Opposite Circular Polarization Research Articles

42.4: Compact Stereo Waveguide Display Using a Polarization‐Multiplexed In‐coupling Metagrating

Three‐dimensional (3D) vision in augmented reality (AR) displays can enable highly immersive and realistic viewer experience, hence attracts much attention. Most current approaches create 3D visions by projecting stereoscopic images to different eyes using two separate projection systems, which are inevitably bulky for wearable devices. Here, we propose a compact stereo waveguide AR display system using a single piece of thin flat glass integrated with a polarization‐multiplexed metagrating in‐coupler and two diffractive grating out‐couplers. Incident light of opposite circular polarization states carrying stereoscopic images are first steered by the metagrating to opposite propagation directions in the flat glass waveguide, subsequently extracted by the diffractive gratings and finally received by different eyes, forming 3D stereo vision. Experimentally we fabricated a display prototype and demonstrated independent projection of two polarization‐multiplexed stereoscopic images. This work paves a novel path towards the implementation of high‐performance stereo displays with compact size, light weight, and multi‐color compatibility.

High-contrast level-crossing resonances in a small cesium vapor cell for applications in atomic magnetometry

Level-crossing (LC) resonances in alkali-metal vapors are widely used for atomic magnetometry. One of the most simple and robust techniques involves a single circularly polarized light wave, while a transverse magnetic field (Bx⊥k) is scanned around zero to observe the subnatural-linewidth resonance of electromagnetically induced transparency (EIT) in the light wave transmission. This technique allows miniaturization of the magnetic field sensor to a great extent, maintaining high sensitivity of measurements. To obtain a high quality factor of the LC resonance and, therefore, high performance of the sensor, either a high temperature (>120 °C) or an extended volume of the vapor cell (V≫1 cm3) is usually required. Here, we propose a slight modification to the commonly used configuration, which can provide high-quality LC resonances in small (≪1 cm3) vapor cells at a relatively low temperature of 60 °C or less. The modification consists in adding the second (pump) counterpropagating light wave with opposite circular polarization (σ+σ− configuration). In our experiments, the waves excite the D1-line ground-state level Fg=4 in cesium atoms in the presence of a buffer gas (Ne, 130 Torr). In the proposed scheme, a subnatural-linewidth electromagnetically induced absorption (EIA) resonance is observed. We compare parameters of the EIA resonance with those obtained in the single-wave scheme to show benefits of using the proposed σ+σ− configuration. The results have good prospects for developing a low-power miniaturized atomic magnetometer with a wide operating range.

Gyrotropic superlattice as a transformer of light polarization

We suggest a novel approach to predict and clarify the transformation of an arbitrarily polarized wave into two circularly polarized ones, based on the peculiar features of their transmission and reflection from a stack with regularly arranged gyrotropic layers. In spite of the fact that in the Faraday geometry gyrotropic media possess four electromagnetic eigenmodes, we have managed to modify the problem to be analytically resolved within the conventional transfer-matrix method. As a result, we have obtained and analyzed the dispersion relation, as well as the transmission and reflection coefficients, which are applicable for any elliptic polarization of a wave irradiating a gyrotropic superlattice with finite number of unit cells. It is shown that there exists a wide range of wave frequency and magneto-optical parameter (proportional to external dc magnetic field) inside of which the wave of given circular polarization is perfectly transmitted, while the wave with the opposite circular polarization is completely reflected. Thus, under certain conditions, the incident plane wave (taken as an appropriate example) splits up into the transmitted and reflected circular waves with mutually inverse rotations. We indicate conditions when a relatively small variation of the magnetization can reverse the polarizations mentioned above.

Vector-apodizing phase plate coronagraph: design, current performance, and future development [Invited

Over the last decade, the vector-apodizing phase plate (vAPP) coronagraph has been developed from concept to on-sky application in many high-contrast imaging systems on 8 m class telescopes. The vAPP is a geometric-phase patterned coronagraph that is inherently broadband, and its manufacturing is enabled only by direct-write technology for liquid-crystal patterns. The vAPP generates two coronagraphic point spread functions (PSFs) that cancel starlight on opposite sides of the PSF and have opposite circular polarization states. The efficiency, that is, the amount of light in these PSFs, depends on the retardance offset from a half-wave of the liquid-crystal retarder. Using different liquid-crystal recipes to tune the retardance, different vAPPs operate with high efficiencies (${\gt}96\%$) in the visible and thermal infrared (0.55 µm to 5 µm). Since 2015, seven vAPPs have been installed in a total of six different instruments, including Magellan/MagAO, Magellan/MagAO-X, Subaru/SCExAO, and LBT/LMIRcam. Using two integral field spectrographs installed on the latter two instruments, these vAPPs can provide low-resolution spectra (${\rm{R}} \sim 30$) between 1 µm and 5 µm. We review the design process, development, commissioning, on-sky performance, and first scientific results of all commissioned vAPPs. We report on the lessons learned and conclude with perspectives for future developments and applications.

