Orbital angular momentum (OAM) has aroused a widespread interest in many fields, especially in telecommunications due to its potential for unleashing new capacity in the severely congested spectrum of commercial communication systems. Beams carrying OAM have a helical phase front and a field strength with a singularity along the axial center, which can be used for information transmission, imaging and particle manipulation. The number of orthogonal OAM modes in a single beam is theoretically infinite and each mode is an element of a complete orthogonal basis that can be employed for multiplexing different signals, thus greatly improving the spectrum efficiency. In this paper, we comprehensively summarize and compare the methods for generation and detection of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications and technical challenges of OAM in communications, including free-space optical communications, optical fiber communications, radio communications and acoustic communications. To complete our survey, we also discuss the state of art of particle manipulation and target imaging with OAM beams.

Orbital Angular Momentum Waves: Generation, Detection, and Emerging Applications

Abstract Optical metasurfaces are judicously engineered electromagnetic interfaces that can control and manipulate many of light’s quintessential properties, such as amplitude, phase, and polarization. These artificial surfaces are composed of subwavelength arrays of optical antennas that experience resonant light-matter interaction with incoming electromagnetic radiation. Their ability to arbitrarily engineer optical interactions has generated considerable excitement and interest in recent years and is a promising methodology for miniaturizing optical components for applications in optical communication systems, imaging, sensing, and optical manipulation. However, development of optical metasurfaces requires progress and solutions to inherent challenges, namely large losses often associated with the resonant structures; large-scale, complementary metal-oxide-semiconductor-compatible nanofabrication techniques; and incorporation of active control elements. Furthermore, practical metasurface devices require robust operation in high-temperature environments, caustic chemicals, and intense electromagnetic fields. Although these challenges are substantial, optical metasurfaces remain in their infancy, and novel material platforms that offer resilient, low-loss, and tunable metasurface designs are driving new and promising routes for overcoming these hurdles. In this review, we discuss the different material platforms in the literature for various applications of metasurfaces, including refractory plasmonic materials, epitaxial noble metal, silicon, graphene, phase change materials, and metal oxides. We identify the key advantages of each material platform and review the breakthrough devices that were made possible with each material. Finally, we provide an outlook for emerging metasurface devices and the new material platforms that are enabling such devices.

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Material platforms for optical metasurfaces

Solar cell efficiency improvement has been one of the major concerns to realize ultimately the cost effective efficient solar cells. Among various ways to improve solar cell efficiency, plasmonic light trapping mechanism has been found to be of immense interests recently. The mechanism of strong scattering into the active materials and guiding of light at the excitation of plasmons at the metal-semiconductor interface play significant role for better photon harvesting. The present review concentrates on the recent advances on the application of plasmonics in inorganic semiconductor solar cell efficiency improvements. Various research groups active in this field have employed various metal nanostructures on to the surface of solar cells to achieve higher efficiency. This review partially also concentrates on surface nanopatterning of solar cells with nonmetallic dielectrics. Finally, a brief account on the dye-sensitized solar cell is presented to show the potential of plasmonics in solar cell research.

Progress in plasmonic solar cell efficiency improvement: A status review

In the free space optical communication, the information can be encoded as the orbital angular momentum (OAM) state of light, which is called OAM shift keying (OAM-SK). This paper has proposed a communication system with OAM-SK, in which an image has been delivered from the transmitter to the receiver successfully in the simulation environment. Specifically, we have carefully designed and implemented the phase holograms used at the transmitter and the receiver for multiplexing and de-multiplexing the OAM states, respectively. At the transmitter, the multiplexing phase hologram designed by the modified Lin's algorithm is loaded on the spatial light modulator 1 (SLM1) to generate the multiplexing vortex beam, which is a superposition of multiple vortex beams with different OAM states. Correspondingly, at the receiver, a novel phase hologram is designed and loaded on the SLM2 to effectively de-multiplex the multiplexing vortex beam in different directions. In our phase hologram used at the receiver, the detected power of each OAM state can be controlled by adjusting the weight coefficient by the modified Lin's algorithm. This way, the incident power can be concentrated to the target OAM states, from which the target OAM states can be detected more effectively than conventional fork grating.

