Vectorial Light Research Articles

Generation of arbitrary vector orbital angular momentum beams on higher-order Poincaré spheres based on an all-dielectric metasurface with complex amplitude modulation

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.

Cooperative Decay of an Ensemble of Atoms in a One-Dimensional Chain with a Single Excitation

We propose a new expression of the cooperative decay rate of a one-dimensional chain of N two-level atoms in the single-excitation configuration. From it, the interference nature of superradiance and subradiance arises naturally, without the need to solve the eigenvalue problem of the atom–atom interaction Green function. The cooperative decay rate can be interpreted as the imaginary part of the expectation value of the effective non-Hermitian Hamiltonian of the system, evaluated over a generalized Dicke state of N atoms in the single-excitation manifold. Whereas the subradiant decay rate is zero for an infinite chain, it decreases as 1/N for a finite chain. A simple approximated expression for the cooperative decay rate is obtained as a function of the lattice constant d and the atomic number N. The results are obtained first for the scalar model and then extended to the vectorial light model, assuming all the dipoles aligned.

Tailoring an arbitrary large vectorial structured light beam array utilizing the tensor theory of multiplexed polarization holograms

Vectorial structured light beams, characterized by their topological charge and non-uniform polarization distribution, are highly promising beam modes for several applications in different domains of optics and photonics. To harness its potential specifically in optical communication, data encryption, and optical trapping, it is necessary to tailor a multitude of these beams with arbitrary and large topological charge and polarization distribution. However, achieving the above-mentioned requires bulky optical setups that necessitate the superposition of two beams or involve complex material fabrication techniques that can directly generate these beams. In this paper, we report the generation of a large structured light beam array by utilizing multiplexed polarization holograms, computer-generated holography, and azo-carbazole polymer film. We have developed a theoretical framework for double-exposure polarization holography that enables the possibility of tailoring such a vectorial light beam array. Utilizing the developed theory, we showcase the experimental generation of a structured vector beam array of size 8 × 8 with arbitrary topological charges and polarization distribution in 3 mm × 3 mm area of the polymer film. Exploiting the large space bandwidth of the polymer film, we also demonstrate the generation of vector vortex beam arrays with exceptionally large topological charges (l=100). All the above has been experimentally realized by simply illuminating the hologram with a plane Gaussian beam, and no additional optics are needed. This reported method offers huge potential and opens up new possibilities for the utilization of vectorial structured light beams.

Controlling the hidden parity in vectorial light with metasurfaces

Controlling the hidden parity in vectorial light with metasurfaces

Polarimetric measurement of temporal coherence in electromagnetic light beams

We present a method to determine the degree of temporal coherence of a quasimonochromatic vectorial light beam by polarimetric measurements. More specifically, we employ Michelson’s interferometer in which the polarization Stokes parameters of the output (interference) beam are measured as a function of the time delay. Such a measurement enables us to deduce the magnitudes of the coherence (two-time) Stokes parameters, and hence the degree of coherence, of the input beam. Compared to existing methods the current technique has the benefits that neither optical elements in the arms of the interferometer nor visibility measurements are needed. The method is demonstrated with a laser diode and a filtered halogen source of various degrees of polarization.

Coherent manipulation of vectorial soliton beam in sodium like atomic medium

A four-level, sodium like atomic medium, driven by a probe and control fields, is used to modify the paraxial vectorial light beam in the medium. A uniform packet of vector arrows is controlled, whose shape is stable and undistorted during propagation, known as vectorial soliton. To the best of our knowledge, significant control over vectorial soliton is reported for the first time in the present work. Optical vectorial solitonic beams are controlled and manipulated with the parameters of the coupled fields with the system. The vectorial soliton is a strong, undistorted function of detunings, Rabi frequencies, and collective phase of the deriving fields, respectively. The modified vectorial solitons may have useful applications in telecommunication technology for delivering data bit streams over long distances and data processing.

Cross-spectral purity of the Stokes parameters in random nonstationary electromagnetic beams.

We consider cross-spectral purity in random nonstationary electromagnetic beams in terms of the Stokes parameters representing the spectral density and the spectral polarization state. We show that a Stokes parameter being cross-spectrally pure is consistent with the property that the corresponding normalized time-integrated coherence (two-point) Stokes parameter satisfies a certain reduction formula. The current analysis differs from the previous works on cross-spectral purity of nonstationary light beams such that the purity condition is in line with Mandel's original definition. In addition, in contrast to earlier works concerning the cross-spectral purity of the polarization-state Stokes parameters, intensity-normalized coherence Stokes parameters are applied. It is consequently found that in addition to separate spatial and temporal coherence factors the reduction formula contains a third factor that depends exclusively on polarization properties. We further show that cross-spectral purity implies a specific structure for electromagnetic spectral spatial correlations. The results of this work constitute foundational advances in the interference of random nonstationary vectorial light.

