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Herein, we present a picosecond-pulsed optical vortex parametric oscillator capable of generating high-power, narrow-linewidth near- and mid-infrared optical vortex outputs. The optical parametric oscillator (OPO), consisting of a KTiOAsO4 (KTA) crystal and a Z-shaped standing wave cavity formed by five mirrors, transferred the orbital angular momentum (OAM) of the pump field to the signal and idler fields. The transmission mechanism of the OAM within the signal singly resonantsingly-resonant KTA-OPO was investigated, and the OAM was controlled and selectively transferred among the pump, signal, and idler fields by adjusting the focus position of the pump beam on the KTA crystal. With an incident pump power of 17 W, the maximum average output power was 2.14 W at 1535 nm (signal vortex field) and 0.95 W at 3468 nm (idler vortex field), respectively, corresponding to optical conversion efficiencies of 20.8% and 9.2%. The spectral linewidths of the signal and idler vortex fields were 0.502 nm and 1.216 nm, respectively. To the best of our knowledge, this is the first instance of a picosecond-pulsed optical vortex parametric oscillator with a KTA crystal. [ABSTRACT FROM AUTHOR]

Optical vortex induced forward transfer (OV-LIFT) allows the crystallization of magnetic nanoparticles enabling the fabrication of a 2-dimensional array of monocrystalline structures with a high spatial resolution. We herein investigate the optimum experimental conditions, such as the viscosity range of the donor and the energy range of the optical vortex pulse required for the crystallization of magnetic nanoparticles through a cavitation processes, including the flight dynamics of donor. [ABSTRACT FROM AUTHOR]

Supramolecular condensation during cooling cycles often transitions through multiple metastable phases before achieving a stable crystalline state. Metastability arises from various competing parameters like symmetrical arrangement, and supramolecular bonding and manifests at different temperatures. Symmetrical physical arrangements can minimize vibrational energy and stabilize the systems at higher temperatures. Further cooling promotes directional supramolecular bonding, such as charge‐assisted hydrogen bonding, resulting in molecular periodicity within metastable structures. Frustration occurs when weaker van der Waals bonds form during further cooling, propagating perpendicularly to stronger one‐dimensional charge‐assisted hydrogen bonds and disrupting lateral periodicity in certain solvents. This makes parallel 1D fibers slidable, adding flexibility to the gel fiber. Eventually, some supramolecular systems attain thermodynamically stable crystalline states by perfectly arranging all the molecules. Throughout the process, metastability results from different symmetrical arrangements, and each rearrangement alters the supramolecular structure‘s symmetry, generating new physicochemical properties. Different supramolecular gels uniquely break symmetry, which can be monitored through various techniques. This perspective analyzes supramolecular thermoreversible, reverse thermal, liquid crystalline, thixotropic, and antisolvent‐induced gels to illustrate spontaneous symmetry reduction processes. Reaching a suprasymmetry condensate can classify big data and be applied in unconventional analogue computing or data storage. [ABSTRACT FROM AUTHOR]

A metalens for detection an incident field with initially a fractional topological charge in the range from –2 to 0 is considered in this work. The metalens is constructed utilizing a spiral zone plate with a topological charge of –1.5. A change in the topological charge of the focused incident beam is shown by simulation to lead to a displacement of its focal spot from the center on the optical axis and to a change in the intensity maximum value, which results in the change in the intensity on the optical axis by 6.9, the change from –0.6 to –1.5 of the topological charge of the incident beam was considered. The intensity at the focus on the optical axis is also affected by the rotation of the beam with a fractional topological charge. This makes it possible using the metalens to measure the tilt angle of the incident beam in the range from 0° to 110°. [ABSTRACT FROM AUTHOR]

In this work, we investigated tight focusing characteristics of beams, which do not contain longitudinal component of intensity. The investigated beams have azimuthal or sector-azimuthal polarization and could contain vortex phase. It was numerically shown that beams with azimuthal and sector azimuthal polarization do not contain longitudinal component of intensity. Moreover, the helical phase added to the beams does not add longitudinal component to the electric field; however, it could be used for manipulation with longitudinal component of spin angular momentum in the tight focus. The possibility of generation of investigated beams was demonstrated using vector waveplates and spatial light modulator. [ABSTRACT FROM AUTHOR]

