Spatial Distribution Of Polarization Research Articles

Generating complex vectorial optical fields via surface lattice resonances.

Vectorial optical fields (VOFs) with extra degrees of freedom hold promise for many photonic applications. However, current methods to generate VOFs are either bulky in size or exhibit limited functionalities. Here, we demonstrate a tunable VOF generator by exciting plasmonic surface lattice resonances (SLRs) with axial symmetry. By meticulously arranging bilayer circular arrays with opposite handedness, we achieve a high Q-factor of 103 via just a few particles despite the general belief that too small array size suppresses the SLRs. This work presents tunable complex VOFs with distinct inhomogeneous spatial polarization distributions, which may enable various applications in integrated and polarization optics.

First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring

The Event Horizon Telescope observed the horizon-scale synchrotron emission region around the Galactic center supermassive black hole, Sagittarius A* (Sgr A*), in 2017. These observations revealed a bright, thick ring morphology with a diameter of 51.8 ± 2.3 μas and modest azimuthal brightness asymmetry, consistent with the expected appearance of a black hole with mass M ≈ 4 × 106 M ⊙. From these observations, we present the first resolved linear and circular polarimetric images of Sgr A*. The linear polarization images demonstrate that the emission ring is highly polarized, exhibiting a prominent spiral electric vector polarization angle pattern with a peak fractional polarization of ∼40% in the western portion of the ring. The circular polarization images feature a modestly (∼5%–10%) polarized dipole structure along the emission ring, with negative circular polarization in the western region and positive circular polarization in the eastern region, although our methods exhibit stronger disagreement than for linear polarization. We analyze the data using multiple independent imaging and modeling methods, each of which is validated using a standardized suite of synthetic data sets. While the detailed spatial distribution of the linear polarization along the ring remains uncertain owing to the intrinsic variability of the source, the spiraling polarization structure is robust to methodological choices. The degree and orientation of the linear polarization provide stringent constraints for the black hole and its surrounding magnetic fields, which we discuss in an accompanying publication.

Polarization near dislocation cores in SrTiO3 single crystals: The role of flexoelectricity

Spontaneous polarization as large as ∼28 μC/cm2 was recently observed around the dislocation cores in non-polar SrTiO3 bulk crystals, and its origin was attributed to the flexoelectric effect, i.e., polarization induced by strain gradients. However, the roles of flexoelectricity, relative to other electromechanical contributions, and the nature of dislocations, i.e., edge vs screw dislocations in the induced polarization, are not well understood. In this work, we study the role of flexoelectricity in inducing polarization around three types of dislocation cores in SrTiO3: b=a(100) edge dislocation, b=a(110) edge dislocation, and b=a(010) screw dislocation, where b is the Burgers vector. For the edge dislocations, polarization can be induced by electrostriction alone, while flexoelectricity is essential for stabilizing the symmetric polarization pattern. The shear component of the flexoelectric tensor has a dominant effect on the magnitude and spatial distribution of the flexoelectric polarization. In contrast, no polarization is induced around the b=a(010) screw dislocation through either electrostriction or flexoelectricity. Our findings provide an in-depth understanding of the role of flexoelectricity in inducing polarization around dislocation cores and offer insights into the defect engineering of dielectric/ferroelectric materials.

Characterization of Instrumental Polarization in the MOPTOP Instrument

MOPTOP is a polarimeter currently installed on the fully robotic 2 m Liverpool Telescope, designed to allow accurate measurements of the variability of the polarization of astronomical objects like gamma-ray bursts and blazars. In this work, we present the characterization of the field position dependent instrumental polarization added by MOPTOP to the acquired images. This characterization is done by the acquisition of images of low polarized standard stars, for different positions over the CCD frame, for the filters BVRIL. These images were reduced using the MOPTOP pipeline, providing us with the q and u Stokes parameters. These values, in turn, were subtracted from the q and u values of the respective star, found in literature, resulting in the spatial distribution of the residual polarization added by the instrument. A plane was fitted to the obtained result, allowing us to calculate the q and u values added by MOPTOP.

