Geometric Metasurfaces for Ultrathin Optical Devices

Abstract

occurring materials, it is difficult to further reduce the thickness of optical elements based on such a design theory. [1] Metamaterials are artificially structured materials which attain their properties from the unit structures rather than the constituent materials, [2] so their optical properties such as the effective electric permittivity and the magnetic permeability can be tuned accordingly. [3] To overcome the fabrication difficulties of metamaterials working in the optical range, a 2D metamaterial, or metasurface has been developed, which consists of a layer of optical antennas to locally modify the phase, amplitude, and polarization of the scattered light. [1b,4] Optical antennas are typically made of metals or high refractive index dielectrics, which can be fabricated through standard nanofabrication process, such as electron beam lithography, liftoff process, focused ion beam milling, or reactive ion etching, thus the complexity of the fabrication is greatly reduced in comparison with that for their 3D counterparts. Besides, metasurfaces can change the properties of the scattered light by using antennas with a dimension smaller than the operating wavelength, which exhibits high resolution and can avoid the higher diffraction orders of the traditional diffractive optical devices. In addition, the thickness of the metasurface is much smaller than the incident wavelength which makes them practical for device miniaturization and system integration.Metasurfaces are generally divided into two categories based on their mechanisms: [1a] the one based on the dispersion of antenna resonance and the one based on the Pancharatnam-Berry phase, namely, geometric phase. [5] The former relies on the delicate design of antenna geometry to obtain the desired phase delay of the scattered light. For example, the V-shaped antennas with various arm lengths and opening angles can provide phase gradient to the cross-polarized light, which verifies the generalized laws of reflection and refraction. [6] In contrast, the latter usually contains antennas with the same structure but spatially variant orientations. [7] Each antenna can be regarded as an anisotropic scatterer, which converts part of the incident circularly polarized (CP) light to its opposite helicity with a geometric phase ± 2ψ. The symbol ψ represents the orientation angle of the antenna, with + sign for the conversion from left-handed circularly polarized (LCP) light to righthanded circularly polarized (RCP) light, and the − sign for RCP to LCP light. Thus, a sampled phase function can be imparted to the scattered light from an array of antennas with designed Metasurfaces, planar metamaterials consisting of a single layer or several layers of artificial structures, not only form the basis for fundamental physics research but also have considerable technological significance. Metasurfaces can locally modify the optical property within a subwavelength range, which can facilitate device miniaturization and system integration. Metasurfaces have shown extraordin...

Colorful Metahologram with Independently Controlled Images in Transmission and Reflection Spaces

“…In this paper, by exploiting the polarization properties of geometric phase–based metasurfaces and the off‐axis illumination, we proposed a full‐space imaging metahologram, where the holographic image can be located in both the transmission and reflection spaces. The proposed metaholograms are consisted of locally rotating nanostructures that generate Pancharatnam–Berry (PB) phase (also referred to as geometric phase) according to the rotation angles . The sign of the PB phase is flipped when the incident polarization is changed from left circular polarization (LCP) to right circular polarization (RCP), or vice versa.…”

“…The sign of the PB phase is flipped when the incident polarization is changed from left circular polarization (LCP) to right circular polarization (RCP), or vice versa. This change leads to a 180° rotation of the images with respect to the center of 2D holographic image16b (the holographic image locates at the Fraunhofer region of the sample). Theoretically, the characteristics of the phase flipping and 2D holographic image's rotation are consistent both in transmission and reflection spaces.…”

“…F denotes the fast Fourier‐transform function. If the incident/detected polarizations change from LCP/RCP to RCP/LCP, the 2D holographic image will change from I ( f x , f y ) to I (− f x , − f y ) = | F (exp(−i Φ ( x , y )))| 2 , which means a 180° rotation of the holographic image 16b. Furthermore, when light illuminates on the metahologram with an oblique incidence angle of θ( θ x , θ y , θ z ) ( θ x , θ y , and θ z are defined as the angles between the wavevectors of the incident light and the positive directions of x ‐axis, y ‐axis, and z ‐axis, respectively, as shown in Figure a), the corresponding holographic image in the Fraunhofer region could be depicted asand…”

Empowered Layer Effects and Prominent Properties in Few‐Layer Metasurfaces

“…A minimized camera with total size of 1.6 mm × 1.6 mm × 1.7 mm has been achieved and can be extended as a tunable lens based on microelectromechanical systems . With the further development of integrated few‐layer metasurfaces, they can enter the practical stage like integrated circuits. (2)Giant nonlinear few‐layer metasurfaces: Although metasurfaces can provide unprecedented manipulation over nonlinear generation processes, including amplitude, polarization, and phase, the nonlinear signal generated by single‐layer metasurfaces is still very weak . The reason is that the interaction length between light and nanostructures is too short due to the ultrathin size of the single‐layer metasurfaces.…”

Ultrawideband Spin‐Decoupled Coding Metasurface for Independent Dual‐Channel Wavefront Tailoring

“…However, such metasurfaces comprising subwavelength‐scaled birefringent phasers can only afford distinct wavefront manipulations built with orthogonal linear polarizations, leaving much potentials unexploited because circularly‐polarized waves are more favorable in applications such as satellite communication, biological sensing. For these reasons, geometric phases have been employed to impart spatial phase gradient for orthogonal spins, unlocking many new phenomena and fascinating meta‐devices such as spin Hall effect, meta‐lens, etc. Although the geometric phase metasurface can realize multiple functionalities by using field‐multiplexing techniques, the spatial phase modulations are exactly opposite for excitations of different spins.…”