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Optical vortex

An optical vortex is a structured beam of light featuring a helical phase wavefront that spirals around a central line singularity, where the intensity drops to zero, producing a characteristic doughnut-shaped transverse intensity profile. This phase singularity is characterized by a topological charge \ell, an integer that quantifies the number of $2\pi phase windings around the vortex core, enabling the beam to carry orbital angular momentum (OAM) of \ell \hbar per photon, where \hbar is the reduced Planck's constant.^[1]^[2]^[3] Optical vortices were first theoretically described in 1989 by Coullet et al., drawing analogies between laser instabilities and fluid dynamics, marking the beginning of their study as a fundamental aspect of structured light.^[2] These beams exhibit unique properties beyond their OAM, including an infinite-dimensional Hilbert space of states due to the unbounded range of \ell (from -\infty to \infty), distinguishing them from the limited spin angular momentum of polarized light (\pm \hbar).^[3] They can be generated through methods such as spiral phase plates, computer-generated holograms, or metasurfaces, with reported OAM values reaching up to 10,010 using advanced phase plates.^[1]^[2] The significance of optical vortices spans diverse applications, including optical tweezers for microparticle manipulation, high-capacity optical communications leveraging OAM multiplexing, super-resolution microscopy, and quantum information processing through entanglement of vortex states.^[2]^[3] In nonlinear optics, they enable phenomena like second-harmonic generation and high-harmonic generation while conserving OAM, facilitating the production of vortex beams across ultraviolet to extreme-ultraviolet wavelengths.^[3] Recent advances, such as metasurface-based generation and interferometric detection techniques, have further expanded their utility in imaging, encryption, and particle trapping.^[2]^[3]

Fundamentals

Definition and Basic Concept

An optical vortex represents a fundamental feature in the structure of light beams, manifesting as a point-like region of zero intensity surrounded by a helical wavefront that twists around this central singularity. This helical structure imparts a corkscrew-like appearance to the propagating light, where the phase of the electromagnetic field changes discontinuously by multiples of $2\pi radians as one encircles the vortex core.^[4]^[2] In contrast to conventional Gaussian beams, which exhibit smooth, paraboloidal wavefronts with uniform intensity profiles, optical vortices feature a distinctive dark core at the phase singularity, where the electric field amplitude vanishes entirely. This zero-intensity point acts as a dislocation in the wavefront, disrupting the otherwise continuous phase progression and creating a localized void in the beam's energy distribution.^[5]^[6] Within the broader framework of singular optics, optical vortices are interpreted as wavefront dislocations, analogous to defects in crystal lattices but occurring in the phase topology of optical fields. These singularities highlight the intricate, non-diffracting propagation behaviors possible in structured light. A prominent example of such vortex beams includes Laguerre-Gaussian modes, which qualitatively display a doughnut-shaped intensity profile with a central helical phase twist, enabling applications in beam manipulation.^[7]^[8]

Mathematical Description

The electric field of an optical vortex beam in the paraxial approximation can be expressed in cylindrical coordinates as E(r, \phi, z) \propto u(r, z) e^{i l \phi} e^{i k z}, where r is the radial distance, \phi is the azimuthal angle, z is the propagation direction, k = 2\pi / \lambda is the wave number, l is the integer topological charge, and u(r, z) describes the radial amplitude profile that varies slowly with z.^[2] This form satisfies the paraxial wave equation $2 i k \frac{\partial E}{\partial z} + \nabla_\perp^2 E = 0, where \nabla_\perp^2 is the transverse Laplacian, assuming the beam envelope changes gradually along the propagation axis.^[2] The phase term e^{i l \phi} introduces a helical wavefront with a phase singularity at the beam axis (r = 0), where the field amplitude vanishes for l \neq 0, rendering the phase undefined.^[9] Around this singularity, the phase undergoes a winding of $2\pi l along a closed path encircling the core, as quantified by the topological charge l = \frac{1}{2\pi} \oint_C \nabla \varphi \cdot d\mathbf{r}, where \varphi is the phase and C is the contour.^[2] Near the core, the field behaves as E \propto r^{|l|} e^{i l \phi}, ensuring the intensity I(r, \phi, z) = |E|^2 \propto |u(r, z)|^2 r^{2|l|} drops to zero at r = 0, forming a dark central region surrounded by a bright ring.^[9] Beam propagation incorporates additional phase factors, including the Gouy phase shift e^{i (2p + |l| + 1) \tan^{-1}(z/z_R)}, where p is a non-negative radial index, z_R is the Rayleigh range, and the term accounts for the (2p + |l| + 1) half-wavelength shifts upon passing through the focus, generalizing the fundamental Gaussian beam Gouy phase.^[10] A prototypical example is the Laguerre-Gaussian (LG) mode, given by \mathrm{LG}_{p,l}(r, \phi, z) = u_{p,l}(r, z) e^{i l \phi}, where the radial function u_{p,l}(r, z) = C_{p,l} \left( \frac{\sqrt{2} r}{w(z)} \right)^{|l|} L_p^{|l|} \left( \frac{2 r^2}{w(z)^2} \right) \exp\left( -\frac{r^2}{w(z)^2} \right) \exp\left( i \frac{k r^2 z}{2 R(z)} \right) \left( 1 + i \frac{z}{z_R} \right)^{-(|l| + 1)/2} involves the beam waist w(z), radius of curvature R(z), normalization constant C_{p,l}, and associated Laguerre polynomial L_p^{|l|}.^[10] This structure ensures LG modes form a complete orthogonal basis for paraxial beams carrying well-defined orbital angular momentum.^[2]

