Structured Light Beams Research Articles

Coherent Transfer of Transverse Optical Momentum to the Motion of a Single Trapped Ion.

Using a structured light beam carrying orbital angular momentum, we demonstrate excitation of the center-of-mass motion of a single atom in the transverse direction to the beam's propagation. This interaction enables quantum control of atomic motion in all axes with a single beam direction, which leads to applications in quantum computing and simulations with ion crystals. Here we demonstrate all the key features required for these applications, namely, coherent dynamics and strong carrier suppression in a configuration with the ion centered in the beam, which allows for single ion addressing and also provides robustness against pointing instabilities. To quantify transverse momentum transfer, we observe coherent dynamics on the sidebands of the S_{1/2} to D_{5/2} transition near 729nm of a singly charged ^{40}Ca^{+} ion, cooled near the ground state of motion in the 3D harmonic potential of a Paul trap, and placed at the center of a first-order Laguerre-Gaussian beam. Exchange of quanta in the perpendicular direction to the beam's wave vector k is observed with a centered vortex shaped beam, together reduction of the parasitic carrier excitation by a factor of 40. This is in sharp contrast to the vanishing spin-motion coupling at the center of the Gaussian beam. Further, we characterize the coherent interaction by an effective transverse Lamb-Dicke factor η_{⊥}^{exp}=0.0062(5) which is in agreement with our theoretical prediction η_{⊥}^{theo}=0.0057(1).

Structured Light Laser Based on Intra-Cavity Modulation

Structured light fields carrying orbital angular momentum (OAM) have been widely studied, and a variety of applications have been found. The exploration of the generation of the structured light beams with higher purity, order, and efficiency has become an inevitable trend. In early years, structured light beams are normally generated with the extra-cavity methods, as it is simpler in principle. However, the intra-cavity generation of the structured light has a better beam purity and higher light conversion efficiency. Here, the current advance in the production of structured light directly from lasers based on spatial light modulators (SLMs), metasurfaces, and microrings is reviewed.

Controlling the Abrupt Autofocusing of Circular Airy Vortex Beam via Uniaxial Crystal

The propagation of many kinds of structured light beams in uniaxial crystal has been investigated. However, the investigation of the evolution of these structured light beams after the uniaxial crystal is lacking. In this paper, an evolution formula of a light beam after passing through a uniaxial crystal is derived. Based on the formula, controlling the autofocusing of a circular Airy vortex beam (CAVB) via a uniaxial crystal is studied. It is found that a uniaxial crystal can prolong the focal length of the autofocusing. By changing the crystal length, the relative weight of the left- and right-hand circular polarization components and the relative value between the orbital and spin angular momentum densities of the beam’s focal plane can be adjusted flexibly. In addition, other optical elements can be inserted between the crystal and the focus to further adjust the focal plane field distribution. The influences of inserting x- and y-polarization polarizers on the intensity distribution are calculated as examples.

Flux trajectory analysis of Airy-type beams.

Airy beams are solutions to the paraxial Helmholtz equation known for exhibiting shape invariance along their self-accelerated propagation in free space. These two properties are associated with the fact that they are not square integrable, that is, they carry infinite energy. To circumvent this drawback, families of so-called finite-energy Airy-type beams have been proposed in the literature and, in some cases, also implemented in the laboratory. Here an analysis of the propagation of this type of structured light beam is presented from a flux trajectory perspective with the purpose of better understanding the mechanisms that make infinite and finite energy beams exhibit different behaviors. As is shown, while the foremost part of the beam can be clearly and unambiguously associated with the well-known accelerating term, the rear part of the beam corresponds to a nearly homogeneous distribution of flow trajectories, particularly for long propagation distances. This is shown to be related to an effective transfer of trajectories between adjacent lobes (gradually, from the fore part of the beam to its rear part), which leads to smearing out the transverse flow along the rear part of the beam. This is in sharp contrast to the situation found in ideal Airy beams, where trajectories belonging to a given lobe of the intensity distribution remain the same all along the propagation. The analysis is supplemented with an also trajectory-based description of Young's experiment performed with finite-energy Airy beams to provide a dynamical understanding of the autofocusing phenomenon observed with circular Airy beams.