In this work, spin-orbit conversion in a vector optical vortex will be considered. The polarization in such a beam corresponds to the polarization of a cylindrical vector beam, that is, it is initially linear at each point. It is shown numerically and analytically using the Richards-Wolf formalism that zones with non-zero longitudinal spin angular momentum are formed in the focal spot, i.e. zones with elliptical polarization. It has been experimentally shown that for the case when the topological charge of the optical vortex coincides with the order of the beam, the observed spin-orbit conversion is large enough to be recorded in the paraxial approximation. [ABSTRACT FROM AUTHOR]

In this study, experiments were conducted aimed at the formation of both standard Laguerre-Gaussian vector beams and more complex structured Laguerre-Gaussian vector beams using a spatial light modulator. The ability to measure the polarization structure of these beams using a differential Stokes polarimeter was experimentally demonstrated. The obtained measurement data on the polarization distribution successfully correspond to theoretical calculations, which confirms the effectiveness and accuracy of this method in the analysis of optical structures. [ABSTRACT FROM AUTHOR]

The sharp focusing of the superposition of a plane wave with the linear polarisation (LP) and an optical vortex with topological charge (TC) m and with the same LP directed along the horizontal axis is considered. Applying the Richards–Wolf formalism, analytical formulas are derived for the intensity and the spin angular momentum (SAM) lengthwise component in the focal plane. It is demonstated that for odd and even TCs m the SAM and the intensity have different symmetries: for even m they are symmetric about both Cartesian axes, and for odd m they are symmetric only about the vertical axis. The intensity pattern has 2m local maxima in the focal plane. The intensity on the optical axis for any TC m is nonzero. The pattern of the lengthwise SAM (spin density) in the focal plane has 2(m + 2) subwavelength areas in the half of which the SAM has positive value while in others it has negative value. The centers of these areas alternately lie on a circle of a certain radius with a center on the optical axis. This spin pattern with different signs shows the spin Hall effect in the focal plane. The total negative and positive spin in the focal plane are mutually compensated and equal to zero. [ABSTRACT FROM AUTHOR]

In recent years, topological waves (TWs), which are characterized by structured phase, polarization, and amplitude, have garnered significant attention across various research fields due to their distinctive physical characteristics. Reflecting on the history of TW research, advancements in TW generation methods have also led to evolution in TW application fields. Photosensitive liquid crystal (LC) polymers have the ability to spatially control optical parameters due to their high flexibility in fabricating three-dimensional anisotropic structures. In this review, we outline our efforts toward generating and applying TWs utilizing the photo-alignment technique of photosensitive LC polymers. [ABSTRACT FROM AUTHOR]

Abstract Recently, vortex beams have been widely studied and applied because they carry orbital angular momentum (OAM). It is widely acknowledged in the scientific community that fractional OAM does not typically exhibit stable propagation; notably, the notion of achieving stable propagation with dual-fractional OAM within a single optical vortex has been deemed impracticable. Here, we address the scientific problem through the combined modulation of phase and polarization, resulting in the generation of a dual-fractional OAM vector vortex beam that can stably exist in free space. Applying this unique characteristic, we derive an integrated analytical model to calculate the focused electromagnetic fields and Poynting vector distributions based on Debye vector diffraction integral. Utilizing phase stitching technology, this research combines two fractional topological charges to investigate the properties of dual-fractional OAM optical vortices with diverse polarization conditions. Furthermore, the transmission characteristics of these optical vortices are meticulously analyzed. This work not only enriches the types of vortex beams but also provides a novel optical tool, potentially transformative for applications in optical communications, optical manipulation, and optical imaging.

In this work, using the Richards-Wolf formalism, we found explicit analytical expressions for the coordinates of the major and minor axes of the polarization ellipse centered in the focal plane for a cylindrical vector beam of integer order n. For such a beam, the major axis of the polarization ellipse lies in the focal plane, whereas the minor axis is perpendicular to the focal plane. Therefore, the polarization ellipse is perpendicular to the focal plane, with the polarization vector rotating clockwise or counterclockwise in this plane (producing 'optical wheels'). Considering that the wave vector is also perpendicular to the focal plane, the polarization ellipse and the wave vector turn out to lie in the same plane, so that at some point of time the polarization vector can coincide with the wave vector, which is not usual for transverse electromagnetic oscillations. For a cylindrical vector beam, the spin angular momentum vector lies in the focal plane. So, when going around a certain circle with the center on the optical axis, the spin vector is directed counterclockwise in some sections of the circle, and clockwise in other sections. This effect can be called the transverse (azimuthal) optical spin Hall effect, in contrast to the well-known longitudinal optical spin Hall effect at the sharp focus. The longitudinal spin Hall effect is understood as the separation in the focal plane of regions with different signs of the longitudinal projection of the spin angular momentum vector. This work shows that there is always an even number of such regions and that when going around a circle, the vector of the major axis of the polarization ellipse forms a two-sided twist surface with an even number of turns.

