Polarization Singularities Research Articles

Merging diverse bound states in the continuum: from intrinsic to extrinsic scenarios

Bound states in the continuum (BICs) in photonic crystal slabs are characterized as vortex centers in far-field polarization and infinite quality (Q) factors, which can be dynamically manipulated in momentum space to construct the singularity configurations with functionalities such as merging BICs for further suppress scattering loss of nearby resonance. However, the vast majority of research focuses on two types of intrinsic BICs for simplicity, because these polarization singularities affect each other, and are even prone to annihilation. Here, we introduce the extrinsic (Fabry-Pérot) BICs and combine them with the intrinsic BICs to merge diverse BICs in momentum space. The extrinsic BICs can move independently of the intrinsic BICs, providing an unprecedented degree of freedom to reduce the complexity of constructing merging BIC configurations. Interestingly, an interaction of oppositely charged BICs that is collision beyond annihilation is revealed, which only exchanges the topological charge of BICs but not affect their existence. Following the proposed strategy, four-types-BICs merging and steerable three-types merging are achieved at the Γ and off-Γ points, further boosting the Q factor scaling rule up to Q∝k x −14 and Q∝k x −6 respectively. Our findings suggest a systematic route to arrange abundant BICs, may facilitate some applications including beam steering, optical trapping and enhancing the light-matter interactions.

Determining triple half-twist of optical Möbius strips in electromagnetic fields with a non-planar structure.

Polarization ellipses in a nonparaxial field form twisted strips when traced along closed contours around a circular polarization singularity line. We found an analytical expression for the twist number of the strip when the contour is coplanar with the polarization ellipse in its center. Necessary and sufficient conditions for strips having one or three half-twists are found. A set of five parameters of electromagnetic field at the polarization singularity point is found which definitely determines the value of the twist number of the strip.

Deformable polarization singularity array by superimposed Ince–Gaussian beams

We present a numerical and experimental study of polarization singularity arrays, which are generated by the coherent superposition of orthogonal, linearly polarized, even and odd Ince–Gaussian (IG) beams. The generated array consists of multiple polarization singularities of different types. The type of the polarization singularities can be adjusted effectively by switching the linear polarization states of two IG modes and by modifying the relative phase factor. In addition, we study the evolution of the arrays with the ellipticity parameter, which leads to the splitting and spatial rearrangement of polarization singularities. This work may have implications for material processing, optical trapping, and manipulation.

Fine structure of polarization and optical Möbius strips topology of laser light in planar metamaterial

It is shown that upon propagation of a plane linearly polarized harmonic wave normally incident on a planar nonlinear metamaterial whose basic cell consists of two rectangular silver plates with characteristic dimensions of hundreds of nanometers, there exists in the local field a circular polarization singularity line (C-line) containing two regions where their segments come to each other to such small distance that it may appear as a self-intersection in the numerical experiment. We found that optical ribbons constructed around the ordinary points of these lines have the topology of Möbius ribbons, and the ribbons constructed in the possible self-intersection region of C-lines and spanning its two segments are orientable surfaces.

Time-varying optical spin-orbit Hall effect in tightly focused femtosecond optical field.

The spin-orbit Hall effect (HE) is dominated by the law of conservation of angular momentum of a beam and is highly significant in light-matter interactions. The electromagnetic field, phase, topological structure, and spin-orbit HE of an azimuthally polarized vortex pulse beam in a tightly focused system are studied theoretically here. Calculations show that the focal field has ultrafast bright-dark alternating characteristics and a distorted phase distribution. Furthermore, the time evolution of the polarization singularity in the focused light field is explained using Stokes parameters. Importantly, the spin-orbit HE of the pulsed beam is shown to be time-varying in a tightly focused system. This time-varying spin-orbit HE is particularly sensitive to the pulse width and central wavelength. Our method has important applications in particle manipulation.

Scattering of partially coherent electromagnetic beams by a sphere.

We examine the scattering of a partially coherent, partially polarized electromagnetic beam by a homogeneous sphere (Mie scattering). The degree of polarization and the Stokes parameters in the far zone are found to be strongly dependent on the state of coherence and polarization of the incident beam. In particular, we demonstrate the emergence of polarization singularities and show that partial spatial coherence gives rise to significant depolarization effects. In addition we explore the symmetry properties of the scattered field.