Observation of high-order imaginary Poynting momentum optomechanics in structured light

The imaginary Poynting momentum (IPM) of light has been captivated as an unusual origin of optical forces. However, the IPM force is predicted only for dipolar magnetoelectric particles that are hardly used in optical manipulation experiments. Here, we report a whole family of high-order IPM forces for not only magnetoelectric but also generic Mie particles, assisted with their excited higher multipoles within. Such optomechanical manifestations derive from a nonlocal contribution of the IPM to the optical force, which can be remarkable even when the incident IPM is small. We observe the high-order optomechanics in a structured light beam, which, despite carrying no angular momentum, is able to set normal microparticles into continuous rotation. Our results provide unambiguous evidence of the ponderomotive nature of the IPM, expand the classification of optical forces, and open new possibilities for levitated optomechanics and micromanipulations.

Probing Spin Dynamics of 2D Excitons with Twisted Light

We propose a mechanism of intravalley spin-flip scattering in spin-valley coupled two dimensional systems by transferring momentum of light into exciton center of mass using optical vortex (OV) beams. By varying the dispersion of light using the topological charge of OV beam, we demonstrate a unique approach to control the intra-valley spin-flip scattering rate of excitons. From our photoluminescence measurements, we demonstrate that the intra-valley scattering rate in W-based TMDs can be tuned externally by OV beams. Variation of photoluminescence intensity with topological charges shows a crossover temperature (> 150 K), indicating competitions among time scales involving radiative recombination, spin-flip scattering, and thermal relaxations. Our proposed technique utilizing a structured light beam can open up a new approach to explore the physics of excitons in 2D systems.

Scattering analysis of two-dimensional Airy beams by typical non-spherical particles.

The scattering of structured light beams by various particles is an important subject of research with myriad practical applications, such as the manipulation, measurement, and diagnosis of small particles. We carry out an analysis of the scattering of two-dimensional (2D) Airy beams by typical non-spherical particles. The electric and magnetic field vectors of the incident Airy beams are derived by introducing a vector potential in the Lorenz gauge. The scattered fields of the particles are obtained by utilizing the method of moments based on surface integral equations. Some numerical simulations for the scattering of 2D Airy beams by several selected non-spherical particles are performed and analyzed. Especially, a spheroidal particle is taken as an example, and the effects of various parameters describing the 2D Airy beams on its differential scattering cross section are examined. It is expected that this work will be helpful for understanding the interactions of 2D Airy beams with non-spherical particles and their further applications.

High-dimensional orbital angular momentum comb

The article comments on a recently reported OAM comb. The proposed approach relies on a simple azimuthal binary phase modulation scheme to transform a Gaussian beam into a structured light beam containing multiple OAM components with equal spacing and uniform power. It provides a convenient way to simultaneously tailor multiple OAM channels, with the potential to impact many fields, including optical information technology.

Concise and explicit expressions for typical spatial-structured light beams beyond the paraxial approximation.

Structured light beams with distinct spatial inhomogeneity of amplitude, phase, and polarization have garnered tremendous attention in recent years. A better understanding of the vectorial structure of such beams is helpful to reveal their important and interesting features for further applications. In this paper, explicit analytical expressions for the electric field components of typical spatial-structured light beams, including fundamental Gaussian beams, Hermite-Gaussian beams, Laguerre-Gaussian beams, Bessel/Bessel-Gaussian beams, and Airy beams, beyond the paraxial approximation are derived on the basis of the vectorial Rayleigh-Sommerfeld diffraction integrals. Compared with the existing expressions in the literature, the expressions given in this paper are very concise. Using the derived analytical expressions, distributions of the electric field components of these typical structured light beams are displayed and analyzed.

Nanoneedle formation via doughnut beam-induced Marangoni effects.

Recently, nanosecond pulsed optical vortices enables the production of a unique chiral and sharp needle-like nanostructure (nano-needle). However, the formation process of these structures has been unsolved although mass transport by angular momentum would contribute to the chirality. Here, we reveal that another key factor in the formation of a sharp nano-needle is the Marangoni effect during the melting condition at high temperature. Remarkably, the thickness and height of the nano-needle can be precisely controlled within 200 nm, corresponding to 1/25 of beam radius (5 µm) beyond the diffraction limit by ring-shaped inhomogeneous temperature rise. Our finding will facilitate the development of advanced nano-processing with a variety of structured light beams.