Historical Development

The study of phase singularities in optical fields traces back to 1974, when J. F. Nye and M. V. Berry described wavefront dislocations in wave trains.^[9] Building on this, the concept of optical vortices was theoretically predicted in 1989 by Pierre Coullet, Luis Gil, and François Rocca, who described phase singularities in laser cavities within the framework of nonlinear optics and solutions to the Maxwell-Bloch equations. This work drew analogies to hydrodynamic vortices and established the foundational idea of stable optical fields with helical phase structures.^[2] Experimental confirmation followed in the early 1990s, with the first generation of optical vortex beams achieved using computer-generated holograms by N. R. Heckenberg and colleagues in 1992, demonstrating observable phase singularities in laboratory settings.^[11]^[2] A pivotal milestone occurred in 1992 when Les Allen and coworkers theoretically linked these vortices to orbital angular momentum (OAM) in paraxial beams, such as Laguerre-Gaussian modes, opening avenues for quantized angular momentum transfer in light-matter interactions. The period from 1989 to 1999 focused on fundamental theory, including refinements to phase singularity models and early explorations of vortex stability in various optical systems.^[2] From 1999 to 2009, research shifted toward application exploration, with demonstrations of vortex-based optical trapping and manipulation, such as particle rotation using OAM beams. The 2009–2019 era emphasized integration with OAM concepts, highlighted by advancements in OAM multiplexing for high-capacity communications; for instance, in 2013, Alan Willner’s group achieved terabit-per-second data rates over free-space links using multiplexed vortex modes. In the 2010s, key milestones included scalable OAM state generation exceeding 10,000 ħ per photon, as shown by Anton Zeilinger’s team in 2016, enhancing quantum information processing potential. Recent developments have integrated optical vortices with metasurfaces for compact generation and quantum optics applications, including on-chip devices for producing vortex beams in entangled photon pairs and nonlinear processes. These advances enable miniaturized systems for quantum sensing and secure communications.^[2]^[12]

Physical Properties

Orbital Angular Momentum

Optical vortices carry orbital angular momentum (OAM) as a distinct component of the total angular momentum of light, arising from the helical phase structure of the beam. Each photon in such a vortex beam, characterized by topological charge l, possesses an OAM of l \hbar, where \hbar is the reduced Planck's constant and l is an integer that can take any value, positive or negative.^[10] This OAM is associated with the azimuthal phase dependence, e^{i l \phi}, which imparts a twisted wavefront to the light propagating along the beam axis. The total angular momentum \mathbf{J} of an electromagnetic field decomposes into OAM \mathbf{L} and spin angular momentum \mathbf{S}, such that \mathbf{J} = \mathbf{L} + \mathbf{S}. Here, \mathbf{L} originates from the spatial distribution and phase gradients of the field, particularly the orbital motion around the propagation axis, while \mathbf{S} stems from the field's polarization, limited to \pm \hbar per photon for circular polarizations. Unlike spin angular momentum, which is bounded and tied to the photon's intrinsic helicity, OAM is unbounded in magnitude since l can be arbitrarily large, allowing for beams with exceptionally high angular momentum density. This distinction enables OAM to provide scalable torque without altering polarization, facilitating unique manipulations in optical systems. In light-matter interactions, OAM conservation governs the transfer of angular momentum, where absorption or scattering processes exchange l \hbar between the beam and the material. For instance, optical vortices impart azimuthal torque to absorbing particles, inducing rotation at rates proportional to l, as first observed with microscopic particles spinning at several hertz under Laguerre-Gaussian beams. This transfer has been verified in various contexts, including the rotation of trapped micro-objects, where the beam's OAM directly couples to the particle's mechanical motion without reliance on spin contributions. Such conservation ensures that the helical phase structure influences the dynamics predictably, distinguishing OAM effects from those of spin in applications involving torque and rotation.