Influence of oxygen vacancies on the domain wall stability in BiFeO3

AbstractOxygen vacancy is the most common type of point defects in functional oxides, and it is known to have profound influence on their properties. This is particularly true for ferroelectric oxides since their interaction with ferroelectric polarization often dictates the ferroelectric responses. Here, we study the influence of the concentration of oxygen vacancies on the stability of ferroelectric domain walls (DWs) in BiFeO3, a material with a relatively narrow bandgap among all perovskite oxides, which enables strong interactions among electronic charge carriers, oxygen vacancies, and ferroelectric domains. It is found that the electronic charge carriers in the absence of oxygen vacancies have essentially no influence on the spatial polarization distribution of the DWs due to their low concentrations. Upon increasing the concentration of oxygen vacancies, charge‐neutral DWs with an originally symmetric polarization distribution symmetric around the center of the wall can develop a strong asymmetry of the polarization field, which is mediated by the electrostatic interaction between polarization and electrons from the ionization of oxygen vacancies. Strongly charged head‐to‐head DWs that are unstable without oxygen vacancies can be energetically stabilized in the off‐stoichiometric BiFeO3−δ with δ ∼ 0.02 where ionization of oxygen vacancies provides sufficient free electrons to compensate the bound charge at the wall. Our results delineate the electrostatic coupling of the ionic defects and the associated free electronic charge carriers with the bound charge in the vicinity of neutral and charged DWs in perovskite ferroelectrics.

Cylindrical vector beam multiplexing holography employing spin-decoupled phase modulation metasurface

Abstract Cylindrical vector beams (CVBs) hold considerable promise as high-capacity information carriers for multiplexing holography due to their mode orthogonality. In CVB holography, phase holograms are encoded onto the wave-front of CVBs with different mode orders while preserving their independence during reconstruction. However, a major challenge lies in the limited ability to manipulate the spatial phase and polarization distribution of CVBs independently. To address this challenge, we propose a spin-decoupled phase modulation strategy by leveraging the propagation and geometric phase of composite phase metasurfaces. By exploiting the polarized Poincaré sphere, we show that CVBs can be decomposed into two circularly polarized components with orthogonal polarization states and conjugate phase distributions. This decomposition enables independent control of the phase and polarization distributions of CVBs by modulating the initial phase and phase difference of these two components. Consequently, two holograms with discrete spatial frequency distributions that carry opposite helical phases are encoded to modulate the wave-front of CVBs by the metasurface consisting of Si nanopillars. This allows for us to achieve successful four-channel CVB multiplexing holography. Benefiting from the non-dispersive nature of geometric phase, this metasurface exhibits a broad operating band spanning the entire visible light spectrum (443 nm–633 nm). These suggest that our proposed method offers comprehensive control over the spatial phase and polarization of CVBs, thereby holding significant potential for advancing their application in holography.

Controlling nonlinear collapse of ellipticity and orientation of a co-variant vector optical field.

A vector optical field with inhomogeneous spatial polarization distribution offers what we believe to be a new paradigm to form controllable filaments. However, it is challenging to steer multiple performances (e.g. number, orientation, and interval) of filaments in transparent nonlinear media at one time. Herein, we theoretically self-design and generate a kind of believed to be novel ellipticity and orientation co-variant vector optical field to interact with Kerr medium to solve this issue. The collapsing behaviors of such a new hybrid vector optical field reveal that, by judiciously adjusting the inherent topological charge and initial phase of incident optical field, we are able to give access to stable collapsing filamentation with tunable numbers, orientations and interval. Additionally, the collapsing patterns presented are immune nearly to the extra random noise. The relevant mechanism behind the collapse of the vector optical field is elucidated as well. The findings in this work may have huge potential in optical signal processing, laser machining, and other related applications.

Multi-scale model of axonal and dendritic polarization by transcranial direct current stimulation in realistic head geometry

