Polarization Vortices Research Articles

Phase-Change Perovskite Microlaser with Tunable Polarization Vortex.

Metasurfaces supporting optical bound states in the continuum (BICs) are emerging as simple and compact optical cavities to realize polarization-vortex lasers. The winding of the polarization around the singularity defines topological charges which are generally set by the cavity design and cannot be altered without changing geometrical parameters. Here, a subwavelength-thin phase-change halide perovskite BIC metasurface functioning as a tunable polarization vortex microlaser is demonstrated. Upon the perovskite structural phase transitions, both its refractive index and gain vary substantially, inducing reversible and bistable switching between distinct polarization vortexes underpinned by opposite topological charges. Dynamic tuning and switching of the resulting vector beams may find use in microscopy imaging, particle trapping and manipulation, and optical data storage.

All-optical generation of Abrikosov vortices by the inverse Faraday effect

Within the framework of the time-dependent Ginzburg-Landau theory we show that circularly polarized terahertz or far-infrared radiation induces a dc supercurrent that influences the dynamics of vortex-antivortex pair formation in a mesoscopic superconductor undergoing rapid thermal quenching. The Lorentz force arising from the supercurrents promotes vortex-antivortex separation and allows survival of the vortex polarity defined by the helicity of light. Based on this idea, we propose a two-stage irradiation scheme that provides a powerful method for controlled all-optical generation of Abrikosov vortices.

Dynamic electrical response of vortex polarization in (Pb0.9Sr0.1)TiO3/SrTiO3 artificial superlattice thin films

Artificial superlattice thin films with alternative Pb0.9Sr0.1TiO3 (PST) and SrTiO3 layers were fabricated via pulsed laser deposition. The vortex domains were formed by adjusting the unit cell number of the PST layer to be 20. The electrical response measured by time-resolved synchrotron x-ray diffraction with the repeated pulse voltages of a time interval of 1000 μs indicates that the reorientation of dipole moments in the vortex domains is susceptible to the applied electric field and is a reversible process. Although the centrosymmetric vortex domain structure is not considered to exhibit piezoelectricity, an evident change in the out-of-plane lattice constant with the applied electric field was confirmed. The magnitude of the electromechanical response at 600 kV/cm was estimated to be 19 pm/V, which is nearly comparable to the piezoelectric constant d33 of bulk PbTiO3.

Theoretical Studies on Controlling the Chirality of Helical Polarization Vortices in Ferroelectric Nanowires: Implications for Reconfigurable Electronic Devices

Theoretical Studies on Controlling the Chirality of Helical Polarization Vortices in Ferroelectric Nanowires: Implications for Reconfigurable Electronic Devices

High-efficiency nonlocal reflection-type vortex beam generation based on bound states in the continuum.

Momentum-space polarization vortices centered at symmetry-protected bound states in the continuum of a periodic structure, e.g. photonic crystal slab, provide a novel nonlocal approach to generate vortex beams. This approach enjoys a great convenience of no precise alignment requirements, although the generation efficiency of the nonlocal generators requires further optimization before the practical application. In this work, we propose a temporal-coupled-mode-theory-based guideline for high-efficiency nonlocal reflection-type vortex generator design. The conversion efficiency of the vortex beam is found to be limited by the ratio of the radiative loss to the intrinsic absorption in practical systems. To increase this ratio through mode selection and structure design, the photonic crystal slabs are theoretically designed and experimentally characterized, showing a maximum on-resonance conversion efficiency of up to 86%. Combining high efficiency with simple fabrication and no requirement for precise alignment, reflection-type photonic crystal slabs could offer a new and competitive way to generate vortex beams flexibly.

High-Efficiency Characterization of Optical Vortices with Arbitrary State of Polarization Using Straight-Line and Parabolic-Line Polarization Gratings

An optical system consisting of a straight-line polarization grating (SPG) and two parabolic-line polarization gratings (PPGs) is presented for the characterization of optical vortices (OVs) with arbitrary states of polarization (SoPs). The PPG is capable of converting an OV with a specific SoP into a bright bar-like spot with 100% efficiency. The number of dark lines and their orientation respectively correspond to the magnitude and sign of topological charge (TC) of the incident OV, thereby enabling characterization of OVs with high efficiency. Furthermore, on combining an SPG with PPGs, the present system can characterize the TC of incident OVs regardless of their SoP. The feasibility of the system was demonstrated in experiments using gratings fabricated by applying the photoalignment method and employing films of a photo-crosslinkable polymer liquid crystal. The experimentally obtained efficiency is 70.2%. We furthermore demonstrate the system’s capability to characterize polarization vortices.