Polarization singularity index determination using wedge plate lateral shear interferometry

Polarization singularity is a point in an inhomogeneous polarization distribution at which the azimuth of the polarization ellipse is indeterminate. The azimuth gradient is solenoidal around the polarization singular point. In metrology, shear interferometry enables the visualization of phase gradients in optical beams. Since the polarization azimuth gradient is related to the Stokes phase gradient, we adopt the shear interferometry technique to visualize these phase gradients for the detection of polarization singularity. We use a wedge plate lateral shear interferometer (LSI) in our experiments for the detection of the ellipse and vector field singularities. Being a self-referencing and intensity-based technique, this method is convenient for practical use. Unlike the traditional Stokes polarimetry method, where the polarization distribution at every spatial point is evaluated, this technique can distinguish degenerate states of polarization singularities simply by visual observation of intensity fringes. Index and helicity measurement of these polarization singularities without using polarization measurement methods are novel aspects of this scheme. Both theoretical and experimental results are presented to support our claim.

Metasurface for Engineering Superimposed Ince-Gaussian Beams.

Ince-Gaussian beams (IGBs) are the third complete family of exact and orthogonal solutions of the paraxial wave equation and have been applied in many fields ranging from particle trapping to quantum optics. IGBs play a very important role in optics as they represent the exact and continuous transition modes connecting Laguerre-Gaussian and Hermite-Gaussian beams. The method currently in use suffers from the high cost, complexity, and large volume of the optical system. The superposition of IGBs can generate complicated structured beams with multiple phase and polarization singularities. A metasurface approach is proposed to realizing various superpositions of IGBs without relying on a complicated optical setup. By superimposing IGBs with even and odd modes, multiple phase, and polarization singularities are observed in the resultant beams. The phase and polarization singularities are modulated by setting the initial phase in the design and controlling the incident linear polarization. The compactness of the developed metasurface devices and the unique properties of the generated beams have the potential to impact many practical applications such as particle manipulation, orbital angular momentum spectrum manipulation, and optical communications.

Three-Dimensional Reconfigurable Optical Singularities in Bilayer Photonic Crystals.

Metasurfaces and photonic crystals have revolutionized classical and quantum manipulation of light and opened the door to studying various optical singularities related to phases and polarization states. However, traditional nanophotonic devices lack reconfigurability, hindering the dynamic switching and optimization of optical singularities. This paper delves into the underexplored concept of tunable bilayer photonic crystals (BPhCs), which offer rich interlayer coupling effects. Utilizing silicon nitride-based BPhCs, we demonstrate tunable bidirectional and unidirectional polarization singularities, along with spatiotemporal phase singularities. Leveraging these tunable singularities, we achieve dynamic modulation of bound-state-in-continuum states, unidirectional guided resonances, and both longitudinal and transverse orbital angular momentum. Our work paves the way for multidimensional control over polarization and phase, inspiring new directions in ultrafast optics, optoelectronics, and quantum optics.

Stacked Polarizing Elements for Controlling Parameters of Surface Relief Gratings Written in Photosensitive Materials.

Photosensitive materials are widely used for the direct fabrication of surface relief gratings (SRGs) without the selective etching of the material. It is known that the interferometric approach makes it possible to fabricate SRGs with submicron and even subwavelength periods. However, to change the period of the written SRGs, it is necessary to change the convergence angle, shift a sample, and readjust the interferometric setup. Recently, it was shown that structured laser beams with predetermined, periodically modulated polarization distributions can also be used to fabricate SRGs. A structured laser beam with the desired polarization distribution can be formed with just one polarizing optical element-for example, the so-called depolarizer, a patterned micro-retarder array. The use of such stacked elements makes it possible to directly control the modulation period of the polarization of the generated laser beam. We show that this approach allows one to fabricate SRGs with submicron periods. Moreover, the addition of q-plates, elements effectively used to generate cylindrical vector beams with polarization singularities, allows the efficient formation of fork polarization gratings (FPGs) and the fabrication of higher-order fork-shaped SRGs. Full control of the parameters of the generated FPGs is possible. We demonstrate the formation of FPGs of higher orders (up to 12) by only adding first- and second-order q-plates and half-wave plates to the depolarizers. In this work, we numerically and experimentally study the parameters of various types of SRGs formed using these stacked polarizing elements and show the significant potential of this method for the laser processing of photosensitive materials, which often also serve as polarization sensors.

Large transmittance contrast via 90-degree sharp bends in square lattice glide-symmetric photonic crystal waveguides.

We demonstrate an intriguing transmittance contrast in a glide-symmetric square-lattice photonic crystal waveguide with a 90-degree sharp bend. The glide-symmetry gives rise to a degeneracy point in the band structure and separates a high-frequency and a low-frequency band. Previously, a similar large transmittance contrast between these two bands has been observed in glide-symmetric triangular- or honeycomb-lattice photonic crystals without inversion symmetry, and this phenomenon has been attributed to the valley-photonic effect. In this study, we demonstrate the first example of this phenomenon in square-lattice photonic crystals, which do not possess the valley effect. Our result sheds new light onto unexplored properties of glide-symmetric waveguides. We show that this phenomenon is related to the spatial distribution of circular polarization singularities in glide-symmetric waveguides. This work expands the possible designs of low-loss photonic circuits and provides a new understanding of light transmission via sharp bends in photonic crystal waveguides.