BackgroundTranscranial direct current stimulation (tDCS) is a non-invasive brain stimulation modality that can alter cortical excitability. However, it remains unclear how the subcellular elements of different neuron types are polarized by specific electric field (E-field) distributions. ObjectiveTo quantify neuronal polarization generated by tDCS in a multi-scale computational model. MethodsWe embedded layer-specific, morphologically-realistic cortical neuron models in a finite element model of the E-field in a human head and simulated steady-state polarization generated by conventional primary-motor-cortex–supraorbital (M1–SO) and 4 × 1 high-definition (HD) tDCS. We quantified somatic, axonal, and dendritic polarization of excitatory pyramidal cells in layers 2/3, 5, and 6, as well as inhibitory interneurons in layers 1 and 4 of the hand knob. ResultsAxonal and dendritic terminals were polarized more than the soma in all neurons, with peak axonal and dendritic polarization of 0.92 mV and 0.21 mV, respectively, compared to peak somatic polarization of 0.07 mV for 1.8 mA M1–SO stimulation. Both montages generated regions of depolarization and hyperpolarization beneath the M1 anode; M1–SO produced slightly stronger, more diffuse polarization peaking in the central sulcus, while 4 × 1 HD produced higher peak polarization in the gyral crown. The E-field component normal to the cortical surface correlated strongly with pyramidal neuron somatic polarization (R2>0.9), but exhibited weaker correlations with peak pyramidal axonal and dendritic polarization (R2:0.5–0.9) and peak polarization in all subcellular regions of interneurons (R2:0.3–0.6). Simulating polarization by uniform local E-field extracted at the soma approximated the spatial distribution of tDCS polarization but produced large errors in some regions (median absolute percent error: 7.9 %). ConclusionsPolarization of pre- and postsynaptic compartments of excitatory and inhibitory cortical neurons may play a significant role in tDCS neuromodulation. These effects cannot be predicted from the E-field distribution alone but rather require calculation of the neuronal response.

Generating vector beams by modulating and superimposing multiple polarization modes within a helium-neon resonant cavity

In recent years, vector beams, which have greater control freedom due to their non-uniform spatial polarization distribution, have received extensive attention from scholars. In this paper, we present a straightforward intracavity modulation method by increasing the volume of the gas gain medium inside the cavity to induce multimode oscillation. Subsequently, Brewster windows and a curved-output mirror are used to achieve polarization control and superposition of multiple modes within the cavity to generate stable and controllable vector beams. In addition, the polarization purity of the radial vector beam and the angular vector beam can reach 87.98% and 85.52%, respectively. This work can not only break the understanding of the traditional vector beam generation method and deepen the understanding of laser cavity regulation technology but also play a guiding role in designing compact vector lasers.

Measurement of the spatial polarization distribution of circularly polarized gamma rays produced by inverse Compton scattering

Measurement of the spatial polarization distribution of circularly polarized gamma rays produced by inverse Compton scattering

Optical magnetic combination method for suppressing the Rb polarization-induced magnetic gradient in Rb-Xe NMR co-magnetometers.

The Rb polarization-induced magnetic field gradient affects the Xe nuclear spin relaxation properties and degrades the long-term stability of the NMR co-magnetometers. This paper proposes a combination suppressing scheme, which uses the second-order magnetic field gradient coils to compensate for the Rb polarization-induced magnetic gradient under counter-propagating pump beams. Based on the theoretical simulation, we find that the spatial distribution of the Rb polarization-induced magnetic gradient and the magnetic field distribution generated by gradient coils are complementary. The experimental results indicate that the compensation effect is 10% higher under the counter-propagating pump beams scheme compared with the compensation effect under the conventional single beam. Besides, due to a more uniform spatial distribution of electronic spin polarization, the Xe nuclear spin polarizability is improved and the signal-to-noise ratio (SNR) of NMR co-magnetometers could be further enhanced. The study provides an ingenious method for suppressing magnetic gradient in the optically polarized Rb-Xe ensemble, which is expected to improve the performance of the atomic spin co-magnetometers.

Generating a Superoscillation Three-Dimensional Hollow Spot by Polarization Manipulation

A wide variety of practical applications utilize three-dimensional (3D) hollow spots. To improve their performance, it is critical to independently control the transverse and longitudinal size of a 3D hollow spot. Here, we propose a method of creating a superoscillation 3D hollow spot by properly engineering the spatial polarization distribution. A cylindrical vector wave can be generated and the transverse and longitudinal sizes of the 3D hollow spot can be independently controlled by individually optimizing the wave fronts of the radial and azimuthal components in the cylindrical vector wave. A cylindrical vector metalens integrated with the functions of polarization conversion and wave-front manipulation is optimized and fabricated. The generated 3D hollow spot is experimentally demonstrated with a transverse size of (0.336 \ifmmode\pm\else\textpm\fi{} 0.005)\ensuremath{\lambda} and a longitudinal size of (1.170 \ifmmode\pm\else\textpm\fi{} 0.005)\ensuremath{\lambda}. Our method provides an easy and practical approach to generate a 3D hollow spot with a subwavelength transverse size, which can be intriguing for nanoparticle manipulation and in nanomicroscopy.

