Nonlinear Metasurface for Structured Light with Tunable Orbital Angular Momentum

Traditional wireless links employing OAM (orbital angular momentum) beams make use of their spatial orthogonality in order to increase their spectral efficiency, by carrying a separate data stream on each beam. This requires that the OAM antenna axes are aligned with the line of sight of the link, and are capable of radiating and receiving high-purity orthogonal OAM beams. Due to the deep spatial nulls of these beams (except the zero-order OAM beam) on their axes, the received signal strengths are much lower than from conventional beams of comparable-sized antennas, limiting the signal-to-noise ratio (SNR) and the reach of such links, or requiring very large receive antennas in terms of wavelengths.Here we describe an alternative technique for recovering received OAM beams based on pseudo-Doppler techniques [1]-[3] which avoid the above limitations of traditional OAM links. In this work, the receiving antennas are not aligned with the OAM beam axes, so the received signals are stronger, the receivers and their antennas are fewer and simpler, and the reach is longer than in traditional wireless OAM links. We demonstrate for the first time, by simulations and experiments performed on an outdoor 100m link at 28 GHz, that it is possible to recover 4 independent data streams carried in a common 20MHz channel on 4 co-aperture OAM beams using the technique developed in [1], with only 2 receiver chains. The transmitting OAM antenna radiates all OAM beams with the same cone angles and the 2 receiving antennas are set on their peaks off-axis in the far field, 20cm apart and tangent to the OAM cones, unlike in traditional OAM wireless links [4]. Salient aspects of the pseudo-Doppler OAM recovery algorithm, its simulation in an end-to-end link, and key design features of the demonstration hardware and signal-processing are presented.

As a pathbreaking approach to generate orbital angular momentum (OAM) beam, uniform circular array (UCA) has been extensively researched and applied. However, the impact of the imperfect UCA (IUCA) converted from a UCA with the position of antenna elements on the purity of the generated OAM beam is rarely considered. In order to verify the effect of position deviation on the OAM beam, this letter deduces the analysis model of the IUCA and the OAM mode purity model. The OAM mode purity refers to the weight of the intensity of the "expected OAM mode." Numerical simulation results show that the OAM beam generated by IUCA is a superposition of OAM beams with a series of different modes, and this results in a certain OAM beam received being contaminated. Compared with the radius deviation, the azimuth deviation of antenna elements has more obvious influence on the OAM mode purity. Moreover, low-mode OAM beams are more suitable for long-distance transmission. Properly increasing the antenna array radius can improve the purity of high-order OAM modes.

Vector orbital angular momentum (OAM) beams, described by higher-order Poincaré (HOP) sphere, are generalized forms of waves carrying OAM with an inhomogeneous polarization of wavefronts. We construct all-dielectric metasurfaces with adjustable amplitude, polarization, and phase to generate arbitrary vector OAM beams. The metasurface is composed of two pairs of silicon nanopillars arranged alternately. Using the interference effect of the four meta-atoms related to the circular polarization, combined with the propagation and geometric phases, two OAM beams with controlled amplitude, phase, and equal topological charge but opposite signs can be obtained under the incidence of orthogonally circularly polarized lights. For the x linearly polarized light, arbitrary vector OAM beams on the HOP sphere are generated via the superposition of the above OAM beams. Additionally, the evolution process of the beam on the longitude and latitude of the Poincaré sphere is revealed by changing the amplitude and phase of the two OAM beams. This work provides a simple, effective, and flexible method for realizing vector OAM beams while having potential implications for the generation and manipulation of vectorial light fields at the micro-nano scale.