Relationship between entanglement and polarization in tripartite states

Entanglement and polarization of pure, two-qubit systems are known to be constrained by the relationship . Here, C stands for concurrence, a standard measure of entanglement, and P refers to the degree of polarization of either of the two, single-qubit subsystems. The above constraint may be understood as reflecting that a system cannot be in a pure state, if it is entangled with another one. In order to explore the connection between entanglement and polarization beyond two-qubit systems, we addressed states in which at least one of the parties has dimensionality greater than two. The case of pure, tripartite states offered itself as a timely candidate. We focussed on three qubits and studied the connection between one qubit’s polarization and its entanglement with the other two taken together. We derived new constraints, akin to the above one, and submitted them to experimental tests using single photons entangled in polarization. The latter provided two qubits, each one attached to one photon. The third qubit was chosen to be the path (momentum) of one of the photons. Our experimental results confirmed the validity of the new constraints. Our theoretical results hold also for classical light. Possible experimental tests could be done with so-called structured light beams.

Implementation of typical structured light beams in discrete dipole approximation for scattering problems

In this work, we report an implementation of typical structured light beams, including the Hermite-Gaussian beams, Laguerre-Gaussian beams, Bessel beams, and Airy beams, in the discrete dipole approximation (DDA) for scattering problems of small particles. The analytical expressions for the electric field components of these beams are presented, and the detailed processes for the implementation of these beams in DDA are given, with particular emphasis on the explicit codes. The calculated result for our codes of Gaussian beams is compared with that from the Amsterdam DDA codes, and very good agreement is observed. The internal and near-surface fields of a sphere illuminated by the typical structured light beams with different beam parameters are illustrated and discussed. Also, the intensity distributions of internal and near-surface fields as well as the scattering efficiency factor for the scattering of structured light beams as mentioned above by sphere, spheroid, and cylinder are presented. The proposed codes can be directly used for the investigation of scattering of typical structured light beams by complex particles within the framework of the DDA.

Thermal lens effect with light's orbital angular momentum.

Thermal lens effect has been well developed and exploited for decades by using the Gaussian intensity distribution of a laser beam. In this paper, a new thermal lens effect by using a laser beam with Orbital Angular Momentum (OAM) is proposed. We find that the dynamic process for the formation of the OAM-thermal lens has reda rapid change towards the evolution direction at the beginning but then slowly approaches to a steady state for a while. This phenomenon is significantly different from the traditional Gaussian-thermal lens, thus it may be used to improve the sensitivity of the absorption spectrum for the chemical and biomedical analysis. Besides, theoretically and experimentally, the factors affecting the steady state of the OAM-thermal lens are also studied, hoping these may provide a useful reference for the research community. We also find a potential slow thermal-optical gate that can control of light passing through or blocking by changing the OAM of the heating beam. Our work opens the door which utilizes the structured light beam to study the thermal-optical effect, and more interesting phenomena remain to be explored.

Deep Learning of Coherent Laser Arrays in Angular Domain for Orbital Angular Momentum Beams Customization

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.

Rotational Doppler Effect With Vortex Beams: Fundamental Mechanism and Technical Progress

Structured light beams such as optical vortices can carry the orbital angular momentum (OAM) with an unbounded quantum number. Recent years have witnessed a growing interest in the rotational Doppler effect with vortex light. Here we present an overview on the technical progress in measuring the rotational Doppler effect associated with OAM. This includes how a high-order OAM light beam is crucial for realizing high-sensitivity remote sensing of rotating objects. The basic physical mechanism of rotational Doppler effect is manifested from both perspectives of the wave property and the conservation law of energy. Besides, we summarize the extension of the rotational Doppler effect from linear optics to nonlinear optics, and to quantum realms. Also, we discuss the main challenges and opportunities of angular remote sensing in a realistic scenario for future applications.

Controllable high-speed rotated femtosecond cylindrical vector beam based on optical heterodyne interference.