Interfacial modification of GO-FePt hybrid film leads to giant ultrafast optical nonlinearity

Excellent optical nonlinearity is highly promising for the development of multifunctional ultrafast photonics techniques and integrated optoelectronic devices. However, existing numerous optical materials commonly suffer from several challenges, such as bad functional integration, slow response speed, and weak optical nonlinearity thus severely hindering their extensive applications. Here, we first engineer new scalable graphene oxide (GO)-FePt films by chemistry induced self-assembly and then demonstrate the enhanced ultrafast optical nonlinearity by resorting to their interfacial modifications. It is found that the FePt magnetic nanoparticles (NPs) are easily assembled on the surface of the GO by exchanging the surface ligands of the former with evaporated solvent, leading to good water solubility and super-paramagnetism. The femtosecond Z-scan test results show that both the nonlinear saturation absorption and optical Kerr refraction of the pure GO are subjected to reversion once the FePt NPs are introduced. More importantly, we can greatly enhance the ultrafast nonlinear optics response of as-prepared GO-FePt hybrid film by promoting the concentration of magnetic NPs. In addition, the associated optical nonlinearity can be further boosted with femtosecond vectorial beams excitation instead of the linearly polarized one. In principle, the enhanced optical nonlinearity may arise from direct metal-to-semiconductor interfacial charge transfer transition (DICTT) related to the difference in work function and Fermi level together with complicated interaction between the vectorial light fields and the hybrid films. The GO-based magnetic hybrid films with giant ultrafast optical nonlinearity have potential wide-ranging applications in integrated optoelectronics, ultrafast nanophotonics, opto-magnetic storage, and beyond.

Spin Hall Effect in Paraxial Vectorial Light Beams with an Infinite Number of Polarization Singularities.

Elements of micromachines can be driven by light, including structured light with phase and/or polarization singularities. We investigate here a paraxial vector Gaussian beam with an infinite number of polarization singularities residing evenly on a straight line. The intensity distribution is derived analytically and the polarization singularities are shown to exist only in the initial plane and in the far field. The azimuthal angle of the polarization singularities is shown to increase in the far field by π/2. We obtain the longitudinal component of the spin angular momentum (SAM) density and show that it is independent of the azimuthal angle of the polarization singularities. Upon propagation in free space, an infinite number of C-points is generated, where polarization is circular. We show that the SAM density distribution has a shape of four spots, two with left and two with right elliptic polarization. The distance to the transverse plane with the maximal SAM density decreases with decreasing distance between the polarization singularities in the initial plane. Generating such alternating areas with positive and negative SAM density, despite linear polarization in the initial plane, manifests the optical spin Hall effect. Application areas of the obtained results include designing micromachines with optically driven elements.

The invariance and distortion of vectorial light across a real-world free space link

Vectorial structured light, where the polarization is inhomogeneously distributed in space, has found a myriad of applications in both 2D and 3D optical fields. Here, we present an experimental study of the invariance and distortion of vectorial light through a real-world medium of atmospheric turbulence. We show that the amplitude and polarization structure are both severely distorted by the turbulent medium, yet the non-separability of these two degrees of freedom remains invariant. We monitor this invariance under a range of beam types and atmospheric conditions, over extended time periods, revealing the unitary nature of atmospheric turbulence in our experiment. Our results provide conclusive evidence that invariance and distortion are not mutually exclusive and that the degree of classical entanglement remains unaltered through such channels, and will be of interest to the large community interested in classical and quantum communication in free space.

A Robust Basis for Multi‐Bit Optical Communication with Vectorial Light (Laser Photonics Rev. 17(6)/2023)

Multi-Bit Optical Communication with Vectorial Light In article number 2200844, Keshaan Singh, Andrew Forbes, and colleagues outline a new optical communication protocol that exploits spatial patterns of light for multi-dimensional encoding in a manner that does not require the patterns to be recognised, thus overcoming the prior limitation of modal distortion in noisy channels. The result is a new encoding state-of-the-art of over 50 vectorial patterns of light sent virtually noise-free across a turbulent atmosphere that can immediately be extended to optical fibre and underwater links.

