Incident Vortex Beam Research Articles

Dynamic control of optical skyrmions in cylindrical waveguide

In recent times, the optical skyrmions have received an increasing amount of interest owing to its applications in optical manipulation, super-resolution imaging and microscopy, quantum technologies. However, few studies are focused on the dynamic control of optical skyrmions. It is found that Neel-type photonic skyrmions were discovered in evanescent electromagnetic waves. Here, we find the Bloch-type skyrmions in a three-dimensional cylindrical waveguide. By modulating the amplitude ratio of two incident vortex beams, the new types of skyrmions such as Twisted-type skyrmions and Planar-type skyrmions can be found. Further more, we have also achieved arbitrary modulation ratios of internal spin components. It is believed that our findings greatly enrich the types of photonic skyrmions and provide a method to freely control the photonic skyrmions.

On-chip terahertz orbital angular momentum demultiplexer

The terahertz regime is widely recognized as a fundamental domain with significant potential to address the demands of next-generation wireless communications. In parallel, mode division multiplexing based on orbital angular momentum (OAM) shows promise in enhancing bandwidth utilization, thereby expanding the overall communication channel capacity. In this study, we present both theoretical and experimental demonstrations of an on-chip terahertz OAM demultiplexer. This device effectively couples and steers seven incident terahertz vortex beams into distinct high-quality focusing surface plasmonic beams, and the focusing directions can be arbitrarily designated. The proposed design strategy integrates space-to-chip mode conversion, OAM recognition, and on-chip routing in a compact space with subwavelength thickness, exhibiting versatility and superior performance.

Momentum-dependent Pancharatnam-Berry phase enabled in- and out-of-plane photonic spin-Hall shifts

The photonic spin-Hall effect (PSHE) is a result of the interaction of spin and orbital angular momentum (OAM). However, previous research on the PSHE has primarily focused on out-of-plane (transverse) shifts. In fact, it encompasses both out-of-plane shifts and relatively weaker in-plane shifts, and the latter of which plays a crucial role in controlling the direction of spin separation. Here, we investigate the in- and out-of-plane shifts of the PSHE based on a full-wave theory, revealing that they have the same physical mechanism. The in- and out-of-plane displacements of the PSHE are determined by a momentum-dependent Pancharatnam-Berry (PB) phase, where the in-plane wavevector kx-dependent phase gradient determines the in-plane shift, and the transverse wavevector ky-dependent phase gradient dictates the out-of-plane shift. We discover that the intrinsic OAM (IOAM) and extrinsic OAM (EOAM) carried by incident off-axis vortex beams can respectively convert into the EOAM and IOAM of abnormal modes in the transmitted beam. The OAM generated during the spin-reversal process, together with the OAM carried by the incident beam, collectively determines the in- and out-of-plane displacements. When off-axis vortex beams are incident, the competition between the IOAM and EOAM is more complex than in those without OAM. Our study unifies in- and out-of-plane shifts into a single theoretical framework, which offers an alternative perspective on the spin–orbit interaction of light.

Monolithic Spiral Metalens for Ultrahigh‐Capacity and Single‐Shot Sorting of Full Angular Momentum State

AbstractSorting orbital angular momentum (OAM) and spin angular momentum (SAM) of photons is crucial for optical communications, optical storage, and quantum information processing, wherein complicated optical setups or bulky devices are conventionally utilized to determine the angular momentum via repetitive interference or diffractive projection measurements. Recently, advancements in compact and single‐shot OAM detection have been made via coordinate transformation metasurfaces. Nevertheless, most of these strategies require precise separation and alignment between multiple components and cannot simultaneously distinguish full angular momentum states. Besides, the single‐shot maximum identifiable OAM mode numbers are generally limited to 60 due to the tradeoff between the mode space and reduced‐cross‐talk mode interval. Here, this work puts forward and experimentally demonstrates a monolithic spiral metalens (SML) for ultrahigh‐capacity sorting full angular momentum states. The SML focus the incident vortex beam into multiple‐turn spiral ring rather than a circle ring on the focal plane according to its topological charge, with the mode order ranging from −250 to 250. Consequently, the maximum identifiable OAM mode number is up to 168 under a single‐shot measurement, which largely surpass the best results reported hitherto. The experimental results obtained in the telecom band underpin the design approach, which may find great potential in high‐capacity communication and storage applications.