We analyze the sharp focusing of the superposition of a vortex laser beam with topological charge n and linear polarization and a plane wave with the same linear polarization directed along the horizontal axis. Using the Richards-Wolf formalism, analytical expressions are obtained for the intensity distribution and longitudinal projection of the spin angular momentum (SAM) in the focal plane. It is shown that for even and odd numbers n the intensity and SAM have different symmetries: for even n they are symmetric with respect to both Cartesian axes, and for odd n they are symmetric only with respect to the vertical axis. The intensity distribution has 2n local maxima at the focus, and the intensity on the optical axis is nonzero for any n. The distribution of the longitudinal SAM (spin density) in the focal plane has (n+2) subwavelength regions with a positive SAM and (n+2) regions with a negative SAM, the centers of which alternately locate on a circle of a certain radius centered at the optical axis. Such an alternating-spin pattern demonstrates the spin Hall effect at the focus. At the focus, the negative and positive spins are mutually compensated, meaning that the total spin is zero.

In this work, using the Richards-Wolf formalism, we consider the sharp focusing of optical vortices with hybrid polarization, which combines the properties of azimuthal and circular polarizations. It is shown that these beams have a number of unique properties: the intensity pattern in such beams is rotating with distance from the focal spot and the longitudinal component of the spin angular momentum is asymmetrically shaped.

Abstract We explore the dynamics of nonlinear parametric generation and light beam propagation in a Landau-quantized graphene structure with three energy levels interacting with two laser pulses, utilizing the Maxwell-Bloch equations. By applying a laser field to one transition of the graphene sample while keeping the second beam initially absent, the distinctive preparation of the graphene sample, coupled with its weak interaction with laser radiation, results in the parametric generation of a new laser beam in a different transition. We investigate the influence of diverse system parameters on both the efficiency of the generated beam and the propagation dynamics of both beams. Our findings reveal that manipulating these parameters can induce oscillations in the intensity of propagated beams, mitigate absorption losses during propagation allowing for earlier relaxation, and enhance the efficiency of energy transfer from the initial to the generated beam. Additionally, we demonstrate the transfer of optical vortices within the graphene ensemble by introducing an optical vortex to the initial beam. This scheme holds promise for applications in high-dimensional quantum information processing.

Chiral plasmonic nanomaterials have been widely utilized to study light‐matter interactions due to its capability to amplify chiroptic signals. Conventionally, chiro‐optic experiments have demonstrated interactions between circularly polarized light and materials. However, employing light with chiral phase, i.e., optical vortex, can generate a strong chiral response and holds the potential to unveil extensive material information owing to the infinite topological numbers. In this work, an array of 3D chiral nanoparticles is employed to demonstrate large helical dichroism (HD). Chiral gold nanoparticle arrays are illuminated by vortex beams of opposite helicity, which revealed the high HD value of 0.93. The chiral interaction is theoretically investigated, and enantioselective interaction can be explained by multipole analysis. It is determined that the strong HD is attributed to the interaction of higher‐order multipole moments such as electric quadrupole and magnetic quadrupole moments. This study provides deeper insight into understanding of the interaction between optical vortex and chiral plasmonic nanostructures and paves the way for next‐generation chiroptical applications ranging from ultrasensitive chiral spectroscopy to chiral quantum optics. [ABSTRACT FROM AUTHOR]

The development of straightforward and effective methods to determine the topological charge of phase singular beams is crucial for broadening the applications of optical vortices. Here, a straightforward and elegant method for determining the topological charge of an optical vortex using an innovative switchable achromatic nematic liquid crystal Fresnel lens is proposed. The approach allows for the unambiguous determination of both the absolute value and the sign of the topological charge of a singular beam across a wide range of the visible spectrum, as demonstrated both experimentally and theoretically. The research outcomes are promising for applications in optical information transmission systems, cryptography, and quantum computing. [ABSTRACT FROM AUTHOR]