On the Spatial Distribution of Polarization and Color of a Distant Comet C/2017 K2 (PanSTARRS)

Comet C/2017 K2 (PanSTARRS) was observed at two wavelengths using two different-sized apertures in two measurement campaigns. We find that all the observations can be interpreted using a two-component mixture of dust composed of Mg-rich silicates and amorphous carbon. The modeling suggests a slight increase in the absorption of the refractory component at shorter wavelengths, which is consistent with the optical properties of Mg-rich silicates, and a change in the size distribution as a function of distance from the cometary nucleus, a result of smaller particles accelerating more rapidly from the expanding-gas and radiative-pressure forces.

Vector beam bending via a polarization gradient.

We propose, analyze and demonstrate experimentally an entirely new optical effect in which the centroid of a coherent optical beam can be designed to propagate along a curved trajectory in free space by tailoring the spatial distribution of linear polarization across the transverse beam profile. Specifically, a non-zero spatial gradient of second order or higher in the linear state of polarization is shown to cause the beam centroid to "accelerate" in the direction transverse to the direction of propagation. The effect is confirmed experimentally using spatial light modulation to create the distribution in linear polarization and then measuring the transverse location of the beam profile at varying propagation distances. The observed displacement of the beam centroid is shown to closely match the theory out to 34m propagation distance.

Noninterleaved Metasurface for Full‐Polarization Three‐Dimensional Vectorial Holography

AbstractMetasurface‐enabled full‐polarization vectorial holography has attracted numerous attention due to the promising applications for multichannel displays, enhanced information capacity, and optical encryption. However, recently showcased design strategies suffer from undesirable cross‐talk resulting from interleaved meta‐atoms and are restricted to a two‐dimensional plane. This work presents a general approach for full‐polarization beam shaping in three‐dimensional space by completely exploiting the capacity of a noninterleaved metasurface. The noninterleaved metasurface is spin‐decoupled, which is capable of achieving independent phase control for the two orthogonal circular polarizations by combining both propagation and geometric phases. As a proof‐of‐concept, a three‐dimensional vectorial holography capable of converting the linear polarized light to holographic images with arbitrary spatial polarization distributions is experimentally demonstrated. Taking use of the extended dimension, the z‐axis distance, which provides a new degree of flexibility for boosting information capacity and security, a double‐level optical encryption scheme is implemented. This work provides a compact and general scheme for full‐polarization channel wavefront steering, unlocking a new door for designing planar polarization functionality devices.

High-resolution, wavefront-sensing, full-field polarimetry of arbitrary beams using phase retrieval.

Recent advances in structured illumination are enabling a wide range of applications from imaging to metrology, which can benefit from advanced beam characterization techniques. Solving uniquely for the spatial distribution of polarization in a beam typically involves the use of two or more polarization optics, such as a polarizer and a waveplate, which is prohibitive for some wavelengths outside of the visible spectrum. We demonstrate a technique that circumvents the use of a waveplate by exploiting extended Gerchberg-Saxton phase retrieval to extract the phase. The technique enables high-resolution, wavefront-sensing, full-field polarimetry capable of solving for both simple and exotic polarization states, and moreover, is extensible to shorter wavelength light.

In-depth understanding of {110}-type domain walls in bismuth titanate ceramics

Bismuth titanate(BIT) ceramics have been widely studied for high temperature applications thanks to their high Curie temperature and large spontaneous polarization. The underlying domain structures play significant roles in regulating the macroscopic properties of ferroelectric materials, while there is only limited knowledge of the domain structures for BIT ceramics until now, substantially bad for the development of BIT-based new materials. In this study, the intriguing domain walls(DWs) in BIT ceramics were investigated in detail. The atomic-scale observation reveals that there primarily exist energetically favorable {110}-type 90° DWs in BIT ceramics. Combined with the theoretical analysis, these DWs are not only elaborated under multiple piezoelectric response modes, but also further corroborated by the quantitative reconstruction of spatial polarization distributions of twin-related ferroelectric variants. This work sheds insight into domain structure and piezoelectric response mechanism in BIT ceramics, and contributes to promoting their more applications in advanced electron devices.