Multibeam Metasurface Antenna Enabled by Orbital Angular Momentum Demultiplexing Feeding for IoT Communication

Multibeam antenna radiations provide linkages to multiple nodes and can bring enhanced communication quality with reduced power consumption for Internet of Things (IoT) applications. In this paper, a novel method providing a low-cost and flexible multibeam antenna solution is proposed. Multiple orbital angular momentum (OAM) modes are used as internal excitations of the antenna, and the generated multiple beams are pencil beams with no OAM mode, which is convenient for IoT nodes connection. First, aperture phase distributions that convert incident vortex beams with different and orthogonal topological orders into designated directional pencil beams are acquired and superimposed to form a metasurface aperture that, under excitations of vortex beams with multiple orders, produces multibeam each with a predefined direction. The proposed multibeam method is then illustrated with a transmissive metasurface design, with four beams pointing at arbitrary half space directions. Techniques such as OAM modes spacing and aperture partitioning are utilized for reducing sidelobe levels (SLLs). Furthermore, a similar multibeam metasurface antenna with additional linear-to-circular polarization conversion function is demonstrated, which opens the possibility of utilizing different polarizations on top of the OAM based multibeam. This paper successfully explores and achieves a novel antenna system with multiple radiations suited for multiple IoT nodes accessing with low cost and low energy consumption, and also provides a new degree-of-freedom for OAM applications.

Diffraction characteristics of optical vortices using annular radial grating with gradually changing period

Using a grating to measure the orbital angular momentum (OAM) of a vortex beam is a straightforward and economical approach. We devised an annular radial grating with a gradually changing period to characterise a vortex beam. By utilising the different off-axis positions of incidence, the spot distribution law for the far-field diffraction pattern can provide information regarding the topological charge magnitude and sign of the incident vortex beam. Our findings demonstrate that the fringe number and orientation of the diffracted spot can be employed to measure the vortex beam. Furthermore, we simulate the highest measurable OAM order, reaching +170. This measurement technique displays good tolerances for beam alignment and parameter selection. Our study offers a vital approach to the verification of high-order OAM beam generation and demultiplexing and wavefront correction in OAM multiplexing communication systems. These applications have significant value for large-capacity free-space optical communications and OAM holography.

Modulation of orbital angular momentum of vortex beam based on ordered pinhole screens

Optical vortex phase beams have unique amplitudes and phase distributions, rendering them sensitive to the position and symmetry of modulating structures. This sensitivity can facilitate their applicability to optical communications. We propose a method for modulating the topological charge of input vortex beams using a pinhole screen. By changing the pinhole arrangement to match the phase of the vortex beam and considering the Fresnel diffraction theory, the transmission light field is calculated numerically. In addition, the intensity map of the adjustable topological charges is measured near the origin, with the experimental results suitably agreeing with the theoretical simulation results. The diffraction field modulated by spiral pinholes has various orbital angular momentum modes, including the increase and decrease of orbital angular momentum, especially when the incident vortex beam is more complex. The proposed method opens new possibilities for the use of vortex beams in advanced optical applications with improved capabilities and functionalities.

Scattering characteristics of electrically large arbitrarily shaped targets illuminated by an off-axis vortex electromagnetic beam

For the application of vortex electromagnetic (EM) beams in practical detection scenes, the scattering characteristics of electrically large arbitrarily shaped targets illuminated by an off-axis Laguerre–Gaussian (LG) vortex beam are investigated and compared to the on-axis incidence case. The vector potential method is used to extract the electric and magnetic field components of the LG beam in different polarization states. The physical optics algorithm is adopted to calculate the scattering fields of four typical targets with the shape of a sphere, NASA almond, blunt cone, and blade model. The results revealed that as the beam center offset and the topological charge of the incident vortex beam increase, the scattering field distorts, and the obvious orbital angular momentum (OAM) spectrum mixing occurs. In addition, OAM spectrum aliasing occurs for asymmetric targets, even at on-axis incidence. These results elucidate the mechanism of vortex EM scattering and provide a reference for applying vortex beams for target detection and recognition.

3D magnetic imaging using electron vortex beam microscopy

Electron vortex beams are free-electron waves that carry orbital angular momentum. There has been growing theoretical and experimental interest in the use of electron vortex beams as a tool for the investigation of magnetic materials. However, due to the complex wavefront of the propagating waves, a deeper understanding of the interaction of electron vortex beams and the magnetic sample is needed. Here we calculate the magnetic phase shift that an electron vortex beam obtains upon transmitting through a magnetic sample. We show that this magnetic phase shift is influenced by the out-of-plane magnetization, which is a unique characteristic of incident electron vortex beams and is proportional to their orbital angular momentum. Finally, we develop a phase retrieval methodology to retrieve the out-of-plane component of magnetization. Based on our theory, we discuss suitable experimental conditions that would enable this imaging capability for magnetic materials and further extend to non-magnetic chiral materials.

Generalized Newton's rings with vortex beams.

The Newton's rings are interference patterns with concentric rings, and Newton's rings experiment is one of the most famous classic optics experiments. Here, we show that if we use a vortex beam, we can obtain generalized Newton's rings. Unlike traditional Newton's rings, the generalized ones are no longer concentric rings but spiral arms, and fork-shaped dislocations appear in spiral arms. More interesting, we reveal that both the number of spiral arms and the number of fork-shaped dislocations are equal to the value of topological charge of incident vortex beams. Our theoretical results are demonstrated experimentally. This novel interference pattern can be used for measuring the topological charge of vortex beams.