Control of the exciton valley dynamics in atomically thin semiconductors by tailoring the environment

The exciton valley dynamics in van der Waals heterostructures with transition metal dichalcogenides monolayers is driven by the long-range exchange interaction between the electron and the hole in the exciton. It couples the states active in the opposite circular polarizations resulting in the longitudinal-transverse splitting of excitons propagating in the monolayer plane. Here we study theoretically the effect of the dielectric environment on the long-range exchange interaction and demonstrate how the encapsulation in the hexagonal boron nitride modifies the exciton longitudinal-transverse splitting. We calculate the exciton spin/valley polarization relaxation due to the long-range exchange interaction and demonstrate that the variation of the monolayer environment results in significant, up to five-fold, enhancement of the exciton valley polarization lifetime.

Femtotesla Direct Magnetic Gradiometer Using a Single Multipass Cell

We describe a direct gradiometer using optical pumping with opposite circular polarization in two ${}^{87}\mathrm{Rb}$ atomic ensembles within a single multipass cell. A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation due to the intrinsic subtraction of two contributions exceeding 3.5 rad from the highly polarized ensembles. We develop analysis methods for the direct gradiometer signal and measure a gradiometer sensitivity of 10.1 $\mathrm{fT}/\mathrm{cm}\sqrt{\mathrm{Hz}}$. We also demonstrate that our multipass design, in addition to increasing the optical depth, provides a fundamental advantage due to the significantly reduced effect of atomic diffusion on the spin-noise time-correlation, in excellent agreement with the theoretical estimate.

Восприятие виртуальных стереообъектов: особенности взаимодействия зрительных механизмов и пространственные перцептивные эффекты

The purpose of our study was investigation of the peculiarities of human visual perception in virtual environment created on the basis of stereo technologies. The participants were 100 adults aged from 17 to 79 years (40 males and 60 females, average age 32,9 years). Observation of virtual stereo objects was provided by computer software “Fusion” created for measuring visual fusion reserves which characterize the quality of binocular mechanisms of stereo perception. Test stimuli were random dot stereograms (RDSs) encoding a square test object moving from the screen to the observer. Separate presentation of the stimuli to the left and right eyes was based on the opposite circular polarization method. The participant’s task was to observe virtual stereo objects and describe perceived vi- sual images: their sizes, positions in depth and directions of movement. It has been found that, in conditions of view- ing the same virtual stereo objects, the participants with normally functioning mechanisms of binocular stereopsis could perceive quite different stereo images. On the basis of the perceived stereo image parameters, all participants were divided into four types. The described phenomena and the identified typology of spatial perceptual stereo effects could be considered as the consequences of restructuring interaction of visual sensory, accommodative and oculo-motor mechanisms involved in visible image formation when adapting to a virtual environment.

Optical spin-dependent beam separation in cyclic group symmetric metasurface

Abstract Cross-polarization scattering of a circularly polarized beam from nano-rod introduces a geometric phase to the outgoing beam with opposite circular polarization. By manipulating the spatial array of subwavelength nano-structure constituting metasurface, the geometric phase can be engineered to generate a variety of beam profiles, including vortex beam carrying orbital angular momentum via a process called spin-to-orbital angular momentum conversion. Here we introduce a cyclic group symmetric metasurface composed of tapered arc nano-rods and explore how azimuthal angular distribution of total phase determines the feature of spin-dependent beam separation. When scattered from a circular array of tapered arc nano-rods possessing varying width with a fixed length, a dynamical phase having non-constant azimuthal gradient is introduced to an incoming Gaussian beam. This leads to a spin-dependent beam separation in the outgoing vortex beam profile, which is attributed to an azimuthal angle dependent destructive interference between scatterings from two plasmonic excitations along the width and the length of tapered arc nano-rod. Relation of cyclic group symmetry property of metasurface and the generated vortex beam profile is examined in detail by experimental measurement and analysis in terms of partial-wave expansion and non-constant azimuthal gradient of total phase. Capability of spatial beam profiling by spin-dependent beam separation in vortex beam generation has an important implication for spatial demultiplexing in optical communication utilizing optical angular momentum mode division multiplexing as well as for optical vortex tweezers and optical signal processing employing vortex beams.