Structured light beams that possess unique polarization distribution could offer a new degree of freedom for a variety of applications, and hence its flexible polarization manipulation is necessary. Here we experimentally report a heterodyne interference-based method for generating femtosecond cylindrical vector beam (CVB) with high-speed controllable rotated polarization states. The femtosecond CVBs are created through the superposition of two optical vortices with opposite handedness. The use of two acoustic-optical modulators (AOMs) with frequency differences allows to achieve polarization rotation in a hopping-free scheme at on demand speed. Up to 1 MHz of the rotation frequency is demonstrated by visualizing the fast rotation events through a fast-frame-rate CCD camera. Moreover, we show our method can be readily extended to produce higher order CVBs with more complex rotated polarization distributions. Such a simple yet versatile femtosecond polarization-controlled laser system has the capability to act as a nonlinear trapping platform, thus opening tremendous potential opportunities in the fields of micromachining, nanofabrication, and so force.

Theoretical and numerical investigation of internal conical refraction of structured light beams.

We present an ab initio numerical investigation of the internal conical refraction of structured light beams in a biaxial crystal. Starting from the solutions of the Fresnel equation, a theoretical analysis is developed without assuming any analytical approximation, thus obtaining a set of exact equations that can be solved by standard methods of integration for any impinging light beam. As examples of applications, we consider the particular cases of linearly and circularly polarized Gaussian and Bessel beams inside a KTP crystal. Numerical calculations follow the evolution of the refracted beam inside the crystal. It is seen that for realistic boundary conditions, a refraction cone appears in a certain range of distances within the crystal, and its shape is rather sensitive to the initial conditions.

Lensless Wavefront Parallel Processing of Vector Beams by Self‐Images of a Self‐Organized Q‐Plates Microarray

Wavefront shaping of structured light beams is attracting considerable attention in optics and related technologies. Due to the potential benefits, it is crucial to develop versatile methods for controlling the wavefront in a compact space with the capacity for high‐throughput parallel processing. Herein, a unique concept is introduced for converting the wavefront of multiple structured light beams, which are periodically packed in a microscopic area. An experimental demonstration is provided by using transmissive planar optical elements consisting of a microarray of geometric phase based on nematic liquid crystals. The periodic microstructure is fabricated by the directed self‐organization of topological defects present in a nanoscale relief obtained by area‐selective surface modification. The conversion of multiple wavefronts is realized by using the Talbot self‐imaging effect of the microarray. The integrated geometric phase microarray has potential applications in vortex beam emitters to harness structured light, such as for optical manipulation and lithography.

Massive Parallel Sorting of Particles Using Unwound Polygonal Vortex Beams

Optical sorting, as one kind of optical tweezers, is used to separate mixed particles in a background environment. This unusual tool has a wide application prospect because the non-contact and non-destructive advantages ideally suit the pressing need of bio-technology. However, most sorting methods and devices have been accomplished based on real-time one-by-one sorting, which ignored the sorting efficiency and is not applicable to high-capacity particles. Although more and more structured light beams are proposed to enhance the sorting efficiency, it is still not enough for desired applications. Here, we propose a method for the massive parallel sorting of particles: polygonal optical vortex (OV) beams are unwound by a geometric transformation to produce linear OV beams with kinked distributions. This structured light is used to greatly enhance the sorting efficiency. We adopt the fractal theory to illustrate the increase of the region over which the beam can interact with particles. We demonstrate that the gradient force of this beam is large enough to manipulate spherical particles in the Rayleigh regime of scattering. These results introduce new possibilities for high-capacity particle sorting.

Structured Light Transmission under Free Space Jamming: An Enhanced Mode Identification and Signal-to-Jamming Ratio Estimation Using Machine Learning

In this paper, we develop new classification and estimation algorithms in the context of free space optics (FSO) transmission. Firstly, a new classification algorithm is proposed to address efficiently the problem of identifying structured light modes under jamming effect. The proposed method exploits support vector machine (SVM) and the histogram of oriented gradients algorithm for the classification task within a specific range of signal-to-jamming ratio (SJR). The SVM model is trained and tested using experimental data generated using different modes of the structured light beam, including the 8-ary Laguerre Gaussian (LG), 8-ary superposition-LG, and 16-ary Hermite Gaussian (HG) formats. Secondly, a new algorithm is proposed using neural networks for the sake of predicting the value of SJR with promising results within the investigated range of values between −5 dB and 3 dB.