Multiple-image encryption using phase jump gradient factors -based OAM multiplexing holography

Orbital angular momentum (OAM) has been widely implemented in information encryption as an independent degree of freedom. In this paper, we propose using phase gradient factor of vortex phase structure as an independent degree of freedom for further boosting OAM information carrying capacity. The orthogonality of different phase gradient factors is firstly demonstrated. Multiple images encoded in different phase gradients even with the same integral or fractional topological charge (TC) can be simultaneously encrypted free of crosstalk. Herein, the safety and the multiplexing capacity are greatly enhanced while keeping the fractional topological charge interval as small as 0.1. In the decryption process, the encoded information can only be reconstructed under the incident vortex beam illumination with inverse TC at correct focus distances and accurate phase jump gradient factors. The feasibility and the security of proposed method are both verified in the experiments and the numerical simulations. This opens up a new avenue for high security and practical information encryption systems.

Orbital Angular Momentum of Superpositions of Optical Vortices Perturbed by a Sector Aperture

In optical communications, it is desirable to know some quantities describing a light field, which are conserved on propagation or resistant to some distortions. Typically, optical vortex beams are characterized by their orbital angular momentum (OAM) and/or topological charge (TC). Here, we show analytically that the OAM of a single rotationally symmetric optical vortex is not affected by an arbitrary-shape aperture or by other amplitude perturbations. For a superposition of two or several optical vortices (with different TCs), we studied what happens to its OAM when it is distorted by a hard-edge sector aperture. We discovered several cases when such perturbation does not violate the OAM of the whole superposition. The first case is when the incident beam consists of two vortices of the same power. The second case is when the aperture half-angle equals π multiplied by an integer number and divided by the difference between the topological charges. For more than two incident beams, this angle equals π multiplied by an integer number and divided by the greatest common divisor of all possible differences between the topological charges. We also show that such a sector aperture also conserves the orthogonality between the complex amplitudes of the constituent vortex beams. For two incident vortex beams with real-valued radial envelopes of the complex amplitudes, the OAM is also conserved, when there is a ±π/2 phase delay between the beams. When two beams with the same power pass through a binary radial grating, their total OAM is also conserved. We hope that these findings could be useful for optical communications since they allow for the identification of incoming optical signals by their OAM by registering only part of the light field within a sector aperture, thus reducing the cost of the receiving devices.

Smile face array: Generating oblique incident and front output vortex beam for both TE and TM waves

A single layer reflective array is designed to generate orbital angular momentum (OAM) beam at microwave frequency. The unit cell of this design is smile face shape distributed over a planar surface. Phase modulation can be realised by changing the size of each unit cell, with very low variation in reflection amplitude. By careful design, oblique incident can be obtained, reducing feed blockage to the output beam. The direction of the output beam can also be intendedly designed. In this work, the front output is chosen as a demonstrative example. In addition, the input beam can be either TE or TM wave, enable dual-mode operation. Furthermore, it is flexible to design the topological mode of the output OAM. The smile array is developed by cutting slots on circular patch array to produce perturbation on TE and TM waves, so that the difference in TE and TM incidence is reduced. As an instance, the topological modes of l = (−2, −1, 1, 2) are simulated, and the mode l = 1 is fabricated and measured. Measurements are conducted for both near field and far field. The measured results show good agreement with the simulated ones, indicating that this dual mode design works satisfactorily when oblique feed is used.

Orbital angular momentum of superpositions of optical vortices after passing through a sector diaphragm

In optical communications, it is desirable to know some quantities describing a light field, that are conserved on propagation or resistant to some distortions. Typically, optical vortex beams are characterized by their orbital angular momentum (OAM) and/or topological charge (TC). Here, we study what happens with the OAM of a superposition of two or several optical vortices (with different TCs) when it is distorted by a hard-edge sector aperture. We discover several cases when such perturbation does not violate the OAM of the whole superposition. The first case is when the incident beam consists of two vortices of the same power. The second case is when the aperture half-angle equals an integer number of π divided by the difference between the topological charges. For more than two incident beams, this angle equals an integer number of π divided by the greatest common divisor of all possible differences between the topological charges. For two incident vortex beams with real-valued radial envelopes of the complex amplitudes, the OAM is also conserved when there is a ±(pi)/2 phase delay between the beams. When two beams with the same power pass through a binary radial grating, their total OAM is also conserved.