A Robust Basis for Multi‐Bit Optical Communication with Vectorial Light

AbstractIncreasing the information capacity of communication channels is a pressing need, driven by growing data demands and the consequent impending data crunch with existing modulation schemes. In this regard, mode division multiplexing (MDM), where spatial modes of light form the encoding basis, has enormous potential but is impeded by noise due to imperfect channels. Here, this challenge is overcome by breaking the existing MDM paradigm of using the modes themselves as a discrete basis and instead exploiting the polarization inhomogeneity (vectorness) of vectorial light as the information carrier. It is shown that this vectorness communication basis is completely impervious to channel noise, which is verified by near perfect data fidelity maintained in multi‐bit information transfer through atmospheric turbulence, with negligible changes on the order of 1%. This allows for demonstration of a new state‐of‐the‐art of 50 vectorial modes in a communications channel with little cross‐talk. This approach replaces conventional amplitude modulation with a novel modal alternative for potentially orders of magnitude channel information enhancement, offering a new paradigm to exploiting the spatial mode basis for optical communication.

Advancing 3D shaping of vectorial light by counter-propagation of self-healing scalar and vector Bessel–Gaussian beams

It is well known that counter-propagation of structured light fields allows shaping of three-dimensional (3D) structures in amplitude, phase, or polarization. Here, we numerically demonstrate the potential of implementing non-diffracting Bessel–Gaussian (BG) beams for advancing this approach by taking advantage of its characteristic propagation behavior. In this context, we investigate the self-healing property in this counter-propagating configuration, observing a spin angular momentum (SAM) variation and the formation of a continuous orbital angular momentum (OAM) gradient in longitudinal direction. Additionally, by counter-propagation of BG beams of different types, namely, scalar and vector BG beams, we are able to increase the complexity of accessible 3D structured fields, revealing combined amplitude, phase, and polarization modulation in all spatial dimensions. Thereby, the SAM and OAM of the input light fields can be used to design the resulting 3D structure and its angular momenta. The presented light fields open new possibilities for customized optical trapping potentials and allow new insights into fundamental spin–orbit interaction in counter-propagating superpositions of structured fields.

Revealing the invariance of vectorial structured light in complex media

Optical aberrations have been studied for centuries, placing fundamental limits on the achievable resolution in focusing and imaging. In the context of structured light, the spatial pattern is distorted in amplitude and phase, often arising from optical imperfections, element misalignment, or even from dynamic processes due to propagation through perturbing media such as living tissue, free-space, underwater and optical fibre. Here we show that the polarisation inhomogeneity that defines vectorial structured light is immune to all such perturbations, provided they are unitary. By way of example, we study the robustness of vector vortex beams to tilted lenses and atmospheric turbulence, both highly asymmetric aberrations, demonstrating that the inhomogeneous nature of the polarisation remains unaltered from the near-field to far-field, even as the structure itself changes. The unitary nature of the channel allows us to undo this change through a simple lossless operation, tailoring light that appears robust in all its spatial structure regardless of the medium. Our insight highlights the overlooked role of measurement in describing classical vectorial light fields, in doing so resolving prior contradictory reports on the robustness of vector beams in complex media. This paves the way to the versatile application of vectorial structured light, even through non-ideal optical systems, crucial in applications such as imaging deep into tissue and optical communication across noisy channels.

A non-separability measure for spatially disjoint vectorial fields

Vectorial forms of structured light that are non-separable in their spatial and polarisation degrees of freedom have become topical of late, with an extensive toolkit for their creation and control. In contrast, the toolkit for quantifying their non-separability, the inhomogeneity of the polarisation structure, is less developed and in some cases fails altogether. To overcome this, here we introduce a new measure for vectorial light, which we demonstrate both theoretically and experimentally. We consider the general case where the local polarisation homogeneity can vary spatially across the field, from scalar to vector, a condition that can arise naturally if the composite scalar fields are path separable during propagation, leading to spatially disjoint vectorial light. We show how the new measure correctly accounts for the local path-like separability of the individual scalar beams, which can have varying degrees of disjointness, even though the global vectorial field remains intact. Our work attempts to address a pressing issue in the analysis of such complex light fields, and raises important questions on spatial coherence in the context of vectorially polarised light.

Transfer and evolution of structured polarization in a double-V atomic system.

We numerically investigate the transfer of optical information from a vector-vortex control beam to an unstructured probe beam, as mediated by an atomic vapour. The right and left circular components of these beams drive the atomic transitions of a double-V system, with the atoms acting as a spatially varying circular birefringent medium. Modeling the propagation of the light fields, we find that, for short distances, the vectorial light structure is transferred from the control field to the probe. However, for larger propagation lengths, diffraction causes the circular components of the probe field to spatially separate. We model this system for the D1 line of cold rubidium atoms and demonstrate that four wave mixing can lead to correlations between the optical polarization structure and the diffraction of light, generating coupled dynamics of the internal and external degrees of freedom.