We explore the dynamics of nonlinear parametric generation and light beam propagation in a Landau-quantized graphene structure with three energy levels interacting with two laser pulses, utilizing the Maxwell-Bloch equations. By applying a laser field to one transition of the graphene sample while keeping the second beam initially absent, the distinctive preparation of the graphene sample, coupled with its weak interaction with laser radiation, results in the parametric generation of a new laser beam in a different transition. We investigate the influence of diverse system parameters on both the efficiency of the generated beam and the propagation dynamics of both beams. Our findings reveal that manipulating these parameters can induce oscillations in the intensity of propagated beams, mitigate absorption losses during propagation allowing for earlier relaxation, and enhance the efficiency of energy transfer from the initial to the generated beam. Additionally, we demonstrate the transfer of optical vortices within the graphene ensemble by introducing an optical vortex to the initial beam. This scheme holds promise for applications in high-dimensional quantum information processing. [ABSTRACT FROM AUTHOR]

It has been shown that the acousto-optic Raman-Nath diffraction on the acoustic wave, which bears an acoustic vortex, is accompanied by the appearance of the diffraction maxima that bear an optical vortex. The charge of the vortices that appears as a result of the diffraction corresponds to the order of diffraction if the incident optical beam is the Gaussian beam and the charge of the acoustic vortex is equal to unity. When the incident optical vortex beam takes part in this process, the charge of the diffracted optical vortices is shifted on the charge value of the incident optical beam. As a result of the analysis, we obtained the relation for the charge of vortices of diffraction maxima. It has been found that the described effect can be used for controlled demultiplexing. [ABSTRACT FROM AUTHOR]

We study a new Laguerre - Gaussian (LG) beam, which differs from conventional LG mode beams that preserve the structure of the intensity distribution up to scale. The proposed beam does not retain its structure upon free-space propagation but shows some interesting properties. This beam is Fourier invariant and has extended dark regions in the initial (waist) plane and in the far field. Thus, while maintaining the beam topological charge, the effective diameter of the central dark intensity spot can be increased or decreased by changing the radial index of the Laguerre polynomial. In addition, this beam has the property of autofocusing, that is, at the Rayleigh distance from the waist, the intensity distribution has the form of a light ring (for any value of the radial index) with a minimum diameter and maximum intensity on the ring. This beam can be used to manipulate microparticles without using an additional focusing spherical lens.

In this paper we study the transmission of higher-order modes, including optical vortices (OVs), through a bus fiber evanescently coupled with a vertical array of ring resonators (VAR), which form a vertically stacked multi-ring resonator. It is shown that the OV transmission curves have a characteristic structure that we explain by the manifestation of the band structure of an infinite stack of coupled ring resonators. We demonstrate a fundamental possibility of using VARs as elements of delay lines for fiber-optic communications using orbital angular momentum. It is shown that the VAR is capable of serving as a delay line element for even and odd Laguerre–Gauss modes.

Herein, we present a picosecond-pulsed optical vortex parametric oscillator capable of generating high-power, narrow-linewidth near- and mid-infrared optical vortex outputs. The optical parametric oscillator (OPO), consisting of a KTiOAsO4 (KTA) crystal and a Z-shaped standing wave cavity formed by five mirrors, transferred the orbital angular momentum (OAM) of the pump field to the signal and idler fields. The transmission mechanism of the OAM within the signal singly resonantsingly-resonant KTA-OPO was investigated, and the OAM was controlled and selectively transferred among the pump, signal, and idler fields by adjusting the focus position of the pump beam on the KTA crystal. With an incident pump power of 17 W, the maximum average output power was 2.14 W at 1535 nm (signal vortex field) and 0.95 W at 3468 nm (idler vortex field), respectively, corresponding to optical conversion efficiencies of 20.8% and 9.2%. The spectral linewidths of the signal and idler vortex fields were 0.502 nm and 1.216 nm, respectively. To the best of our knowledge, this is the first instance of a picosecond-pulsed optical vortex parametric oscillator with a KTA crystal.