Location and Topology of the Fundamental Gap in Photonic Crystals

The fundamental, or first, band gap is of unmatched importance in the study of photonic crystals. Here, we address precisely where this gap can be opened in the band structure of three-dimensional photonic crystals. Although strongly constrained by symmetry, this problem cannot be addressed directly with conventional band-symmetry analysis due to the existence of a photonic polarization vortex at zero frequency. We develop an approach for overcoming the associated symmetry singularity by incorporating fictitious, auxiliary longitudinal modes. Our strategy also enables us to extend recent developments in symmetry-based topological analysis to the fundamental gap of three-dimensional photonic crystals. Exploiting this, we systematically study the topology of the minimal fundamental gaps. This reveals the existence of topological gap-obstructions that push the fundamental gap higher than what a conventional analysis would suggest. Our work demonstrates that topology can play a crucial role in the opening of the fundamental photonic gap and informs future theoretical and experimental searches for conventional and topological band gaps in three-dimensional photonic crystals.

Exploring the ellipticity dependency on vector helical Ince-Gaussian beams and their focusing properties.

We present a numerical study of the intensity and polarization structure of vector helical Ince-Gaussian (VHIG) modes, which present a distinct subclass of vector Ince-Gaussian modes with defined parameter settings. The intensity profile of VHIG beams has an elliptic hollow structure, while the polarization distribution shows multiple single-charge polarization vortices arranged along a line. By selecting the mode order, phase factor and ellipticity of the VHIG beams, we can control the number of elliptic rings, the number of polarization vortices, and the topology of the vector singularity. Furthermore, we simulate the focusing properties of VHIG beams based on vector diffraction theory. Our results indicate that the ellipticity parameter of VHIG beams could be a valuable degree of freedom to generate attractive transverse profiles and longitudinal distributions under focusing, which may have implications for lithography, material processing, optical communication, and even optical trapping and manipulation.

Melting of crystals of polarization vortices and chiral phase transitions in oxide superlattices

We study the equilibrium arrangements of polarization vortices in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices by means of second-principles simulations. We find that, at low temperatures, polarization vortices organize in a regular arrangement in which clockwise and counter-clockwise vortices alternate positions, leading to a crystal-like structure with well defined handedness. This chiral crystal melts at a critical temperature $T_\mathrm{M}$ into a chiral liquid, where long-range order is lost but handedness is preserved. At even higher temperatures, $T_\mathrm{C}$, a second phase transition occurs, at which the chiral liquid of polarization vortices loses its handedness. Both phase transitions can be readily identified by the adequate choices of order parameters.

From the flow to the polarization field: A cognitive way for ferroelectric vortex structures

The generation of topological domain structures can significantly influence the behaviors of ferroelectric materials, and, however, strategies and perspectives for treating and analyzing them are still lacking. Inspired by the fluid lines and vorticity in hydromechanics, a characterization method of polarization lines and polarization vorticity is proposed herein for characterizing the vortex structure evolution. The polarization vorticity has the same unit as charge density, and the polarization vortex size is identified as a structure for vorticity concentration. Observing the radius and vorticity of vortices, we find that the domain evolution can be divided into three stages and the vortices react to mechanical loading in two ways: increasing the vorticity and enlarging the radius. Moreover, vortices exhibit three-dimensional shape, just like the vortex tube, and their conservation is proved. This paper provides a basis for further research on topological domain structures and the mechanism of vortex evolution, which might help improve high energy density devices.

Fast switching of magnetic vortex state under an alternating magnetic field

Vortex is considered to be a promising candidate for nonvolatile magnetic random storage devices due to its unique binary state and high thermal stability. The control of vortex polarity and chirality is key to devices based on vortex. In this work, the switching of the magnetic vortex state in Permalloy nanodisks has been investigated by micromagnetic simulation employing in-plane alternating magnetic fields. During the switching of magnetic vortex state, an antivortex is formed, and the antivortex and vortex will not annihilate together. The formation of antivortex and the annihilation of initial vortex lead to the switching of magnetic vortex state. The switching of vortex state depends on magnetic field and disk size. The switching time of magnetic vortex state declines with the enhancement of magnetic field amplitude or magnetic field frequency. All the switching can be completed within 1.4 ns, indicating that the alternating magnetic field driving can induce the rapid switching of vortex state. This work provides an effective method for the fast switching of magnetic vortex state, which benefits the potential storage applications in vortex-based spintronic devices.

