality flat optical elements require efficient light deflection to large angles and over a wide wavelength spectrum. Although phase gradient metasurfaces achieve this by continuously adding phase shifts in the range of 0-2π to the electric field with subwavelength-sized scatterers, their performance is limited by the spatial resolution of phase modulation at the interface. Here, a new class of metasurfaces is introduced, where the phase shifts cover less than the full 0-2π range, offering significant adva...

Optical Metasurfaces: Evolving from Passive to Adaptive

“…Wavefront manipulation with such plasmonic structures will thus induce spurious diffraction orders. However, due to a different working principle, their efficiencies can surpass the theoretical limit of efficiency of plasmonic PB‐phase surfaces . The full phase range (0–2π) can be attained with plasmonic resonant scatterers if a metallic backplane and a separating spacer are used .…”

“…Here, ( x , y ) = (0, 0) corresponds to the optical axis at the center of the lens. Recent demonstrations include metalenses with focusing efficiencies of up to 86% at visible wavelengths, with achromatic focusing over 200 nm of the visible spectrum and metalenses for imaging with extreme numerical apertures of up to 1.4 in immersion configuration (Figure d–f). In addition to beam deflectors or lenses, an arbitrary 2D or 3D electric field distribution representing, for example, an image can be created in space using a designed distribution of amplitude and phase on a metasurface .…”

“…However, due to a different working principle, their efficiencies can surpass the theoretical limit of efficiency of plasmonic PB-phase surfaces. [29][30][31] The full phase range (0-2π) can be attained with plasmonic resonant scatterers if a metallic backplane and a separating spacer are used. [32] A patch antenna metal-insulator-metal (MIM) geometry of this kind creates an effective mirrored scatterer with respect to the backplane, which results in two resonance modes (electric and magnetic resonances) and, therefore, a maximum phase pick up of 2π.…”

“…In contrast to conventional diffractive optical elements, metalenses show no spurious diffraction orders, can achieve a higher efficiency, and do not suffer from effects such as multiple real and virtual focal points. The independent manipulation of different polarizations [272] and the simultaneous realization of multiple functionalities on a single surface, [31,273] which have been demonstrated by metasurfaces, are difficult to achieve with conventional optical elements. In addition to this, the dynamic adaptation of metasurfaces brings with it a number of added benefits as compared to conventional active optical elements.…”

Quadratic Meta-Reflectors Made of HfO2 Nanopillars with a Large Field of View at Infrared Wavelengths

“…However, the conventional configurations of wide-angle-view optical systems are always achieved via a 3D configuration, such as Luneburg lens [ 27 ] and compound eyes [ 20 , 28 ], leading to bulky and expensive systems. Recently, some works are reported on the metalens with large FOVs in the visible near-infrared ranges based on compact nanoscatters [ 29 ] and asymmetric nanodimers [ 30 ]. Moreover, a metallic metasurface with a quadratic phase profile realized a wide range of operation angles [ 31 ].…”