Topological Charge and Phase Structure

The topological charge l of an optical vortex is an integer-valued topological invariant that quantifies the number of complete $2\pi phase windings around the phase singularity at the beam's core. This charge determines the helical structure of the wavefront, where positive values of l correspond to a left-handed screw sense (counterclockwise phase progression with increasing azimuthal angle \phi), and negative values indicate a right-handed helix. The sign and magnitude of l thus encode the handedness and strength of the vortex, distinguishing it from other beam singularities.^[2] The phase structure of an optical vortex arises from this helical wavefront, which manifests as a continuous twist in the optical phase along the propagation direction, resulting in a characteristic doughnut-shaped intensity profile with a central intensity null at the singularity. For a fundamental vortex with |l| = 1, the phase forms a single helix encircling the beam axis, leading to destructive interference at the core and a ring-like intensity distribution.^[2] This structure is topologically protected, ensuring that the charge l remains conserved during free-space propagation in linear media, as the phase winding cannot unwind without crossing the singularity; deviations occur only through vortex reconnections, where higher-charge vortices split or merge while preserving the total topological charge.^[2] In higher-order vortices where |l| > 1, the phase profile consists of |l| intertwined helices, creating a more complex singularity that is dynamically unstable and prone to splitting into multiple lower-order vortices of unit charge during propagation or in perturbed environments. Such splitting dynamics maintain overall charge conservation, as the emergent vortices collectively sum to the original l, reflecting the topological robustness of the system. For instance, a vortex with l = 2 may bifurcate into two l = 1 vortices, altering the local phase topology without violating global invariance.^[2] The topological charge can be quantitatively measured through the line integral of the phase gradient around a closed path enclosing the singularity:

l = \frac{1}{2\pi} \oint_C \nabla \phi \cdot d\mathbf{l},

where \phi is the phase of the optical field and C is a contour encircling the vortex core once.^[2] This integral yields the integer l due to the single-valuedness of the field amplitude outside the singularity, providing a direct probe of the vortex's topological properties independent of intensity variations.^[2]

Generation Methods

Conventional Techniques

Spiral phase plates (SPPs) are refractive or diffractive optical elements designed to impose a helical phase profile on an incident beam, converting a fundamental Gaussian mode into a Laguerre-Gaussian mode carrying orbital angular momentum with topological charge l. These plates feature a continuous spiral ramp in thickness that provides a phase shift proportional to the azimuthal angle \phi, specifically l \phi, enabling the generation of high-purity optical vortices with efficiencies approaching 100% for low-order charges. SPPs are typically fabricated from materials such as fused silica glass or polymers using techniques like electron-beam lithography, direct laser writing, or inductively coupled plasma etching to achieve the required sub-wavelength precision in the phase profile. Early demonstrations used etched glass plates to produce helical wavefronts directly from a laser beam, establishing SPPs as a robust, passive method for vortex generation in laboratory settings.^[13]^[14]^[15] Computer-generated holograms (CGHs) provide a versatile approach to optical vortex generation by encoding a fork-like dislocation pattern in a phase or amplitude hologram, which diffracts an input beam into the desired vortex mode upon reconstruction. These holograms are created by computing the interference between a reference plane wave and the target Laguerre-Gaussian field, often incorporating a blazed grating to direct the first-order diffraction into the vortex beam while suppressing unwanted orders. CGHs can be implemented on spatial light modulators for dynamic control of the topological charge l or printed on static transparencies for fixed configurations, achieving mode purities exceeding 90% with appropriate filtering. This method, pioneered in the early 1990s, allows for the creation of complex vortex arrays and was instrumental in initial experimental studies of orbital angular momentum transfer.^[11]^[16] Mode conversion using astigmatic transformers, typically consisting of a pair of cylindrical lenses, enables the transformation of Hermite-Gaussian modes into Laguerre-Gaussian modes by introducing a \pi/2 phase shift between orthogonal components of the input beam. In this setup, a fundamental Gaussian beam is first converted to a higher-order Hermite-Gaussian mode via mode decomposition, and the astigmatism imparted by the lenses then couples the modes to produce a vortex beam with topological charge l = \pm 1, with higher charges achievable through cascaded conversions. This all-optical technique offers near-unity efficiency without lossy elements and has been widely adopted for generating doughnut-shaped beams in optical tweezers applications. The method relies on precise alignment to minimize mode impurities, typically below 5% for optimized systems. Q-plates are thin liquid crystal devices with a spatially varying optical axis patterned in a q-plate geometry, where the parameter q determines the imparted orbital angular momentum, allowing conversion from spin angular momentum (via circular polarization) to orbital angular momentum in the output beam. For a q-plate with q=1, a left-circularly polarized input yields a right-circularly polarized Laguerre-Gaussian beam with l = +2, and vice versa, enabling efficient Pancharatnam-Berry phase manipulation for vortex generation. These plates are fabricated by photo-aligning nematic liquid crystals on substrates, offering tunability through voltage control or wavelength dependence, with conversion efficiencies up to 95%. Q-plates extend conventional techniques by linking polarization and phase structure, facilitating compact vortex sources for quantum optics experiments.^[17] Early laser-based methods for optical vortex generation involved intracavity engineering in end-pumped solid-state lasers, such as Nd:YAG or Nd:YVO4, using mode-selecting mirrors or intracavity elements to favor Laguerre-Gaussian transverse modes over the fundamental Gaussian. These mirrors, often curved with specific radii to match the doughnut profile of higher-order modes, suppress competing modes through differential gain and loss, enabling direct output of vortex beams with topological charges up to l=3 at powers in the watt range. Demonstrated in the late 1990s, such configurations achieved mode purities greater than 80% without external conversion, paving the way for high-power, stable vortex sources in continuous-wave operation. This approach leverages the laser's gain medium to amplify the vortex mode intrinsically, reducing complexity compared to post-laser processing.^[18]