Electrons in circular motion emit electromagnetic radiation and lose their energy and angular momentum, both of which are carried away by the radiation field. Electromagnetic radiation from such electrons is not only circularly polarized but also, in general, possessing helical phase structure, the former of which corresponds to spin angular momentum and the latter orbital angular momentum. Based on the classical electrodynamics, we show that the chiral topological property related to the orbital angular momentum arises from deformation of the electromagnetic field due to the relativistic effect. [ABSTRACT FROM AUTHOR]

Chiroptical response, demonstrating chiral interaction between optical vortex and chiral structure, plays an important role in variety of fields like optics and material science. However, the flexibility and efficiency of chiral structure fabrication are limited due to mask requirement and a point‐by‐point constructing strategy. In this paper, a novel chiral lithography method is proposed that utilizes optical vortex phase beam shaping to achieve chiral structure processing with high efficiency and flexibility. By programming topological charges of the vortex phase, chiral structures with adjustable appearance, rotation, and chirality can be produced using femtosecond laser single pulse exposure. Transmittance measurements of fabricated chiral structures array confirm a 66% helical dichroism that is predicted by simulation. Moreover, with the aid of convolutional neural networks (CNN), an accuracy of 98% in Orbital Angular Momentum (OAM) recognition can be achieved. This chiral lithography method provides an alternative for chiral structure fabrication and holds promise in the development of chiral optics, optical communications, and next‐generation optical devices. [ABSTRACT FROM AUTHOR]

This work theoretically tested the possibility of expanding the spectrum of vortex charges generated using multi-cascade optical systems based on LiNbO3 crystals twisted around their optical axis. As a result, analytical expressions for the electric field parameters of the output optical wave were obtained for the four main types of elementary cascades that allow the generation of vortices with charges +1 and –1 and also provide the matching of adjacent cascades in the multi-cascade optical system. At the same time, it was shown that when using the appropriate number of sequentially located cascades, each of which belongs to a certain type from the above, it is possible to generate a vortex beam with arbitrary integer vortex charges, including zero. Using the proposed cascade system, one can operate the outgoing beam parameters. This system can be used at the vortex beam multiplexing for information transfer. [ABSTRACT FROM AUTHOR]

Vortex beams with orbital angular momentum play a crucial role in increasing the information capacity in optical communications. The magnitude of orbital angular momentum determines the ability of information encoding. In practice, a vortex beam can encounter random objects or turbulence during free-space propagation, resulting in information damage. Therefore, accurately measuring the orbital angular momentum of a randomly fluctuated and obstructed vortex beam is a considerable challenge. Herein, we propose a single-shot method for the phase retrieval of a randomly fluctuated and obstructed vortex beam by combining the phase-shift theorem and self-reference holography. Experimental results reveal that the sign and magnitude of the initial orbital angular momentum can be simultaneously determined based on their quantitative relation with the number of coherence singularities on the observation plane, thus addressing the effects of random occlusion and atmospheric turbulence. The proposed method considerably improved the accurate decoding of orbital angular momentum information in nonideal freespace optical communications. [ABSTRACT FROM AUTHOR]

In this paper, tight focusing of a superposition of a vortex laser beam with topological charge n with linear polarization and a plane wave with the same linear polarization directed along the horizontal axis is considered. Using the Richards–Wolf formalism, analytical expressions are obtained for the intensity distribution and longitudinal projection of the spin angular momentum in the focal plane. It is shown that for even and odd numbers n, the intensity and the spin angular momentum have different symmetries: for even n they are symmetric about both Cartesian axes, and for odd n they are symmetric only about the vertical axis. The intensity distribution has n local maxima at the focus, and it is nonzero on the optical axis for any n. The distribution of the longitudinal spin angular momentum (spin density) in the focal plane has (n + 2) subwavelength regions with a positive spin angular momentum and (n + 2) regions with a negative spin angular momentum, the centers of which alternately lie on a circle of a certain radius with a center on the optical axis. This spin distribution with different signs demonstrates the spin Hall effect at the focus. Negative and positive spins are mutually compensated, and the total spin is equal to zero at the focus. We have shown that by changing the topological charge of the optical vortex, it is possible to control the spin Hall effect at the focus, that is, to change the number of regions with spins of different signs. [ABSTRACT FROM AUTHOR]

In this paper, we present a picosecond pulsed, synchronously pumped optical parametric oscillator producing vortex beam output with tunable wavelengths in the near- to mid-infrared range. The system utilizes a Nd:YVO4 picosecond pulsed solid-state laser emitting at a wavelength of 1.064 µm to pump a Z-shaped, singly resonant OPO which contains a MgO:PPLN crystal with a fan-shaped grating. The wavelength tuning characteristics of the OPO output are examined both as a function of the MgO:PPLN grating period and crystal temperature. The orbital angular momentum of the pump field can be selectively transferred to either the signal or idler fields by appropriately adjusting the location of the MgO:PPLN crystal within the OPO cavity. The maximum output power of the signal and idler vortex fields are 5.12 W and 3.46 W, respectively, for an incident pump power of 19 W. [ABSTRACT FROM AUTHOR]