Orbital angular momentum (OAM) beams have attracted great attention owing to their excellent performances in imaging and communication. In this paper, a dual-arm Archimedean spiral antenna (DASA) is proposed to generate multiple OAM states with positive and negative values by feeding at the inner and outer ends, respectively. The topological charge of radiated vortex waves is reconfigurable by tuning the operating frequency. Dual-mode OAM states are generated at different working frequencies (l = ±1 at 3 GHz, l = ±2 at 4 GHz, and l = ±3 at 4.8 GHz). Both the simulation and measurement results demonstrate that OAM beams can be generated effectively by the DASA. In addition, a conical cavity is used to increase the gain of the proposed DASA for more than 5 dBi in comparison to the traditional cylindrical cavity. Furthermore, the qualities of the generated OAM modes by the proposed DASA have been evaluated at different operating frequencies of 3 GHz, 4 GHz, and 4.8 GHz, respectively. The OAM modes purities of l = −1, −2, −3, 1, 2, and 3 are predominate with the proportion of about 81%, 70%, 74%, 78%, 77%, and 75%, respectively. Our results demonstrate that the proposed DASA has great potentials in OAM multiplexing communication systems.

Orbital angular momentum (OAM) beams allow for increased channel capacity in free-space optical communication. Conventionally, these OAM beams are multiplexed together at a transmitter and then propagated through the atmosphere to a receiver where, due to their orthogonality properties, they are demultiplexed. We propose a technique to demultiplex these OAM-carrying beams by capturing an image of the unique multiplexing intensity pattern and training a convolutional neural network (CNN) as a classifier. This CNN-based demultiplexing method allows for simplicity of operation as alignment is unnecessary, orthogonality constraints are loosened, and costly optical hardware is not required. We test our CNN-based technique against a traditional demultiplexing method, conjugate mode sorting, with various OAM mode sets and levels of simulated atmospheric turbulence in a laboratory setting. Furthermore, we examine our CNN-based technique with respect to added sensor noise, number of photon detections, number of pixels, unknown levels of turbulence, and training set size. Results show that the CNN-based demultiplexing method is able to demultiplex combinatorially multiplexed OAM modes from a fixed set with >99% accuracy for high levels of turbulence-well exceeding the conjugate mode demultiplexing method. We also show that this new method is robust to added sensor noise, number of photon detections, number of pixels, unknown levels of turbulence, and training set size.

Artificial intelligence methods for the phase control of laser arrays have been extensively developed in the recent years. To tailor high-power and fast switchable orbital angular momentum (OAM) beams by the coherent combination of laser arrays, complex phase control difficulties arise, and the introduction of deep-learning (DL) methods to learn the mapping from the intensity information to the relative phases of array elements offers a promising solution. In this work, we theoretically propose a DL-based phase control scheme that can customize OAM beams with desirable topological charges from laser array systems, which relies on the acquisition of the optical field information in the view of angular domain. When the system in closed loops, the optical field and phase errors information of laser array are extracted and decoupled in the angular domain perspective by using an OAM demultiplexer. The processed optical field of the coherently combined beam serves the DL-based phase control servo to realize the efficient phase stabilization, thus ensuring the controllable customization of OAM beams. This work could provide a useful reference on the intelligent control of laser array systems for structured light beams customization.

In this paper, a design of circular single-sideband (SSB) time-modulated array (TMA) for generating orbital angular momentum (OAM) beams is proposed. It increases signal bandwidth significantly compared with conventional TMAs for generating vortex waves. Theoretical analysis and simulated results of the proposed SSB TMA modulated by the optimized four-step pulse are reported. Higher harmonic efficiency and total efficiency (98.73% and 64.15%) are achieved simultaneously. And mode 1, 2, and 3 OAM beams with mode purity up to 97.3%, 97.2%, and 79.6% are generated at the first positive harmonic frequency.