Advanced Techniques

Metasurfaces and nanophotonic structures leverage the Pancharatnam-Berry (PB) geometric phase to enable compact, on-chip generation of optical vortices with high efficiency and polarization sensitivity. These flat, subwavelength-patterned devices impart a spatially varying phase to incident circularly polarized light, converting spin angular momentum into orbital angular momentum (OAM) without relying on bulky elements like spiral phase plates. For instance, PB metasurfaces can produce quasi-perfect vortices with ring radii tunable from 10 to 100 micrometers and topological charges up to 20, achieving efficiencies exceeding 80% in the visible spectrum.^[19]^[20] Advancements in 2024 introduced spiral lens metasurfaces that extend this capability to broadband operation, covering wavelengths from 450 to 650 nanometers with minimal phase distortion. These structures, often fabricated using dielectric nanoantennas, generate perfect vortex beams with topological charges as high as 32, maintaining beam quality over a 40% bandwidth and enabling applications in integrated optics.^[21]^[22] Such designs exploit the PB phase's independence from wavelength for dispersion-free performance, contrasting with propagation-phase alternatives.^[23] Spatial light modulators (SLMs) facilitate dynamic generation of optical vortices through digital holography, allowing real-time reconfiguration of phase patterns for arbitrary topological charges. By displaying computer-generated holograms on liquid-crystal SLMs, users can produce Laguerre-Gaussian beams with charges exceeding 100, with diffraction efficiencies up to 90% after optimization. This method supports multiplexed vortices, where multiple OAM modes are encoded in a single hologram, enabling rapid switching in under milliseconds.^[24]^[2] SLMs excel in laboratory settings for their versatility, though pixelation limits resolution for ultra-high charges beyond 200.^[25] Nonlinear optical processes, such as second-harmonic generation (SHG) in birefringent crystals, produce vortex beams where the topological charge doubles due to OAM conservation in the interaction. In type-I SHG, a fundamental beam with charge l yields a harmonic beam with charge $2l, as the nonlinear polarization couples two photons of the same OAM. Experiments with potassium titanyl phosphate crystals have demonstrated this doubling for charges up to 10, achieving conversion efficiencies of 20-30% for femtosecond pulses.^[26]^[27] OAM is conserved in the process, ensuring the harmonic vortex inherits twice the angular momentum per photon from the pump.^[28] Integrated photonics on silicon chips integrates vortex generation directly into waveguides, using forked grating couplers or mode converters for compact, scalable sources. Silicon photonic devices can emit vector vortex beams with charges ±1 to ±5 at telecom wavelengths (1550 nm), with outcoupling efficiencies over 50% into free space. Recent 2025 designs achieve all-on-chip vortex lattices via inverse-engineered nanostructures, supporting up to six independent OAM modes with crosstalk below -20 dB.^[29]^[30] Plasmonic vortices in waveguides exploit surface plasmon polaritons to confine optical vortices to subwavelength scales, enabling nanoscale manipulation. Gold or silver nanoantennas patterned along dielectric waveguides generate plasmonic vortices with charges up to 5, propagating losses under 1 dB/mm at near-infrared wavelengths. These structures support multiplication of OAM in resonant cavities, where successive round trips double the charge, as demonstrated in hybrid metal-insulator platforms.^[31]^[32] Waveguide-based plasmonic vortices maintain phase stability over millimeters, ideal for on-chip routing.^[33] In 2025, quantum dots emerged as on-demand sources of vortex emission, with colloidal PbS quantum dot LEDs producing broadband vortex beams under electrical injection. These non-lasing devices emit directional vortices with charges ±1 to ±3 across 100 nm bandwidths centered at 1300 nm, with external quantum efficiencies up to 10%. The vortex structure arises from anisotropic emission patterns in dot arrays, enabling compact, electrically tunable sources without external optics.^[34] Exciton-polariton condensates in microcavities also advanced vortex generation in 2025, forming stable vortex molecules under nonresonant pumping. These condensates, realized in GaAs-based structures, spontaneously emit vortices with paired charges up to 4, tunable via asymmetric ring potentials with lifetimes exceeding 100 picoseconds. High-dimensional vortices, including lines and rings, were dynamically realized through emergent nonlinearities, offering coherent sources for quantum simulations.^[35]^[36] Such polariton-based methods provide dissipation-tolerant, room-temperature operation in perovskite lattices.^[37]