Measuring the orbital angular momentum mode of vortex beam based on phase matching sequence

Measuring the orbital angular momentum(OAM) mode of vortex beams is of great significance in applications based on vortex beams.we propose a method of phase matching characteristics of vortex beams to measure the OAM mode. The method first use a high-speed spatial light modulator(SLM) to sequentially load a set of helical phase sequence images, so that the vortex beam is modulated by this SLM. Then obtain the modulated optical field through pinhole filtering, and then the synchronized tilt phase modulation is performed by the 4f system and high-speed SLM. When the OAM mode of vortex beam is opposite to the topological charge of helical phase image, that is, the incident vortex beam degenerates to the fundamental Gaussian mode beam,and through our optical system, the output plane obtained a indicating spot with the position related to the OAM mode. The Simulation and experimental results show that the method obtains a clear OAM mode indicating spot on the final output plane, which verifies the theoretical derivation.

Measuring High-Order Optical Orbital Angular Momentum With a Petal-Like Zone Plate

Orbital angular momentum (OAM) detection is a key technique in the applications of vortex beams. In this letter, we propose a simple method to flexibly determine the high-order OAM state of a vortex beam with a petal-like zone plate (PZP). It is theoretically demonstrated that the far-field diffraction of the product of vortex beam and PZP is a Hermitian-like form of the vortex beam. Therefore, the magnitude of the topological charges (TCs) as well as the sign of incident vortex beam can be read from the numbers and direction of the dark lines in the Hermite–like pattern. Possessing the multiple focusing characteristic, the Hermite-like pattern of detected vortex beam can be found in a range of distance instead of a certain location. Meanwhile, to promise a high detection efficiency, the key parameters of PZP have been studied in details. Experimental results agree well with the theoretical prediction. It is testified that the measured TCs can be up to 100.

Influence of orbital angular momentum of vortex light on lateral shift behavior

We suggest a scheme that consists of a cavity with a four-level double Λ atomic medium as an intracavity medium. The reflection of an optical vortex light beam field that is incident on a cavity has been investigated by consideration within the coherence theory framework. We study the influence of the orbital angular momentum (OAM) of the vortex light beam on the lateral or Goos–Hänchen shift (GHS) for the reflected beam. It has been investigated that different values of OAM from the incident vortex beam enhanced positive and negative GHS. Also, we consider two fields (including incident) that carry an optical vortex interacting with the cavity. We obtain giant positive and negative GHS via manipulation of the OAM corresponding to the two optical vortex beams. To the best of our knowledge, no one has ever studied such a large GHS in a reflected beam.

Light scattering of Laguerre-Gaussian vortex beams by arbitrarily shaped chiral particles.

Laguerre-Gaussian (LG) beams with vortex phase possess a handedness, which would produce chiroptical interactions with chiral matter and may be used to probe structural chirality of matter. In this paper, we numerically investigate the light scattering of LG vortex beams by chiral particles. Using the vector potential method, the electric and magnetic field components of the incident LG vortex beams are derived. The method of moments (MoM) based on surface integral equations (SIEs) is applied to solve the scattering problems involving arbitrarily shaped chiral particles. The numerical results for the differential scattering cross sections (DSCSs) of several selected chiral particles illuminated by LG vortex beams are presented and analyzed. In particular, we show how the DSCSs depend on the chiral parameter of the particles and on the parameters describing the incident LG vortex beams, including the topological charge, the state of circular polarization, and the beam waist. This research may provide useful insights into the interaction of vortex beams with chiral particles and its further applications.

A metamaterial lens based on transformation optics for horizontal radiation of OAM vortex waves

Vortex electromagnetic waves carrying orbital angular momentum (OAM) have been widely discussed for potential applications in wireless communications. Belonging to the Laguerre–Gaussian beams family, such type of waves present a hollow conical shape and divergence characteristics along with a directional radiation. In this paper, an innovative method to produce omnidirectional OAM beams based on spatial transformation is proposed at microwave frequencies. As a proof-of-concept demonstration, a lens with omnidirectional radiation in the horizontal plane is designed and simulated with an incident vortex beam carrying the OAM mode l = +2. The designed lens can be potentially implemented with an all-dielectric medium showing a gradient permittivity distribution. Furthermore, the proposed lens presents good performances over a wide operational bandwidth spanning from 8 to 17 GHz. By converting the directional beam to an omnidirectional one, the proposed method opens the door to the potential development of microwave vortex antenna systems.

Focusing characteristics of optical vortex passing through a Fresnel zone plate

The focusing characteristics of Fresnel zone plate (FZP) on vortex beams at 1550 nm are investigated. Employing the Fresnel diffraction integral, the diffraction characteristics on different circular structures are calculated. Many calculations and measurements of the transmission light field-phase distribution demonstrate that the focusing characteristics of the vortex beam through FZP are similar to Gaussian beam. The effect of radius-dependent phase delay on the focus position is verified. At the same time, it is proved that the topological charge has nothing to do with the major focal position and the FZP will not change the topological charge of the incident vortex beam.