We experimentally demonstrate turbulence-resilient free-space optical (FSO) coherent communications with multiple multiplexed data channels in different orthogonal domains, including mode, polarization, and wavelength. The turbulence resiliency is enabled by utilizing pilot-assisted optoelectronic mixing of the received beams, wherein one pilot is being mixed with its corresponding beam. By transmitting additional continuous-wave (CW) pilot tones, whose frequencies are offset from the data-carrying beams, and mixing all pilot tones and beams in a single free-space-coupled photodetector (PD) at the receiver, the turbulence-induced modal coupling could be efficiently suppressed. The paper first discusses an experimental demonstration of a turbulence-resilient mode-division-multiplexed (MDM) FSO communication link using two orbital angular momentum (OAM) beams. Each OAM beam carries an independent 2-Gbit/s quadrature-phase-shift-keying (QPSK) data channel. Experimental results indicate that (i) the channel crosstalk under both the weaker and stronger turbulence distortions are measured to be lower than ∼−25.7 dB, (ii) the turbulence-induced OAM modal coupling could be effectively suppressed for the emulated 15 random turbulence realizations, and (iii) the error-vector magnitudes (EVM) of the multiplexed QPSK data channels are measured to be lower than by 17.4% and 19.8% under the relatively weaker and stronger turbulence effects, respectively. The paper subsequently describes an experimental demonstration of a turbulence-resilient FSO link with eight 0.5-Gbit/s QPSK data channels multiplexing two OAM beams ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = +1 & <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = −2), two polarizations, and two wavelengths. The measured bit-error rates (BERs) of all data channels under turbulence effects are below the 7% forward-error-correction (FEC) limit, with power penalties less than ∼3.7 dB.

As the dominant information carrier in water, the acoustic wave is widely used for underwater detection, communication and imaging. Even though underwater acoustic communication has been greatly improved in the past decades, it still suffers from slow transmission speeds and low information capacity. The recently developed acoustic orbital angular momentum (OAM) multiplexing communication promises a high efficiency, large capacity and fast transmission speed for acoustic communication. However, the current works on OAM multiplexing communication mainly appear in airborne acoustics. The application of acoustic OAM for underwater communication remains to be further explored and studied. In this paper, an impedance matching pentamode demultiplexing metasurface is designed to realize multiplexing and demultiplexing in underwater acoustic communication. The impedance matching of the metasurface ensures high transmission of the transmitted information. The information encoded into two different OAM beams as two independent channels is numerically demonstrated by realizing real-time picture transfer. The simulation shows the effectiveness of the system for underwater acoustic multiplexing communication. This work paves the way for experimental demonstration and practical application of OAM multiplexing for underwater acoustic communication.

Structured light carrying orbital angular momentum (OAM) opens up a new physical dimension for studying light–matter interactions. Despite this, the complex fields created by OAM beams still remain largely unexplored in terms of their effects on surface plasmons. This paper presents a revelation of anomalous plasmon excitations in single particles and plasmon couplings of neighboring nanorods under OAM beams, which are forbidden using non-OAM sources. The plasmon excitation of single nanoparticles is determined both by photon spin angular momentum (SAM) and OAM and influenced by the locations of the nanoparticles. Specifically, when SAM and OAM are equal in magnitude and opposite in direction, a pure plasmon excitation along light propagation direction is achieved. Two plasmon dipoles show end-to-end antibonding coupling and side-by-side bounding coupling, which are the opposite of the typical couplings. Furthermore, we observe Fano resonance with a nanorod dimer: one aligned along light propagation direction acting as the bright mode and the other aligned along the global polarization direction of light acting as the dark mode, which is the opposite of the usual plasmonic Fano resonance. By taking advantage of the unique property of the OAM source, this investigation presents a novel way to control and study surface plasmons, and the research of plasmon behavior with OAM would open new avenues for controlling electromagnetic waves and enriching the spectroscopies with more degrees of freedom.

Orbital angular momentum (OAM) mode division multiplexing and encoding have been widely investigated for achieving higher information capacity and performance in both classical and quantum communication systems [1]. A critical component in such systems is an OAM mode sorter that can efficiently sort different OAM modes exp(ilθ) according to the mode index l. To this end, an elegant scheme is to transform the vortex phase exp(ilθ) of OAM modes to a linear phase exp(ilv / β) of tilted plane waves through geometrical transformations, so that they can be separated in the focal plane of a lens. A log-polar transformation was firstly employed [2] and recently a spiral transformation has been proposed to achieve high-resolution OAM mode sorting by the authors [3].