Detection and Characterization

Interferometric Methods

Interferometric methods provide a phase-sensitive approach to visualize and quantify optical vortices by superimposing the vortex beam with a reference beam, revealing phase singularities through characteristic interference patterns. These techniques exploit the helical phase structure of vortex beams, where the topological charge l manifests as dislocations or bifurcations in the fringes. Fork interferometry is a fundamental technique involving the superposition of an optical vortex beam with an inclined plane wave reference, producing a fork-like fringe pattern. The central dislocation in the interferogram appears as a fork, where the number of extra fringes or prongs at the bifurcation equals the magnitude of the topological charge |l|, and the direction of the fork tilt indicates the sign of l (left-handed for positive, right-handed for negative). This method was experimentally demonstrated using simple beam splitters and mirrors, enabling straightforward identification of single vortices with charges up to several units.^[38] Mach-Zehnder interferometer setups facilitate off-axis holography for reconstructing phase maps of vortex beams. In this configuration, the vortex beam in one arm interferes with a tilted reference beam in the other, recording a hologram that separates the phase information via spatial frequency filtering. Digital reconstruction yields the unwrapped phase, pinpointing singularities and their charges; for instance, high-order vortices with |l| > 10 have been resolved by analyzing the helical phase gradient around the core. Such setups enhance resolution for complex beams containing multiple vortices. Self-interference methods improve vortex visibility without external references by incorporating elements like Dove prisms or spiral phase plates in the interferometer arms. A Dove prism in a modified Mach-Zehnder setup generates a laterally sheared conjugate copy of the vortex beam, yielding petal-like interference patterns where the number of bright petals corresponds to $2|l|, and off-axis adjustment produces forks for sign determination. Alternatively, placing a spiral phase plate in the reference arm imparts a known charge l_r, creating moiré-like patterns upon interference with the unknown vortex; the resulting charge difference l - l_r is quantified from fringe shifts, enhancing contrast for low-intensity vortices. These approaches are particularly useful for vector-vortex beams. Quantitative analysis of interferograms involves measuring fringe bifurcations to extract charge details. In fork patterns, the bifurcation angle and prong count directly yield |l|, while asymmetry in the fork opening determines the sign; for example, simulations and experiments confirm resolutions better than 0.1 for fractional charges.^[2] Statistical methods, such as fitting vortex core positions in multi-vortex lattices, further refine measurements by averaging over multiple dislocations. These methods are limited by sensitivity to precise alignment of beams, where misalignment can distort fringes and obscure singularities, and by the coherence length of the light source, which must exceed the beam path differences to maintain high-contrast patterns. For broadband or pulsed vortices, temporal coherence issues further degrade resolution.