Interference of the scattered vector light fields from two optically levitated nanoparticles.

We experimentally study the interference of dipole scattered light from two optically levitated nanoparticles in vacuum, which present an environment free of particle-substrate interactions. We illuminate the two trapped nanoparticles with a linearly polarized probe beam orthogonal to the propagation of the trapping laser beams. The scattered light from the nanoparticles are collected by a high numerical aperture (NA) objective lens and imaged. The interference fringes from the scattered vector light for the different dipole orientations in image and Fourier space are observed. Especially, the interference fringes of two scattered light fields with polarization vortex show the π shift of the interference fringes between inside and outside the center region of the two nanoparticles in the image space. As far as we know, this is the first experimental observation of the interference of scattered vector light fields from two dipoles in free space. This work also provides a simple and direct method to determine the spatial scales between optically levitated nanoparticles by the interference fringes.

Magnetic modulation of topological polarization singularities in momentum space.

The polarization singularities in momentum space, rather than in real space, are capturing interest for active singular optics with exotic light scattering and various topological phenomena, which have potential applications in vortex nano-lasers, valley exciton emission, and others. Here, we propose to magnetically control the polarization singularities in momentum space in the photonic crystal slabs with inversion spatial symmetry (P symmetry). A pair of C points (circular polarization points) with the same topological charge is spawned from a V point (polarization vortex center), and they can be dynamically shifted in momentum space with the variation of the magnetic field. Moreover, the coupling between transverse electric (TE) and transverse magnetic (TM) modes induced by the magnetic field gives rise to a hybrid mode, which can close certain leaky channels to achieve an accidental V point. Such active manipulation of polarization singularities with magnetic field is promised for various applications in light-matter interactions and reveals novel phenomena and physics in singular optics and topological photonics.

Photocurrent detection of radially polarized optical vortex with hot electrons in Au/GaN

We report a GaN based metal–semiconductor–metal (MSM) infrared photodetector enabled with azimuthally distributed sub-wavelength gratings fabricated on one of the working electrodes. Under illumination, hot electron transfer is introduced by the plasmonic resonance in the infrared waveband formed at the interface of Au/GaN. Without the help of using any external optical polarizers, the device is able to detect radial polarization vortices in the form of photocurrents with a prescribed response spectrum. The detector exhibits a 10%–90% rise and fall time of 0.9 ms under modulated light, much faster than that of conventional ultraviolet GaN MSM photodetectors based on the band edge absorption. This work provides a viable way to measure spatially variant polarization beams with a compact plasmonic photodetectors fabricated from wide bandgap semiconductors.

Stability of chiral polarization vortex in strained ferroelectric superlattices

As a geometric property that cannot overlap with its mirror image, chirality is becoming more and more common in electronic structures. The recent discovery of chiral topological polar structures in artificial ferroelectric superlattices is not only of scientific importance but also holds promise for application in tunable frequency oscillators. Although the tubular chiral polarization vortex has been observed in PbTiO3/SrTiO3 superlattices, the conditions for its stability remain elusive. Here, we construct a temperature-strain phase diagram for the (PbTiO3)10/(SrTiO3)10 superlattices by using phase field simulation in which four domain states exist at different temperatures and misfit strains. It is found that the chiral polarization vortex appears only in a small range of misfit strain, elucidating the absence of chirality in most ferroelectric superlattices. The chiral polarization vortex is an intermediate state between the ordinary achiral polarization vortex and the mixed domain structure. In addition, by changing strain or temperature, chiral vortices can be reversibly transformed into achiral vortices, demonstrating its tunability. The present work provides theoretical guidance for creating the chiral vortex in ferroelectric superlattices.

Controlling Topology and Polarization State of Lasing Photonic Bound States in Continuum

AbstractRecently, optical bound states in continuum (BICs) incorporated with optical gain have been reported to exhibit lasing. Here, it is shown that each of the four BICs supported by C symmetric photonic crystal slab can be made to lase, allowing the control over the topological charge of the resulting laser beam. The type of each BIC and their topological charges are identified by imaging the far‐field polarization vortices of the lasing signal. Results are compared with experimentally obtained dispersions, finite element method simulations, and multipole decomposition method based on the microscopic polarization currents in the photonic crystal plane. A demonstration of multimode lasing of two non‐degenerate BICs with opposite topological charges is presented. The momentum space overlap of the BICs results in a unique polarization pattern. The study provides a generalizable example for engineering the topological properties of coherent light.