Influence of Primary Coma on the Tightly Focusing Characteristics of Circular Basis Hybrid Order Poincaré Sphere Beams

Analogous to the Poincaré sphere, a hybrid order Poincaré sphere is used to represent the ellipse field singularities (C-points). We study the tight focusing properties of generic bright and dark hybrid order Poincaré sphere beams in the presence of primary coma. The role of the polarization singularity index and handedness of the polarization of the hybrid order Poincaré sphere beams on the focused structure has been discussed. Results have been presented for the total intensity, component intensities, and component phase distributions for left- and right-handed bright and dark star and lemon types singularities. The presence of primary coma distorted the focal plane intensity distributions for both positive and negative index generic C-points. Coma is known to disturb the circular symmetry of the focal plane intensity distribution. Similarly in tight focusing polarization is known to disturb the symmetry. Therefore, a beam with structured and inhomogeneous polarization distribution tightly focused under the influence of coma is a fit case to study. It is found that the presence of primary coma aberration in the focusing system produces a positional shift of the high-intensity peaks and a reduction of the intensity on one side of the center. As the strength of the primary coma increases, the focal plane intensity distributions shift more and more toward the right from the initial position. Unlike the scalar vortex case, in the case of hybrid order Poincaré sphere beams, the focal plane intensity distribution undergoes rotation, as the helicity of the hybrid order Poincaré sphere beams is inverted, in addition to shifting. All the component phase distributions are found to be embedded with phase vortices of charge ±1.

All-silicon metasurface-enabled non-coaxial superposition of vector vortex beams

Vector vortex beams (VVBs) possess great applications in fundamental science, from particle trapping to quantum communication. Recently, the complex system for evaluating VVBs has been simplified by implementing metasurfaces and demonstrating the evolutionary trends of its coaxial polarization states. Nevertheless, it is quite challenging to obtain the polarization singularities generated in the terahertz (THz) band when non-coaxial superposition is performed by two multiplexed VVBs. Herein, a minimalist all-silicon metasurface platform is proposed, capable of being implemented to evaluate the non-coaxial superposition states of two identical VVBs. The employment of spin-polarization multiplexing techniques gives higher encoding degrees of freedom within the orthogonal circular polarization channel, enabling the superposition behavior of inhomogeneous polarization states to be performed at predefined focal planes. A systematic analysis of the newly generated polarization singularities in the interaction region is performed by applying the full Stokes polarization vector. With the ultra-compact configuration, the experimental results obtained by the proposed strategy under THz wave illumination exhibit good agreement with the numerical simulations. Thus, such mechanisms offer potential applications for multiple particle capture based on non-coaxial superimposition of VVB and provide efficient meta-platform in data encryption, high-resolution imaging, and quantum communications.

Topological polarization singular enables multiple-system excellent circular dichroism via chiral bound states in the continuum with all-dielectric metasurfaces

The advancements in chiroptics have propelled applications beyond traditional optics, expanding its reach into domains such as heat and fluid mechanics. The current research predominantly focused on utilizing plasmonic materials in the photothermal and photofluidic systems. However, these systems face limitations in circular dichroism due to inherent Ohmic loss associated with plasmonic materials. Herein, we introduce the concept of topological polarization singularity in an all-dielectric structure, which supports chiral bound states in the continuum. The introduction of topological polarization singularity gives the structure an impressive chiral light absorption response. The BICs resonant mode achieves a 147-fold field enhancement, significantly improving the photothermal conversion efficiency of dielectric material. Moreover, the g factors of photothermal and optical fluids reach 1.9, approaching the theoretical limit value. Consequently, an exceptionally high degree of circular dichroism is realized in multiple systems including light absorption, photothermal, and photofluidic systems. Our proposed structures can potentially be used in chiral photothermal and chiral nanoparticle capture, as well as photofluidic applications related to biochemistry and clinical biology.