Suppression of intervalley exchange coupling in the presence of momentum-dark states in transition metal dichalcogenides

Monolayers of transition metal dichalcogenides are promising materials for valleytronic applications, since they possess two individually addressable excitonic transitions at the non-equivalent $K$ and $K'$ points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucial importance to understand the elementary processes determining the lifetime of these optically injected valley excitons. In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism to investigate the efficiency of the intervalley coupling in the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low lying momentum-dark exciton states. While we predict a valley exciton lifetime on the order of some hundreds of fs in the absence of low lying momentum-dark states we demonstrate a strong quenching of the valley lifetime in the presence of such states.

Split-off states in tunnel-coupled semiconductor heterostructures for ultrafast modulation of spin and optical polarization

We present a theoretical analysis of the split-off states emerging due to a tunnel coupling between a remote bound state and a semiconductor quantum well (QW). The on-site Coulomb repulsion and the spin splitting of the bound state have been considered. The split-off states emerge in the band gap of the QW and reveal themselves as two solitary peaks in the photoluminescence (PL) from the QW. The peaks have opposite circular polarization and their spectral position strongly depends on the tunnel coupling strength. We suggest a mechanism of ultrafast PL polarization switching by means of electrical modulation of the tunnel coupling by an external gate. The obtained results open a new possibility for the spin and optical polarization control in nanoscale systems.

Localized solutions of the Dirac equation in free space and electromagnetic space-time crystals

Localized solutions of the Dirac equation for an electron moving in free space and electromagnetic field lattices with periodic dependence on space-time coordinates (electromagnetic space-time crystals) are treated using the expansions in basis wave functions. The techniques for calculating these functions with any prescribed accuracy are presented. It is shown that in the crystals created by two counterpropagating plane electromagnetic waves with the same or the opposite circular polarizations, the Dirac equation describing the basis functions reduces to matrix ordinary differential equations. These functions and the corresponding mean values of velocity, momentum, energy, and spin operators are found for the both types of crystals. Localized solutions describing the families of orthonormal beams in electromagnetic space-time crystals and free space, defined by a given set of orthonormal complex scalar functions on a two-dimensional manifold, are obtained. By way of illustration the orthonormal beams in free space and various localized states with complex vortex structure of probability currents, defined by the spherical harmonics, are presented. The obtained solutions have high probability density only in very small core regions. The evolution of wave packets with one-dimensional localization in the both types of crystals created by two circularly polarized waves is described.

Coupling characteristics of the spun optical fiber with multiple stress elements

Abstract Possible coupling between fundamental modes and higher order modes in a spun optical fiber with multiple stress elements is analyzed. It shows that only the higher order H E mn mode with the opposite circular polarization state can be coupled with the fundamental H E 11 mode. The coupling between the fundamental H E 11 mode and the TE, TM or EH modes can never be occurred. In the possible coupling between H E mn mode and H E 11 mode, m should be equal to N-1, where N is the number of the stress elements. As a comparison, the coupling property in a spun optical fiber with single stress element is also demonstrated. In this case, the fundamental H E 11 mode can not only be coupled with the higher order H E 2 n mode but also can be coupled with the higher order T E mode.

Collective motion of chiral Brownian particles controlled by a circularly-polarized laser beam.

We demonstrate the emergence of circular collective motion in a system of spherical light-propelled Brownian particles. Light-propulsion occurs as consequence of the coupling between the chirality of polymeric particles - left (L)- or right (R)-type - and the circularly-polarized light that irradiates them. Irradiation with light that has the same helicity as the particle material leads to a circular cooperative vortical motion between the chiral Brownian particles. In contrast, opposite circular-polarization does not induce such coupling among the particles but only affects their Brownian motion. The mean angular momentum of each particle has a value and sign that distinguishes between chiral activity dynamics and typical Brownian motion. These outcomes have relevant implications for chiral separation technologies and provide new strategies for optical torque tunability in mesoscopic optical array systems, micro- and nanofabrication of light-activated engines with selective control and collective motion.

Metasurface hologram for polarization measurement.

