Beam‐Editing Coding Metasurfaces Based on Polarization Bit and Orbital‐Angular‐Momentum‐Mode Bit

191

107

introducing abrupt phase changes via an ultrathin sheet of subwavelength resonators (usually known as meta-atoms). The first such metasurface was proposed by Yu and Capasso [2] Afterward, significant number of novel metasurfaces have been proposed in microwave, [3] terahertz, [4] near-infrared, [5] and visible ranges. [6] These metasurfaces are utilized for realizing many applications such as focusing with subwavelength planar lenses, [7] optical cloaking, [8,9] generating nondiffracting beams, [10] manipulating phase profiles having orbital angular momentum (OAM), [11] photon spin Hall effect, [12] and performing computation with the coding metasurfaces. [13,14] Although metasurfaces have proved their unique freedom in wavefront manipulation, most metasurfaces generally suffer from low efficiencies. This is especially critical for transmissive metasurfaces where both the magnetic and electric resonances should be precisely controlled. For example, the efficiency of the first ever demonstrated metasurface was only 5%. [2] A few transmissive metasurfaces have since been introduced with higher efficiencies based on the Huygens' principle [15,16] or meta-transmit arrays. [17,18] However, the operation of these metasurfaces is severely restricted, e.g., may require specific polarization, added complexity in manufacturing, or demand complex, costly computations for each distinct phase.Recently, it has been demonstrated that metasurfaces can be realized based on the photon spin Hall effect. [19][20][21][22] Spin Hall effect is a phenomenon where moving electrons with opposite spins can be transversely separated. Spin Hall effect can be intrinsic based on spin-orbit coupling of electrons [23] or extrinsic due to the spin-dependent scatterings by impurities. [24] Both of these could be utilized to achieve extraordinary wave propagation. It is known that the experimentally observed intrinsic phenomena are usually very weak. [23] On the contrary, extrinsic spin Hall is observed in a special class of metasurfaces, [12] which utilizes subwavelength scatterers to achieve full phase control in the range of 0-2 π. The transmission/ reflection phase control of a cross-polarized wave is achievable through the rotation of the designed scatterers, which in fact make it more promising than the other types of meta-atoms that require phase dispersions to achieve phase control. The maximum achievable efficiency of such metasurfaces was theoretically predicted to be 25% in a single layer structure since only electric responses can be realized in such structures. [25,26] This limitation was overcome by designing reflective metasurfaces with ground planes, [27][28][29] where both the electric and the Metasurfaces offer unprecedented freedom to manipulate electromagnetic waves at deeply subwavelength scales. However, realizing a highly efficient metasurface, yet simple enough to conceptualize, design, and fabricate, is a challenging task. In this paper, a novel approach is proposed for designing meta-atoms which can achieve full phase cont...

Section: Introductionmentioning

confidence: 99%

High Efficiency Ultrathin Transmissive Metasurfaces

Akram

Mehmood

Bai

et al. 2019

191

107

“…In contrast to the bulky optical components with wavelength‐dependent phase shift based on optical path difference, in the Pancharatnam–Berry phase optical elements (PBOEs), the PB phase in broadband operation originating from the geometric phase and accompanied polarization conversion is introduced . The PBOE‐based metasurfaces have been widely designed for wavefront shaping applications in making on‐chip structured beam generators, flat lenses, ultrathin wave plates, and holograms …”