Ultrafast Spin‐to‐Orbit and Orbit‐to‐Local‐Spin Conversions of Tightly Focused Hybridly Polarized Light Pulses

Spin–orbit interaction (SOI) has provided a new viable roadmap for the development of spin‐based photonics devices. However, existing strategies to control the SOI focus commonly on tailoring the spatial dimension of light fields yet neglecting the inherent temporal one. Herein, ultrafast temporal effects on both spin‐to‐orbit and orbit‐to‐local‐spin conversions based on the time‐assistant vectorial diffractive theory and the fast Fourier transformation are first presented. Such interconversions depend upon whether the incident hybridly vectorial light pulse carries vortex phase or not in a single high numerical aperture geometry. For the case of the absence of vortex phase, it is found that it enables orbit angular momentum‐carrying transverse component fields, and the resultant orbit angular momentum embedded within focused light fields remains constantly revolving as time elapses, which indicates that the controllable spin‐to‐orbit conversion occurs. By contrast, it is revealed that hybridly polarized vectorial‐vortex light pulses allow access to the locally excited circular polarization at the focus, and this induced local circular polarization is independent of ultrafast varying time, whereas the resulting spin angular momentum density components experience the alternation between the appearance and annihilation over time, thus giving rise to the tunable orbit‐to‐local‐spin conversion. These exotic ultrafast interconversions not only breathe a new life into the area of ultrafast photonics, but refresh our understanding on the paradigm of photonic SOI.

Robust and Scalable Flat‐Optics on Flexible Substrates via Evolutionary Neural Networks

Flexible Flat-Optics This work, presented in article 2100105 by Andrea Fratalocchi and co-workers, implements an artificial intelligent, end-to-end pipeline for the experimental realization of flexible, broadband ultra-flat optics for vectorial light control. The system takes advantage of “physical” neural networks, engineered into suitably designed optical nanoresonators. The design framework also considers fabrication robustness, furnishing devices that maintain their optical response even under strong stress perturbations and deformations.

Twisting Polarization‐Tunable Subdiffraction‐Limited Magnetization through Vectorial Beam Coupling

All‐optical control of magnetization vectors is considered as a powerful solution that empowers the development of multifunctional integrated optomagnetic devices. The interaction between vectorial light and magnetism has recently experienced a surge of interest due to its prominent ability to steer the magnetic polarization orientations and spatial textures within the subwavelength domain, which, however, usually requires complex wavefront coding optimizers or arduous material fabrication procedures. Herein, an optimization‐free all‐vectorial‐optical strategy for first realizing spatially twist‐controllable and successively polarization‐tunable magnetization at the subdiffraction scale through the inverse Faraday effect is conceived and demonstrated. This facile method relies on the coherent coupling of double crossed azimuthally polarized doughnut‐Gaussian vortex beams in a single optically configured geometry. It is found that both the magnetic twisting manifolds and polarization states, rotating from perpendicular (longitudinal), via hybrid (3D) to in‐plane (transverse) orientations, can be on‐demand mediated by judiciously tuning the crossing angles. It is further unraveled that the orbital angular momenta associated with topological charges of the vortex beams can be viewed as a vital knob to energetically maneuver magnetic twisting and guide magnetization switching. It is believed that the proposed route and presented findings are not only of great theoretical implication in twistronics and optomagnetic topology but also of considerable technological significance in multidimensional high‐density and low‐energy consumption optomagnetic storage and engineering of magnetic topologic materials.

Demonstrating Arago-Fresnel laws with Bessel beams from vectorial axicons.

Two-dimensional Bessel beams, both vectorial and scalar, have been extensively studied to date, finding many applications. Here we mimic a vectorial axicon to create one-dimensional scalar Bessel beams embedded in a two-dimensional vectorial field. We use a digital micro-mirror device to interfere orthogonal conical waves from a holographic axicon, and study the boundary of scalar and vectorial states in the context of structured light using the Arago-Fresnel laws. We show that the entire field resembles a vectorial combination of parabolic beams, exhibiting dependence on solutions to the inhomogeneous Bessel equation and asymmetry due to the orbital angular momentum associated rotational diffraction. Our work reveals the rich optical processes involved at the interplay between scalar and vectorial interference, opening intriguing questions on the duality, complementarity, and non-separability of vectorial light fields.