Direct and Indirect Measurement Techniques

Direct imaging techniques for optical vortices rely on capturing the characteristic doughnut-shaped intensity profile, where the central intensity null corresponds to the phase singularity. High-resolution charge-coupled device (CCD) cameras are commonly employed to record these transverse intensity distributions, enabling visualization of the vortex core and surrounding annular ring without requiring phase-sensitive interferometry. For instance, in experiments with nanostructured gradient index lenses, CCD imaging confirmed the formation of optical vortices by displaying the expected donut-like intensity patterns, allowing determination of the topological charge through radial intensity analysis.^[39] To probe the vortex core more precisely, where intensity is minimal but phase gradients are steep, multiphoton absorption processes have been utilized. In tightly focused vortex beams, two-photon absorption in nonlinear media reveals the core structure, as the absorption rate depends on the local intensity squared and thus accentuates the singularity.^[40] This method has been demonstrated with Laguerre-Gaussian modes, where multiphoton ionization of atoms near the focus distinguishes orbital angular momentum (OAM) states through photoelectron angular distributions, providing indirect access to the core's topological features.^[41] OAM spectrum analysis decomposes vortex beams into a basis of Laguerre-Gaussian (LG) modes to quantify the modal content and purity. Coordinate transformation methods, such as applying a logarithmic spiral transformation to the beam's intensity profile, convert the helical phase into a linear one, facilitating Fourier-domain analysis of the OAM spectrum. Alternatively, mode projectors using phase masks or spatial light modulators project the input beam onto individual LG basis states, measuring the overlap via coupling efficiency into single-mode fibers; this approach has achieved high-fidelity decomposition for beams with mixed OAM, resolving components up to topological charges of ±10.^[42]^[43] Indirect inference of OAM through momentum transfer is achieved by observing the rotational dynamics of particles trapped in vortex beams. In optical tweezers, absorbing microparticles experience torque from the beam's azimuthal phase gradient, leading to rotation rates proportional to the OAM; for example, silica spheres trapped in Laguerre-Gaussian modes rotate at angular velocities scaling linearly with the topological charge, allowing calibration of OAM from the observed rotation without direct phase measurement. This technique has been extended to elastic particles in acoustical analogs, confirming OAM transfer through stable 3D trapping and rotation.^[44]^[45] For vortices involving spin-orbit coupling, polarization-sensitive detection employs Stokes parameter imaging to map the intertwined spin and orbital components. By measuring the full Stokes vector (S0, S1, S2, S3) across the beam profile using polarimeters, singularities in the polarization ellipse orientation reveal the vortex structure; in spin-orbit hybrid beams, these Stokes vortices exhibit C-point morphologies where the polarization state degenerates. Such imaging has characterized perfect vortex beams, distinguishing pure OAM from spin contributions via the spatial variation of circular polarization (S3).^[46]^[47] Recent advancements include weak measurement protocols for amplifying subtle OAM signals in noisy environments. In weak measurement setups, a weakly interacting probe beam coupled to the vortex experiences a post-selected shift proportional to the OAM spectrum, enabling super-resolution detection; for instance, orbital angular momentum has amplified weak signals by factors exceeding 100 in structured light, applied to vortex beams for precision topological charge estimation up to 2024. Complementing this, quantum tomography reconstructs the density matrix of single-photon vortex states using projective measurements in the LG basis. Metasurface-based tomographs project single photons onto OAM modes via geometric phase elements, achieving fidelities over 90% for states with topological charges up to 5, as demonstrated in 2023 experiments with heralded single-photon sources. These methods extend to high-dimensional OAM entanglement, providing full state characterization for quantum applications. As of 2025, integrated all-on-chip platforms have enabled reconfigurable vector vortex detection and sorting, advancing compact applications in communications and sensing.^[48]^[49]^[29]

Applications

Optical Manipulation and Tweezers

Optical vortices, particularly those carried by Laguerre-Gaussian (LG) beams, have revolutionized optical tweezers by enabling non-contact manipulation of microscopic particles with enhanced precision and reduced perturbations. In traditional Gaussian beam tweezers, particles are trapped at the high-intensity focus, but LG beams produce a doughnut-shaped intensity profile with a dark central core, allowing particles—especially absorbing ones—to be stably trapped on-axis without direct exposure to peak intensity, thereby minimizing heating effects that could damage sensitive samples like biological cells or nanoparticles. This configuration facilitates three-dimensional positioning and rotation of trapped objects, such as rotating entire cells or aligning nanoparticles for assembly, as demonstrated in early experiments using mode-converting optics to generate LG beams in tweezers setups. A key advantage of optical vortices in tweezers is their ability to impart orbital angular momentum (OAM), which induces rotational motion in trapped particles. For birefringent particles, the OAM transfers torque proportional to the topological charge l of the vortex, causing controlled spinning around the beam axis at rates that scale linearly with l, enabling studies of rotational dynamics without mechanical contact. This torque application has been pivotal in orienting anisotropic particles and probing their mechanical properties, with experimental verification showing rotation frequencies up to several hertz for micron-sized calcite particles under low-power illumination. Vortex lattices, formed by superimposing multiple LG beams or using spatial light modulators, extend this capability to simultaneous trapping and orbiting of particle arrays, creating dynamic configurations like rings or chains where particles circulate in orbital paths defined by the lattice geometry. Such arrays allow for collective manipulation of dozens of particles, useful in sorting or assembling microstructures, with stability enhanced by the phase singularities that prevent unwanted scattering. In biological contexts, vortex tweezers enable non-invasive rotation of spermatozoa for motility analysis or twisting of DNA strands to investigate supercoiling in microfluidic environments, providing insights into cellular mechanics without altering sample viability.^[2]^[50]^[51] Recent advancements integrate optical vortices with acoustic fields in hybrid tweezers, combining OAM-driven rotation with acoustic radiation forces for robust trapping in turbid or viscous media, such as biological fluids, where pure optical methods may falter due to scattering. These hybrid systems achieve greater versatility, for instance, by using ultrasound to pre-position particles before optical vortex refinement, enhancing throughput in applications like cell sorting or nanoparticle delivery.^[52]