Section: Introductionmentioning

confidence: 99%

Generating Focused 3D Perfect Vortex Beams By Plasmonic Metasurfaces

Zhang

Liu

Gao

et al. 2018

136

from the Fourier transformation of the Bessel-Gauss (BG) beam, [13,14] by utilizing a series of free-space bulky optical components including axicon, spiral phase plate, Fourier transform lens, and spatial light modulator. This fact greatly increases the optical system complexity and also hinders the device integration in OAM-based photonic circuits.Recently, plasmonic metasurfaces made of subwavelength nanoantenna arrays in thin metallic films have provided a powerful and efficient platform for manipulating the intensity, wavefront, and polarization of light beams beyond the diffraction limit. [26,27] In contrast to the bulky optical components with wavelengthdependent phase shift based on optical path difference, in the Pancharatnam-Berry phase optical elements (PBOEs), the PB phase in broadband operation originating from the geometric phase and accompanied polarization conversion is introduced. [28][29][30][31][32][33][34] The PBOE-based metasurfaces have been widely designed for wavefront shaping applications in making on-chip structured beam generators, [35][36][37][38][39][40][41][42][43][44][45] flat lenses, [46][47][48][49] ultrathin wave plates, [50][51][52] and holograms. [53][54][55][56][57] In this work, we present PBOE-based plasmonic metasurfaces made of rectangular-hole nanoantenna arrays as integrated optical beam converters for the direct generation of the focused 3D PV beams, without using bulky optical components, such as axicon, lens, and spatial light modulator. The PB phase profiles encoded on metasurfaces are uniquely designed with the combined phase distributions of axicon, spiral phase plate, and Fourier transform lens. The single ultrathin plasmonic metasurface device with compact area of 50 µm × 50 µm can create 3D PV beam in a long propagation distance and operate in a broad wavelength range from 600 to 1000 nm, which is useful in wavelength-multiplexed OAM-based optical fiber communication. For comparison, the previously demonstrated PB phase element system used to produce PV beams contains three in-sequence metasurface plates with efficient diameters ≥6 mm fabricated in glass boards, with the single operating wavelength at 632.8 nm. [13] It is shown that the obtained PV beams have the almost constant vortex ring radius and the same beam divergence for different TCs. We also demonstrate that the PV beam structures can be easily adjusted by varying several control parameters in the metasurface design including axicon period, lens focal length, and operation wavelength. Furthermore, multiple PV beams with Perfect vortex (PV) beams possessing annular intensity profiles independent of topological charges promise significant advances in particle manipulation, fiber communication, and quantum optics. The PV beam is typically generated from the Fourier transformation of the Bessel-Gauss beam. However, the conventional method to produce PV beams requires a series of bulky optical components, which greatly increases the system complexity and also hinders the photonic device integration. Here, ...

“…Because digital metasurfaces introduce the information theory to tailor both near‐field distributions and far‐field radiation patterns digitally and flexibly by using coding sequences, they have received great attention in the past several years . Various applications such as anomalous scattering, radar‐cross‐section (RCS) reduction, and beam emission with orbital angular momentum (OAM), have been presented in microwave, terahertz, and even acoustics frequencies …”

Section: Introductionmentioning

confidence: 99%

Digital Metasurface with Phase Code and Reflection–Transmission Amplitude Code for Flexible Full‐Space Electromagnetic Manipulations

Zhang

Bao

et al. 2019

Self Cite

113

IntroductionMetasurfaces have attracted much attention due to a number of unique properties to manipulate electromagnetic (EM) waves flexibly and artificially. [1][2][3][4][5] Different from 3D metamaterials, metasurfaces are 2D periodic or nonperiodic arrays of subwavelength Digital coding metasurfaces have been introduced to describe the properties of metasurfaces digitally and provide a new approach to link the physical world and information science. However, most of the current digital metasurfaces are focused on phase code, limiting their functions and applications. Here, the concept of reflection-transmission (R-T) amplitude code is established, and novel digital metasurfaces for full-space electromagnetic (EM) manipulations are proposed, in which the digital states of the R-T amplitude code "0" and "1" represent the total reflection and transmission, respectively. The introduction of the R-T amplitude code enables a full-space communication system and larger information capacity by associating with phase codes. For this purpose, first a novel polarization-dependent R-T amplitude code is proposed, which is distinguished by the polarization of the incidence. Further positive-intrinsic-negative (PIN) diodes are adopted to realize dynamic R-T amplitude code, resulting in real-time controls of far-field radiation characteristics. Good agreements between simulations and experiments demonstrate the powerful ability to manipulate the EM waves by combining the R-T amplitude and phase codes, indicating great potentials in new digital metasurface-based information, radar, and imaging systems.