The influence of heating on the appearance of polarization singularities during the self-action of a Gaussian beam in the isotropic phase of a nematic liquid crystal in the pre-transition temperature region

Heating of the medium occurs during the self-action of elliptically polarized light in the isotropic phase of a nematic liquid crystal near the transition temperature to the mesophase. We studied the influence of heating of the medium, on the propagation modes of laser radiation and the dynamics of the formation of singularity lines of circular polarization in the cross sections of the beam, which is closely related to the multifocal structure of the electric field. It is shown that an increase in the incident radiation intensity leads to a decrease in distance from the front end of the cylindrical cuvette to the beam cross section, containing the primary (brightest and most stable) polarization singularity lines, as well as in the radius of the circle they form. The range of parameters of the incident radiation and medium at which the multifocal structure of the electric field of the propagating beam is formed, as well as the intensity threshold values at which it appears, decreases with the growth of the thermal conductivity coefficient of the liquid crystal.

Efficient generation and amplification of intense vortex and vector laser pulses via strongly-coupled stimulated Brillouin scattering in plasmas

The past decade has seen tremendous progress in the production and utilization of vortex and vector laser pulses. Although both are considered as structured light beams, the vortex lasers have helical phase fronts and phase singularities, while the vector lasers have spatially variable polarization states and polarization singularities. In contrast to the vortex pulses that carry orbital angular momentum (OAM), the vector laser pulses have a complex spin angular momentum (SAM) and OAM coupling. Despite many potential applications enabled by such pulses, the generation of high-power/-intensity vortex and vector beams remains challenging. Here, we demonstrate using theory and three-dimensional simulations that the strongly-coupled stimulated Brillouin scattering (SC-SBS) process in plasmas can be used as a promising amplification technique with up to 65% energy transfer efficiency from the pump beam to the seed beam for both vortex and vector pulses. We also show that SC-SBS is strongly polarization-dependent in plasmas, enabling an all-optical polarization control of the amplified seed beam. Additionally, the interaction of such structured lasers with plasmas leads to various angular momentum couplings and decouplings that produce intense new light structures with controllable OAM and SAM. This scheme paves the way for novel optical devices such as plasma-based amplifiers and light field manipulators.

Tight focusing of azimuthally polarized Laguerre–Gaussian vortex beams by diffractive axicons

This study presents a comprehensive theoretical investigation into the focusing properties of azimuthally polarized Laguerre–Gaussian vortex (APLGV) beams when interacting with different optical elements, including a linear axicon, binary axicon, and lens based on the Debye approximation. The research findings highlight the intriguing combination of polarization and vortex singularities within the APLGV beam, which result in distinctive focal shapes when interacting with these optical elements. The focal shapes achieved include multiple tightly focused spots and optical needles, which can be controlled by adjusting optical system parameters and beam characteristics such as the numerical aperture (NA), truncation parameter, beam order, and annular obstruction. These parameters can be carefully selected to achieve specific focal shapes with applications in multi-optical manipulation, particle acceleration, and trapping. By harnessing the unique properties of APLGV beams and optimizing the optical setup, researchers can explore new possibilities for advanced optical manipulation and control.

The Propagation Characteristics of Circular Airy Beams with Propagational Fractional-Order Optical Vortices

We investigate the propagation properties of circular Airy beams (CABs) with propagational fractional-order optical vortices (OVs). The superposition of the phase singularity and polarization singularity from a vortex vector beam (VVB) plays a significant role in creating a propagational fractional vortex beam. Propagational fractional vortex beams can be considered as a superposition of left and right circularly polarized vortex beams with different integer topological charges (TCs). We study the propagation characteristics of two kinds of propagational fractional vortex CABs, and the results show that both of the two kinds of beams can stably propagate in free space, and they exhibit an “abruptly auto-focusing” property and “self-healing” property during the propagation. The intensity distribution of the first kind of propagational fractional vortex CAB has an odd number of petals (2m + 1), while the second kind of beam has a crescent-shaped intensity distribution. The influence of turbulence on the beam propagation through atmosphere under different turbulence strengths is also numerically studied in this paper. A fractional vortex CAB with an initial radius r0 = 10 mm can retain its shape after propagating 20 m when the atmospheric refractive-index structure constant CN2=0.2×10−12m−2/3. Our results are expected to broaden the application of CABs.

Deep-learning enabled simultaneous detection of phase and polarization singularities of CVVBs and its application to image transmission

Deep-learning enabled simultaneous detection of phase and polarization singularities of CVVBs and its application to image transmission

Lissajous singularities in Young's interference experiment.

We explore the interference of two bichromatic vector beams in Young's interference experiment. Our analysis focuses on determining the conditions under which the superposition of such beams, emerging from the pinholes, can give rise to Lissajous-type polarization singularities on the observation screen. Two independent sufficiency conditions are derived. This analysis aids in comprehending the inherent characteristics of Lissajous singularities. To the best of our knowledge, this is the first demonstration of the singular behavior of polarization in a two-frequency field in Young's interference experiment.