Hybridly polarized (HP) vector vortex Raman lasers dramatically extend their applications on optical microscopy, optical communication, and quantum information. Spatial light modulators and waveplates are widely used for generating HP vector vortex lasers, however, the performance and beam quality of HP vector vortex lasers are restricted by diffraction loss and low damage threshold of these optical elements. Here, HP vector vortex Raman microchip lasers constructed with Yb3+:Y3Al5O12 (Yb:YAG) and vanadate (YVO4) crystals is demonstrated. The states of polarization (SoP) of HP vector vortex lasers are combination of radial and anti‐radial polarizations (RP‐ARP), azimuthal and anti‐azimuthal polarizations (AP‐AAP). The SoP of HP vector vortex lasers can be controlled by adjusting the length of YVO4 crystal and applying pump power. Maximum output powers are 456 and 586 mW with optical efficiency of 7.1% and 9.2% for HP vector vortex lasers with SoP of RP‐ARP and AP‐AAP. The HP vector vortex Raman lasers with SoP of RP‐ARP and AP‐AAP oscillate ≈1076 nm with bandwidths of 11.4 and 10.8 nm. High beam quality is achieved for HP vector vortex lasers with measured M2 nearly equal to theoretical value. The broadband HP vector vortex Raman lasers with high beam quality extend applications on optical trapping, and quantum information processing. [ABSTRACT FROM AUTHOR]

We propose a method for creating parallel Bessel-like vortical optical needles with an arbitrary axial intensity distribution via the superposition of different cone-angle Bessel vortices. We analyzed the interplay between the separation of individual optical vortical needles and their respective lengths and introduce a super-Gaussian function as their axial profile. We also analyzed the physical limitations to observe well-separated optical needles, as they are influenced by the mutual interference of the individual beams. To verify our theoretical and numerical results, we generated controllable spatial arrays of individual Bessel beams with various numbers and spatial separations by altering the spectrum of the incoming laser beam via the spatial light modulator. We demonstrate experimentally how to implement such beams using a diffractive mask. The presented method facilitates the creation of diverse spatial intensity distributions in three dimensions, potentially finding applications in specific microfabrication tasks or other contexts. These beams may have benefits in laser material processing applications such as nanochannel machining, glass via production, modification of glass refractive indices, and glass dicing. [ABSTRACT FROM AUTHOR]

Aiming at the propagation of asymmetric vortex beam, we investigate the spreading and wander characteristics of asymmetric Bessel-Gaussian beam in oceanic turbulence. Based on the cross-spectral density and Wigner distribution function of beam field, the theoretical expressions of spreading factor and beam center displacement are established to evaluate spreading and wander. The anisotropic turbulent condition is adopted to analyze turbulence impacts with more generality. The turbulence dominated by salinity fluctuation with lower anisotropy, larger temperature dissipation and smaller kinetic energy dissipation induces more beam spreading and wander. As for asymmetric Bessel-Gaussian beam, the increment of asymmetry can improve beam directionality. Besides, smaller beam scale but larger Gaussian waist radius, vortex order and beam wavelength also mitigate spreading and wander. The investigations are instructive for the underwater applications of asymmetric vortex beams.

Sharp focusing of a light field with double (phase and polarization) singularity is studied. Using the Richards-Wolf method, an exact analytical expression for the longitudinal projection of the spin angular momentum (SAM) vector at the focus is obtained. The expression derived suggests that 4 (n –1) subwavelength regions are formed at the focus, where n is the cylindrical vector beam order, with their centers located on a certain circle centered on the optical axis. Notably, the SAM projections are found to have the opposite sign in the neighboring regions. This means that in the neighboring focal regions, the light has alternating left or right elliptical polarization (manifestation of a spin Hall effect). At the center of the focal spot near the optical axis, the field is right-handed elliptically polarized at m > 0, or left-handed elliptically polarized at m < 0, where m is the vortex charge. The total longitudinal spin, i.e., the longitudinal SAM component averaged over the beam-cross section, is zero and preserved upon focusing. Due to the beam containing an optical vortex with charge m, the transverse energy flow rotates on a spiral path near the focal plane, rotating on a circle in the focal plane. The rotation direction near the optical axis is counterclockwise for m > 0, and clockwise for m < 0.