Orbital Angular Momentum Shift Keying Based Optical Communication System

Nanoscale compact optical vortex generators promise substantially significant prospects in modern optics and photonics, leading to many advances in sensing, imaging, quantum communication, and optical manipulation. However, conventional vortex generators often suffer from bulky size, low vortex mode purity in the converted beam, or limited operation bandwidth. Here, we design and demonstrate gradient–rotation split-ring antenna metasurfaces as unique spin-to-orbital angular momentum beam converters to simultaneously generate and separate pure optical vortices in a broad wavelength range. Our proposed design has the potential for realizing miniaturized on-chip OAM-multiplexers, as well as enabling new types of metasurface devices for the manipulation of complex structured light beams.

/pdf/generating-and-separating-twisted-light-by-gradient-rotation-3s34gr2qeg.pdf

Generating and Separating Twisted Light by gradient-rotation Split-Ring Antenna Metasurfaces.

We propose an ultra-thin metasurface of the metal rectangular split-ring resonators (MRSRR) array which can modulate and analyze the wavefront of circularly polarized light efficiently. An incident circularly polarized light could be converted into the corresponding cross-polarized light which would be bent to±23°at a wavelength of 808nm for the normal incidence. And a linearly polarized light would be decomposed into two lights of left and right-handed circular polarizations in the directions of∓23°respectively. These phenomena have also been observed at 1200nm with different geometric parameters. And these results depend on controlling the optical-axis profile of the resonators in a subwavelength scale by precisely modulating two degrees of freedom in our nanostructures.

Ultra-thin circular polarization analyzer based on the metal rectangular split-ring resonators

The ultra-thin optical vortex phase plate (VPP) has been designed and investigated based on the metasurface of the metal rectangular split-ring resonators (MRSRRs) array. The circularly polarized incident light can convert into corresponding cross-polarization transmission light, and the phase and the amplitude of cross-polarization transmission light can be simultaneously governed by modulating two arms of the MRSRR. The MRSRR has been arranged in a special order for forming an ultra-thin optical VPP that can covert a plane wave into a vortex beam with a variety of the topological charges, and the transformation between spin angular momentum (SAM) and orbital angular momentum (OAM) has been discussed in detail. The multi-spectral characteristics of the VPP have also been investigated, and the operating bandwidth of the designed VPP is 190 nm (in the range of 710–900 nm), which enable a potential implication for integrated optics and vortex optics.

https://iopscience.iop.org/article/10.1088/2040-8978/17/4/045102/pdf

Ultra-thin optical vortex phase plate based on the metasurface and the angular momentum transformation

We have proposed a carefully designed metasurface constructed by the elongated apertures in gold film. This metasurface structure can support only single anomalous re- fractions for both circular polarization (CP) and linear polari- zation (LP) incidence, which is also following the generalized Snell's law, whereas the normal refraction can be completely suppressed nearly. Furthermore, the circularly polarized light converted to cross-polarized light and the linearly polarized light decomposed into two CP states with opposite transmis- sion angle after passing through the metasurface with a high conversion efficiency of 87.5 %. The concept of single anom- alousrefractioncanbeexploitedforcreatinghigh-performance

Anjun Zhang

Papers

Ultra-thin circular polarization analyzer based on the metal rectangular split-ring resonators

Ultra-thin optical vortex phase plate based on the metasurface and the angular momentum transformation

High-Efficiency Cross Polarization Converters by Plasmonic Metasurface

Calculating the torque of the optical vortex tweezer to the ellipsoidal micro-particles

Gold nanoshells with gain-assisted silica core for ultra-sensitive bio-molecular sensors