Generation and characterization of complex vector modes with digital micromirror devices: a tutorial

Complex vector light modes with a spatial variant polarization distribution have become topical of late, enabling the development of novel applications in numerous research fields. Key to this is the remarkable similarities they hold with quantum entangled states, which arises from the non-separability between the spatial and polarisation degrees of freedom (DoF). As such, the demand for diversification of generation methods and characterization techniques have increased dramatically. Here we put forward a comprehensive tutorial about the use of digital micromirrors devices (DMDs) in the generation and characterization of vector modes, providing details on the implementation of techniques that fully exploits the unsurpassed advantage of DMDs, such as their high refresh rates and polarisation independence. We start by briefly describing the operating principles of DMD and follow with a thorough explanation of some of the methods to shape arbitrary vector modes. Finally, we describe some techniques aiming at the real-time characterization of vector beams. This tutorial highlights the value of DMDs as an alternative tool for the generation and characterization of complex vector light fields, of great relevance in a wide variety of applications.

Topological properties of non-isotropic two-dimensional SSH model

<sec>The one-dimensional (1D) Su-Schrieffer-Heeger (SSH) chain is a model that has been widely studied in the field of topological physics. The two-dimensional (2D) SSH model is a 2D extension of the 1D SSH chain and has many unique physical properties. It is a higher-order topological insulator (HOTI), in which corner states with bound states in the continuum (BIC) properties will arise between the second energy band and the third energy band. There are two different topological phases in the isotropic 2D SSH model, and a topological phase transition will happen when the intracell coupling strength is equal to the intercell coupling strength.</sec><sec>In this paper, we first break the isotropy of the isotropic 2D SSH model, defining the ratio of the <i>x</i>-directional coupling strength to the <i>y</i>-directional coupling strength as <i>α</i> and the ratio of the intercell coupling strength to the intracell coupling strength as <i>β</i>, which represent the strength of the topological property and anisotropy respectively. We use <i>α</i> and <i>β</i> to calibrate all possible models, classify them as three different types of phases, and draw their phase diagrams.Then we argue when the energy gap between the second energy band and the third energy band emerges over the entire Brillouin zone.</sec><sec>Meanwhile, we use a method to calculate the spatial distribution of polarization when the model is half-filled, and it is shown that there is 1/2 polarization localized at the edges in the direction with larger intracell coupling, but no edge polarization in the other direction. The edge polarization excites the edge dipole moment, giving rise to a topological edge state in the energy gap. At the same time, when the model has an entire open boundary, the dipole moment directs the charge to accumulate on the corners, which can be observed from the local charge density distribution. This type of fractional charge is a filling anomaly and formed spontaneously by the lattice to maintain electrical neutrality and rotational symmetry simultaneously. This fractional charge induces the aforementioned corner state. And by its nature of filling anomaly, this corner state is better localized and robust. It will not couple with the bulk state as long as the rotational symmetry or chirality of the model is not broken.</sec><sec>Finally, we construct an acoustic resonant cavity model: a rectangular shaped resonant cavity is used to simulate individual lattice points and the coupling strength between the lattice points is controlled by varying the diameter of the conduit between the resonant cavities. According to the Comsol calculation results, we can see that the topological properties of the anisotropic two-dimensional SSH model are well simulated by this model.</sec>

Tailoring spatially unpolarized light on the Poincaré sphere: From equator states via meridian states to generalized great-circle states

In this paper we investigate the generation, manipulation and invariance properties of spatially unpolarized light in the context of classical optics. We generate spatially randomly polarized light by exploiting the optical activity of a so-called Cornu depolarizer and demonstrate tailored, spatial polarization distributions on the Poincar\'e sphere. We begin with the generation of equator states of spatially unpolarized light, i.e., the manifold of the superposition of all linearly polarized light states in the spatial domain. A quarter-wave plate is utilized to transform these equator polarization states into meridian states and finally a second, subsequent quarter-wave plate results in tilted meridian states or generalized great-circle states on the Poincar\'e sphere. These results of all realized unpolarized states are visually confirmed by tomographic reconstruction of the spatially distributed polarization states on the Poincar\'e sphere by extracting the data from the spatially resolved Stokes parameter measurements. Furthermore, all these experimental results are in excellent agreement with classical Mueller formalism calculations based on the Mueller matrices for the Cornu depolarizer and the wave plates, yielding the tailored manifold of polarization states on the Poincar\'e sphere's surface, confirming the naming of the unpolarized light states in accordance with geodesy. Therefore, we have realized a scheme for the tailored generation of spatially depolarized light based on the Cornu depolarizer and quarter-wave plates with interesting application perspectives and giving further insight into polarization.