Optical Communications

Optical vortices, characterized by their helical phase fronts and associated orbital angular momentum (OAM), enable multiplexing in optical communications by exploiting the orthogonality of modes with different topological charges l. This allows multiple independent data channels to be encoded on spatially overlapping beams, extending capacity beyond traditional polarization-division multiplexing (PDM) and wavelength-division multiplexing (WDM). Seminal demonstrations have achieved petabit-scale aggregate rates, such as 1.036 Pbit/s over short free-space links using 26 OAM modes combined with WDM and PDM.^[53]^[54] In free-space optical systems, OAM multiplexing supports high-capacity links over long distances, with a landmark experiment transmitting OAM superposition modes (l = \pm1, \pm2, \pm3) over 143 km between La Palma and Tenerife in the Canary Islands, achieving mode recognition accuracies of 80–84% despite strong atmospheric turbulence. Turbulence induces mode distortion and crosstalk through beam spreading and scintillation, but mitigation via adaptive optics—such as wavefront correction with deformable mirrors and tip-tilt stabilization—has enabled robust bidirectional OAM transmission over 1 km with reduced bit error rates below forward error correction thresholds. These techniques, including multiple-input-multiple-output (MIMO) equalization, address misalignment and partial aperture effects, facilitating applications in satellite-to-ground and underwater communications.^[55]^[56]^[53] For fiber-optic integration, ring-core fibers are designed to propagate pure OAM modes with minimal intermodal coupling, supporting mode-division multiplexing (MDM). Early implementations achieved 1.6 Tbit/s over 1.1 km using four OAM modes in a custom ring-core fiber, while recent advances have demonstrated 4.32 Tbit/s via three OAM modes with intensity-modulation direct-detection and nonlinear equalization.^[57]^[57] By 2025, weakly coupled 19-core ring-core fibers supporting five OAM mode groups have enabled multiplexing across three cores over 3.5 km in the C-band, scaling toward terabit capacities on kilometer distances through optimized index profiles and low-loss designs.^[58] Demultiplexing OAM modes faces challenges from mode-dependent loss, arising from differential beam divergence, and crosstalk due to fiber imperfections or turbulence-induced mode scrambling. Photonic lanterns, which adiabatically transition multimode inputs to single-mode outputs, effectively solve these by selectively coupling OAM modes with low insertion loss (<1 dB) and crosstalk suppression (>20 dB), as demonstrated in annular multicore variants for multi-order OAM groups. These devices integrate seamlessly with standard fibers, enabling scalable MDM systems.^[53]^[59] OAM modes enhance security in optical communications due to their fragility under perturbation; even slight disturbances scramble the helical structure, making unauthorized interception difficult without precise mode reconstruction. This property positions OAM as an encryption key in schemes like dynamic speckle conversion, where modes are mapped to complex patterns via diffusers, achieving bit error rates as low as 0.008% upon deep-learning-assisted decryption while resisting eavesdropping. High-security implementations, such as OAM-chaotic carrier laser multiplexing, have demonstrated 100 Gbit/s robust transmission with enhanced photon efficiency for key distribution.^[60]^[61]^[53]

Imaging and Microscopy

Optical vortices enhance imaging and microscopy by leveraging their helical phase structure and central intensity null to achieve sub-diffraction resolution and improved contrast, surpassing the limitations of conventional Gaussian beam illumination. In stimulated emission depletion (STED) microscopy, vortex beams generate doughnut-shaped depletion profiles that selectively de-excite fluorophores in the outer regions of the excitation spot, confining emission to a central region smaller than the diffraction limit. This approach yields lateral resolutions as fine as 103 nm, approximately four times better than standard confocal imaging, while maintaining high contrast through an extinction ratio exceeding 17.5 dB in the depletion beam. Vortex fibers, which convert Gaussian modes to vortex modes with over 98% purity, enable robust implementation even in flexible endoscopic setups.^[62] Vortex scanning techniques utilize the orbital angular momentum of vortex beams to trace helical paths during confocal microscopy, facilitating faster volumetric imaging with reduced artifacts compared to linear raster scanning. In the optical vortex scanning microscope (OVSM), a focused vortex beam interacts with phase objects, producing detectable disturbances at the vortex core that allow reconstruction of 3D structures with nanometer precision and minimal photobleaching. This method enables artifact-free imaging of complex samples by exploiting the beam's sensitivity to phase perturbations, achieving reconstruction accuracy for features as small as 1 μm in biological specimens.^[63] For phase contrast enhancement, optical vortices convert subtle phase variations in transparent biological samples into detectable intensity changes, revealing hidden structures such as cellular edges and internal features that are invisible in standard bright-field imaging. Fractional vortex filters, with topological charges between 0.5 and 1, provide gradual edge enhancement, inverting edge brightness relative to the background and improving isotropic contrast for objects like epithelial cheek cells. This technique enhances resolution of phase jumps by orders of magnitude, enabling clear visualization of dynamic biological processes without staining.^[64] In super-resolution applications, optical vortices integrated with structured illumination microscopy (SIM) generate patterned illumination that extends the detectable spatial frequencies, achieving nanoscale resolution in live-cell imaging. Perfect optical vortices combined with plasmonic SIM create standing wave patterns on metal films, reducing fluorescence background noise and enabling resolutions below 200 nm for wide-field dynamic observation of cellular structures. Vortex-speckle illumination in quantitative phase microscopy further refines this by synergizing dynamic speckles with vortex phases, improving lateral resolution from 780 nm (Gaussian) to better than 540 nm across a 300 × 260 μm² field of view in biological samples.^[65]^[66] Recent advances in 2024 have introduced vortex light field microscopy (VLFM) for volumetric imaging, adapting light-sheet principles with twisted phase encoding to achieve simultaneous 3D spatial and spectral localization of single molecules. VLFM employs a microlens array and prism to displace point spread functions radially and azimuthally, delivering 25 nm spatial and 3 nm spectral precision over a 4 μm depth of field in light-sheet illuminated samples like COS-7 cells. This enables four-color tracking and dSTORM super-resolution of spectrally similar dyes separated by just 15 nm, advancing artifact-free volumetric analysis of live specimens.^[67]