Light beams carrying orbital angular momentum (OAM) have important implications for future classical and quantum systems. In many applications, controlled switching of the OAM state at high speed is crucial, while accelerating the switching rate presents a long-standing challenge. Here we present a method for flexibly switching the OAM state of light based on a coherent laser array system. In the system, the output structured light beam is tailored by the coherent combination of array elements. By employing an OAM mode sorting assisted phase control subsystem, which continuously performs the optimization algorithm, the dynamic wavefront distortion of the combined OAM beam could be compensated. Meanwhile, our approach allows one to achieve fast states switching of the combined OAM beam via programming the cost function of the algorithm. The results of Monte-Carlo simulations demonstrate the feasibility of the proposed method, and the mode purity and power scaling potential of the controllably generated OAM beam are discussed. This theoretical work could be beneficial to the future implementation of rapidly switchable OAM beams at practical output power.

Orbital angular momentum (OAM) multiplexing has recently been proposed as a solution to the ultimate goal of increasing the channel capacity of wireless communication links because of the existence of infinite orthogonal modes. It can be combined with the existing conventional multiplexing techniques to boost up the data rate multiple times for future wireless communication systems. We analyze the fundamental behavior of the channel capacity of OAM channels based on two concepts for the most common Laguerre-Gaussian OAM beams: paraxiality and orthogonality. We also confirm our general theory by applying the proposed method to the evanescing OAM beams, circular transverse electric beams. Our study is based on calculating the paraxial estimator of these OAM beams and relating it as a parameter that limits the degree of freedom in OAM channels. In particular, we investigate the dependency on physical parameters, such as transmitter and receiver sizes, in our calculation to define the capacity of OAM channels.

With the rapid growth in the number of mobile devices and user connectivity, the demand for higher system capacity and improved quality-of-service is required. As the demand for high-speed wireless communication grows, numerous modulation techniques in the frequency, temporal, and spatial domains, such as orthogonal frequency division multiplexing (OFDM), time division multiple access (TDMA), space division multiple access (SDMA), and multiple-input multiple-output (MIMO), are being developed. Along with those approaches, electromagnetic waves’ orbital angular momentum (OAM) is attracting attention because it has the potential to boost the wireless communication capacity. Antenna electromagnetic radiation can be described by a sum of Eigen functions with unique eigenvalues, as is well known. In order to address such issues, the millimeter-wave (mmWave) communication is proposed which is considered as one of the potential technology for 5G wireless networks. The intrinsic feature of all electromagnetic waves is OAM. The OAM beams’ unique qualities have led to a slew of new uses. Broadband OAM generators, on the other hand, have gotten very little attention, especially in the mmWave frequency band. The use of OAM in conjunction with mmWave can reduce the beam power loss, enhance the received signal quality, and hence increase the system capacity. The transmitter and receiver antennas must be coaxial and parallel to achieve precise mode detection. The proposed mmWave integrated with OAM system model is discussed in this study. The channel model is created using the channel transition characteristics. The simulation results demonstrate that the proposed system model is a good way to boost the system capacity.

The wireless imaging radar need more orbital angular momentum (OAM) modes to irradiate targets alternately to obtain higher azimuth resolution. For that, this present paper investigates the fast-switching technique of radio frequency orbital angular momentum (RF-OAM) beam with the use of an optical-controlled system. Compared with the method using pure electrical device, the optical-controlled method offers conveniences in processing same-frequency radio signal, which will contribute to make the system compact. Moreover, the fast-responding speed of the optical device provide a fast speed of switching. The fast-switching system of RF-OAM beams with different mode and radiation direction is proposed and constructed, and proof-of-concept experiments are performed. The proposed system is realized by employing a program-controlled optical switch and the wavelength multiplexing technique integrated into an optical-controlled circular antenna array (CAA). In the demonstration experiments, the switching between +1-mode and −1-mode RF-OAM beams without steering control is detected within 20 ms, then the system achieves the switching between a +1-mode OAM beam with radiation direction −3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> and a −1-mode OAM beam with radiation direction 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> at the same rate.