Optical vortices (OVs) have rapidly varying spatial phase and optical energy that circulates around points or lines of zero optical intensity. Manipulation of OVs offers innovative approaches for various fields, such as optical sensing, communication, and imaging. In this work, we demonstrate the correlation between OVs and absorption enhancement in two types of structures. First, we introduce a simple planar one-dimensional (1D) structure that manipulates OVs using two coherent light sources. The structure shows a maximum of 6.05-fold absorption gap depending on the presence of OVs. Even a slight difference in the incidence angle can influence the generation/annihilation of OVs, which implies the high sensitivity of angular light detection. Second, we apply inverse design to optimize two-dimensional (2D) perfect ultrathin absorbers. The optimized free-form structure achieves 99.90 % absorptance, and the fabricable grating structure achieves 97.85 % at 775 nm wavelength. To evaluate OV fields and their contribution to achieving absorption enhancement, we introduce a new parameter, OV circularity. The optimized structures generate numerous OVs with a maximum circularity of 95.37 % (free-form) and 96.14 % (grating), superior to our 1D structure. Our study reveals the role of high-circularity localized OVs in optimizing nano-structured absorbers and devices for optical sensing, optical communication, and many other applications.

Hall effect of light is a result of symmetry breaking in spin and/or orbital angular momentum (OAM) possessing optical system and is caused by e.g. refractive index gradient/interface between media or focusing of a spatially asymmetrical beam, similar to the electric field breaking the symmetry in spin Hall effect for electrons. The angular momentum (AM) conservation law in the ensuing asymmetric system dictates redistribution of spin and orbital angular momentum, and is manifested in spin-orbit, orbit-orbit, and orbit-spin conversions and reorganization, i.e. spin-orbit and orbit-orbit interaction. This AM restructuring in turn requires shifts of the barycenter of the electric field of light. In the present study we show, both analytically and by numerical simulation, how different electric field components are displaced upon tight focusing of an asymmetric light beam having OAM and spin. The relation between field components shifts and the AM components shifts/redistribution is presented too. Moreover, we experimentally demonstrate, for the first time, to the best of our knowledge, the spin-orbit Hall effect of light upon tight focusing in free space. This is achieved using azopolymers as a media detecting longitudinal or z component of the electrical field of light. These findings elucidate the Hall effect of light and may broaden the spectrum of its applications.

Optical information and communication technologies are essentially based on the ability to shape the light field, either at the transmission or at the reception of the optical signal. Nowadays, wavelength and polarization state are already implemented in current telecom architectures and adding the spatial structuring of the light field as a novel degree of freedom to enhance data rate is desirable. In this context, the development of optical devices to shape optical modes belonging to orthogonal basis remains challenging. Here, in the context of singular optics, this work reports on the design, fabrication, and characterization of modal optical vortex beam shaping of paraxial light in the visible domain, in the framework of the Laguerre–Gauss basis. Both the azimuthal and radial degrees of freedom are univocally shaped via optical spin‐orbit interaction mediated by dielectric metasurfaces by combining the spatial modulation of amplitude, geometric phase, and dynamic phase. These results not only demonstrate the realization of new optical components, but also highlight how metasurface‐based technologies can contribute to optical information processing in the classical or quantum regime. [ABSTRACT FROM AUTHOR]

Vortex beams are unique in that they have annular spatial profiles and carry orbital angular momentum. This has led to their use in applications including laser processing, microparticle manipulation and signal transmission. Off-axis vortex beams, which may be considered a subset of vortex beams, display a broader spectrum of physical characteristics in comparison with their conventional (integer-order) counterparts. In this work, we derive the equations which describe the intensity distribution of off-axis vortex beams and use these to theoretically model their spatial profile. These models are supported by experimental generation of both integer and off-axis vortex beams, and the presence of orbital angular momentum is investigated through the use of the cylindrical lens transformation method. [ABSTRACT FROM AUTHOR]