Emerging and Quantum Applications

In quantum optics, optical vortices carrying orbital angular momentum (OAM) have enabled the generation of entangled photon pairs with high-dimensional vortex states, facilitating applications in quantum teleportation. For instance, deterministic all-optical quantum teleportation has been multiplexed across multiple OAM channels, achieving parallel transfer of unknown quantum states with fidelities exceeding 90% in experimental setups.^[68] Similarly, high-dimensional spatial modes encoded in perfect optical vortices have demonstrated quantum teleportation of qudit states up to dimensionality 10, preserving OAM entanglement over free-space links. These vortex states also serve as a basis for OAM-based qubits and qudits, with room-temperature on-chip sources producing single photons in pure OAM modes (purity g^(2)(0) ≈ 0.22), enabling scalable quantum information processing.^[69] Recent advances include qudit-based variational quantum eigensolvers using photonic OAM states to solve molecular ground-state problems, highlighting the role of vortex modes in hybrid quantum algorithms.^[70] Quantum vortices in exciton-polariton condensates provide an optical analog to superfluid vortices, allowing simulation of quantum turbulence in driven-dissipative systems. In these microcavity-based platforms, vortex-antivortex pairs form and interact under non-equilibrium conditions, mimicking quantized circulation in superfluids with healing lengths on the order of micrometers.^[71] Scattering dynamics reveal reconnection events and velocity fields analogous to classical turbulence, but quantized by the polariton superfluid phase.^[71] Recent theoretical models of driven-dissipative turbulence in polariton fluids predict cascade energy transfer to higher OAM modes, enabling studies of two-dimensional quantum turbulence with vortex densities up to 10^4 cm^(-2). Topological pathways in these systems further control vortex nucleation and decay, offering insights into non-equilibrium phase transitions. OAM-enhanced interferometry leverages the helical phase of optical vortices for precision metrology, surpassing classical limits in rotation sensing. Vortex beams in Sagnac interferometers amplify rotational phase shifts by the topological charge l, achieving sensitivities down to 10^(-8) rad/s/√Hz for angular velocities.^[72] Compact rotational Doppler velocimeters using OAM modes enable reciprocal detection of rotating objects with resolutions improved by factors of l up to 100, applicable in inertial navigation.^[73] These techniques exploit the immunity of OAM to linear perturbations, providing quantum-enhanced precision in noisy environments beyond the standard quantum limit.^[72] In biomedical applications, optical vortices facilitate twist-free imaging in curved tissues by maintaining OAM propagation through multimode fibers and scattering media. Vortex beams generated in fibers enable super-resolution endoscopy, such as in STED two-photon setups where azimuthally polarized vortices create doughnut-shaped foci, breaking diffraction limits in biological samples with resolutions below 100 nm.^[74] Multi-mode vortex illumination through turbid tissues, like skin or brain phantoms, enhances contrast in deep imaging by exploiting phase singularities to reduce scattering crosstalk, achieving signal-to-noise ratios up to 20 dB higher than Gaussian beams.^[75] In twisted multicore fibers for flexible endoscopes, OAM modes compensate for fiber torsion, preserving helical wavefronts for undistorted volumetric imaging in curved anatomical paths.^[76] By 2025, developments in long-distance fiber OAM networks have extended transmission beyond 1 km, with ring-core erbium-doped fibers amplifying multiple OAM modes for capacities exceeding 1 Pbps over 10 km.^[77] Hybrid quantum-classical processors incorporating vortex modes have emerged, using OAM qudits for quantum gates interfaced with classical optical neural networks, demonstrating error-corrected state preparation in dimensions up to 16.^[70] These integrations support secure quantum key distribution over fiber links greater than 1 km, combining OAM multiplexing with entanglement distribution.

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