Polarization measurement is crucial for many optical applications in science and technology. Geometric metasurfaces have been used to develop polarization-sensitive holograms, providing a new opportunity for polarization measurement. We propose and experimentally demonstrate a hologram method to measure the polarization state of light. A reflective-type metasurface hologram is used to generate holographic images of graphene pattern. The ellipticity and helicity of the incident light are measured based on the intensities of the neighboring light spots, corresponding to two opposite circular polarization states. Benefiting from the advantages of reflective geometric metasurfaces, this device can operate in broadband.

Optically driven effective Faraday effect in instantaneous nonlinear media

Faraday rotation is arguably the most widely studied nonreciprocal phenomenon, observable in the form of polarization rotation as waves travel in magneto-optical materials. It is at the basis of the realization of various forms of isolators and circulators. However, magneto-optical materials are bulky and difficult to integrate, and inherently associated with losses. Here we leverage nonlinear phenomena in conventional optical crystals to mimic Faraday rotation in a resonant cavity excited with two pumps with opposite circular polarizations and slightly detuned frequencies. The effect can be observed in basic resonant structures with no need for material resonances or tailored dispersion, arbitrary frequency of the pumps, and low loss, yielding an interesting path towards the realization of compact magnet-free nonreciprocal optical elements.

Anomalous thermal expansion and chiral phonons in BiB3O6

The origins of anomalous thermal expansion in the chiral monoclinic solid $\alpha$-BiB$_{3}$O$_{6}$ have been studied through ab initio calculations. Positive and negative axial thermal expansion are shown to be driven by librations of borate units, elastic anisotropy, and most notably by chiral acoustic phonons involving elliptical revolutions of bismuth atoms. The chirality of the lattice gives rise to these modes by allowing the transverse acoustic branches to have opposite circular polarizations, only one of which couples strongly to the lattice strains. These results further understanding of relationships between crystallographic symmetry and physical properties.

Mode demultiplexing of vector beams using crossed-fork-shaped polarization grating fabricated by photoalignment of photo-crosslinkable liquid crystal polymer

A crossed-fork-shaped polarization grating (crossed-FPG) is presented for use in vector beam division demultiplexing. The crossed-FPG was fabricated in a thin photocrosslinkable liquid crystal polymer film by recording multiple polarization holograms between optical vortices and Gaussian beams that have opposite circular polarizations to each other. The recorded holograms work individually to act as FPG with a polarization-dependent topological charge conversion property that corresponds to a recorded pair of vortices. The feasibility of mode demultiplexing was demonstrated experimentally. The proposed crossed-FPG should be used as a mode demultiplexer for optical communications based on vector beam division multiplexing.

The Low-Frequency Solar Corona in Circular Polarization

We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 - 240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5% to nearly 100%. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.

Generation of an optical ball bearing facilitated by coupling between handedness of polarization of light and helicity of its phase

We report on mutual transformation of spin and orbital angular momenta (SAM and OAM) of light upon tight focusing of circularly polarized vortex beams. In some regions of the focal spot, the OAM of light changes by a value ±2ℏ, while the circular polarization (CP) there reverses its rotation direction, resulting in adjacent rings (or core and adjacent ring) of opposite CP handedness. Moreover, the SAM has a transverse angular momentum (AM) component. We show that while the whole pattern rotates around the optical axis at the rate of the optical frequency divided by the value of total AM, the 3D SAM “balls” between adjacent rings (or between a ring and a core) spin in an opposite direction at the rate of the optical frequency, constituting an optical analog of a ball bearing. The applications include manipulation of particles, e.g., optical tweezers or spanners.

Observation of a Giant Nonlinear Chiro‐Optical Response in Planar Plasmonic–Photonic Metasurfaces

AbstractPlasmonic nanosystems and metamaterials have recently attracted considerable attention because of their ability to enhance the light–matter interactions. One of such optical phenomena is the chiral‐ or handedness‐dependent response which typically requires 3D samples. Planar structures that can exhibit chiral response are highly desirable because of their facile fabrication, however fundamental challenges arising from the 2D nature of these systems prevent the generation of strong chiro‐optical effects. In this work, giant enhancement of the handedness‐dependent optical response in planar metallic nanostructures is shown by exploring the hybridization of plasmonic–photonic modes in a chiral metasurface. The resulting planar hybrid metasurface exhibits over an order of magnitude difference in nonlinear optical response when illuminated with excitation light of opposite circular polarizations. The unique properties of the hybridized plasmonic–photonic modes are shown to be responsible for the giant chiral nonlinear response. This platform allows to study the fundamental framework of chiral optical effects that arise from the combination of planar chirality and collective interaction of discrete nanosystems.