Symmetry‐Protected Spoof Localized Surface Plasmonic Skyrmion

AbstractElectromagnetic (EM) skyrmions are an EM analogue of the skyrmions in condensed matter physics, offering new degrees of freedom to structure light and manipulate light–matter interactions and thus promising various groundbreaking applications in optics and photonics. Recently, there is a growing interest in composing EM skyrmions based on different field vectors of EM waves. Here, an EM skyrmion is realized, i.e., a spoof plasmonic skyrmion (SPS), using the electric field vectors of spoof localized surface plasmons (LSPs) in a planar microwave resonator with rotational and mirroring symmetries. The SPS is constructed by synthesizing a scalar vortex (a topological charge 0) and a polarization vortex (a topological charge 1) in the in‐plane and the out‐of‐plane component of the fields, respectively. Besides an experimental demonstration, group theory is employed and pinpoints the symmetry origin of the skyrmion. This investigation demonstrates the ubiquity of the existence of the skyrmion in any planar EM resonator holding rotational and mirroring symmetries, regardless the dimensions and the operating frequencies. This skyrmion design not only promises novel microwave applications for sensing and transferring information, but also lays down a general guideline for devising skyrmions operating over a broad range in the EM spectra.

Size and temperature-dependent stability of polarization vortex in PbTiO3 nanosheet under uniaxial tension or compression

ABSTRACT The stability of polarization vortex is absolutely crucial to the service performance of promising ferroelectric storage devices. Some phase field simulations were conducted on the PbTiO3 nanosheets with an initial polarization vortex under uniaxial tension or compression to investigate the conditions and types of vortex instability as well as the effects of the nanosheet size and temperature on them. The instability of polarization vortex has strong size and temperature dependence. The critical tensile stress decreases with increasing nanosheet size, and only the α-type vortex instability is observed in tensile nanosheets. However, the critical compressive stress first increases and then decreases with increasing nanosheet size, and there exists a peak size. The β-type (including β1 and β2-type) instability occurs when the nanosheet size is less than the peak size, but the α-type instability occurs when the nanosheet size is larger than the peak size. An elevated temperature reduces the critical tensile and compressive stresses, but increases the peak size. These are very important for design of promising nano-ferroelectric devices as high density memories.

Realization of Polaritonic Topological Charge at Room Temperature Using Polariton Bound States in the Continuum from Perovskite Metasurface

AbstractExciton‐polaritons are mixed light–matter excitations resulting from the strong coupling regime between an active excitonic material and photonic resonances. Harnessing these hybrid excitations provides a rich playground to explore fascinating fundamental features, as out‐of‐equilibrium Bose–Einstein condensation and quantum fluids of light, plus novel mechanisms to be exploited in optoelectronic devices. The formation of exciton‐polaritons arising from the mixing between hybrid inorganic–organic perovskite excitons and an optical bound state in a continuum (BIC) of a subwavelength‐scale metasurface, are experimentally investigated at room temperature. These polaritonic eigenmodes, hereby called polariton BICs (pol‐BICs) are revealed in reflectivity, resonant scattering, and photoluminescence measurements. Although pol‐BICs only exhibit a finite quality factor bounded by the nonradiative losses of the excitonic component, they fully inherit BIC peculiar features: a full uncoupling from the radiative continuum in the vertical direction, which is associated to a locally vanishing farfield radiation in momentum space. Most importantly, the experimental results confirm that the topological nature of the photonic BIC is perfectly transferred to the pol‐BIC. This is evidenced by the observation of a polarization vortex in the farfield of polaritonic emission. The results pave the way to engineer BIC physics of interacting bosons and novel room temperature polaritonic devices.

Совместное влияние магнитного поля и спин-поляризованного тока на переключение полярности магнитных вихрей в спин-трансферном наноосцилляторе

The combined influence of a spin polarized current and an external magnetic field for switching the polarity of vortices in spin-transfer nanooscillators 400 nm in diameter is studied. A diagram is constructed of the dependence of the magnitude of the spin-polarized current on the magnitude of the magnetic field, which separately switches the polarity of the vortex in the magnetic layers of the spin-transfer nanooscillator.