The paper briefly presents some essential concepts and features of light fields with strong spatial inhomogeneity of amplitude, phase, polarization, and other parameters. It contains a characterization of optical vortices, speckle fields, polarization singularities. A special attention is paid to the field dynamical characteristics (energy, momentum, angular momentum, and their derivatives), which are considered not only as mechanical attributes of the field but also as its meaningful and application-oriented descriptive parameters. Peculiar features of the light dynamical characteristics in inhomogeneous and dispersive media are discussed. The dynamical properties of paraxial beams and evanescent waves (including surface plasmon -- polaritons) are analyzed in more detail; in particular, a general treatment of the extraordinary spin and momentum, orthogonal to the main propagation direction, is outlined. Applications of structured light fields for optical manipulation, metrology, probing, and data processing are described. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach for generating structured light using a diffractive beam‐shaping element that can generate multiple beams with distinct spatial structures in the focal plane. The element is constructed by restructuring a spiral zone plate with fork gratings, providing a tunable and controllable means for generating structured light. The element can create linear and 2D arrays of various structured beams, including vortices with adjustable charge and vorticity, petallike beams, and ring‐lattice structures. To control the generation of these beams, it introduces several parameters of the fork gratings, enabling tailoring of the beam properties to specific applications. The proposed approach involves significant mathematical analysis, including the derivation of complex equations and expressions to understand the behavior of the proposed element and its impact on the generated beams. It conducts simulations and experiments to validate the mathematical analysis and demonstrate the feasibility of the proposed approach. [ABSTRACT FROM AUTHOR]

Vortex beams carrying orbital angular momentum (OAM) have gained much interest in optical communications because they can be used to expand the number of multiplexing channels and greatly improve the transmission capacity. However, the number of states used for OAM-based communication is generally limited by the imperfect OAM generation, transmission, and demultiplexing methods. In this work, we proposed a dense space-division multiplexing (DSDM) scheme to further increase the transmission capacity and transmission capacity density of free space optical communications with a small range of OAM modes exploiting a multi-ring perfect vortex (MRPV). The proposed MRPV is generated using a pixel checkerboard complex amplitude modulation method that simultaneously encodes amplitude and phase information in a phase-only hologram. The four rings of the MRPV are mutually independent channels that transmit OAM beams under the condition of occupying only one spatial position, and the OAM mode transmitted in these spatial channels can be efficiently demodulated using a multilayer annular aperture. The effect of atmospheric turbulence on the MRPV was also analyzed, and the results showed that the four channels of the MRPV can be effectively separated under weak turbulence conditions. Under the condition of limited available space and OAM states, the proposed DSDM strategy exploiting MRPV might inspire wide optical communication applications exploiting the space dimension of light beams. [ABSTRACT FROM AUTHOR]

We study numerically the statistics of the formation of local intensity maxima in the focal plane of a collecting lens after a Gaussian beam passes through a spiral phase transparency in the presence of amplitude‒phase noise in the beam. It is shown that when the amplitude of field fluctuations is about 15%, two hot spots are most likely to be formed on the annular profile of an optical vortex with a topological charge m = 1. Moreover, in more than half of the implementations, these hot spots are located diametrically opposite to the phase dislocation region. In an elliptical beam, regions of increased intensity in the cross section of the optical vortex are located predominantly along the shorter axis. [ABSTRACT FROM AUTHOR]

Theoretical, numerical, and experimental means are used to study the propagation of three-Airy beams with and without an optical vortex in free space. These light fields are objects of interest for use in modern photonics, due to strong localization of the transverse intensity of the field upon propagation, self-healing, autofocusing, and a change in the intensity structure. This is useful for various applications. [ABSTRACT FROM AUTHOR]

Topological spin textures are field arrangements that cannot be continuously deformed to a fully polarized state. In particular, merons are topological textures characterized by half-integer topological charge ±1/2 and vortex-like swirling patterns at large distances. Merons have been studied previously in the context of cosmology, fluid dynamics, condensed matter physics and plasmonics. Here, we visualized optical spin angular momentum of phonon polaritons that resembles nanoscale meron spin textures. Phonon polaritons, hybrids of infrared photons and phonons in hexagonal boron nitride, were excited by circularly polarized light incident on a ring-shaped antenna and imaged using infrared near-field techniques. The polariton field reveals a half-integer topological charge determined by the handedness of the incident beam. Our phonon polaritonic platform opens up new pathways to create, control, and visualize topological textures.

It is shown that a squared Laguerre-Gaussian (LG) vortex beam is Fourier-invariant and retains its structure at the focus of a spherical lens. In the Fresnel diffraction zone, such a beam is transformed into superposition of conventional LG beams, the number of which is equal to the number of rings in the squared LG beam. If there is only one ring, then the beam is structurally stable. A more general beam, which is a “product” of two LG beams, is also considered. Such a beam will be Fourier-invariant if the number of rings in two LG beams in the “product” is the same. The considered beams complement the well-known family of LG beams, which are intensively studied as they remain stable during their propagation in turbulent media.