Optical Spin Hall Effect Research Articles

Spin-Orbital Conversion of a Strongly Focused Light Wave with High-Order Cylindrical-Circular Polarization.

We discuss interesting effects that occur when strongly focusing light with mth-order cylindrical–circular polarization. This type of hybrid polarization combines properties of the mth-order cylindrical polarization and circular polarization. Reluing on the Richards-Wolf formalism, we deduce analytical expressions that describe E- and H-vector components, intensity patterns, and projections of the Poynting vector and spin angular momentum (SAM) vector at the strong focus. The intensity of light in the strong focus is theoretically and numerically shown to have an even number of local maxima located along a closed contour centered at an on-axis point of zero intensity. We show that light generates 4m vortices of a transverse energy flow, with their centers located between the local intensity maxima. The transverse energy flow is also shown to change its handedness an even number of times proportional to the order of the optical vortex via a full circle around the optical axis. It is interesting that the longitudinal SAM projection changes its sign at the focus 4m times. The longitudinal SAM component is found to be positive, and the polarization vector is shown to rotate anticlockwise in the focal spot regions where the transverse energy flow rotates anticlockwise, and vice versa—the longitudinal SAM component is negative and the polarization vector rotates clockwise in the focal spot regions where the transverse energy flow rotates clockwise. This spatial separation at the focus of left and right circularly polarized light is a manifestation of the optical spin Hall effect. The results obtained in terms of controlling the intensity maxima allow the transverse mode analysis of laser beams in sensorial applications. For a demonstration of the proposed application, the metalens is calculated, which can be a prototype for an optical microsensor based on sharp focusing for measuring roughness.

Dynamic Optical Spin Hall Effect in Chitosan-Coated All-Dielectric Metamaterials for a Biosensing Platform

The optical spin Hall effect, which describes the spin-dependent and transverse shift of light, has been steadily investigated with the development of sensing applications such as polarization-dependent sensors, material interface analysis, and refractive index spectroscopy. However, practical optical spin Hall effect sensing platforms have not been reported since the previous reports have only focused on passive platforms. Here, we propose a biosensing platform using chitosan-coated all-dielectric metamaterials, which can detect the relative humidity of the surroundings by analyzing the displacement of the spin Hall effect. The chitosan coating provides a high sensitivity of relative humidity with a dynamic optical spin Hall effect in all-dielectric metamaterials. The maximum displacement of the split beam is calculated as 350 nm when the relative humidity changes by 60%, resulting in 8.75 nm shift per 1% of relative humidity change. This mechanism can be further exploited for a biosensing platform since the chitosan-coated all-dielectric metamaterials are biocompatible, biodegradable, and hydrophilic. Considering the chitosan-coated all-dielectric metamaterials have high feasibility with low-cost, high-throughput nanoimprinting methods, the dynamic optical spin Hall effect-based metamaterials will be a candidate of a promising real-time and ultra-sensitive sensing platform for detecting biomolecules.

Influence of medium column type on edge states in all-media topological photonic crystals

We have constructed photonic crystals (PCs) of a standard honeycomb lattice composed of solid dielectric columns, hollow dielectric columns, and composite dielectric columns and studied the influence of their dielectric constant and filling ratio on the topological boundary states. The optical quantum spin Hall effect is realized by compressing and stretching the standard honeycomb lattice. In the waveguide structure with sharp bends, cavity, and disorder, the electromagnetic (EM) wave is transmitted unidirectionally and robustly, which verifies the transmission characteristics of the topological edge states. In addition, by replacing several solid dielectric columns near the junction of PCs with different topological states with composite dielectric columns, disturbance is introduced to the edge states, and the influence of the relative effective permittivity of the composite dielectric columns on the edge states is studied. It is found that the EM wave in the gap between the upper boundary state and the lower boundary state cannot be transmitted, and with the increase of the relative effective permittivity of the composite dielectric columns, the gap becomes larger and larger, and the position of the gap gradually moves down. Based on this, a photonic crystal (PC) waveguide structure that has higher transmission efficiency, robust unidirectionality, and higher optical localization performance, is designed. It has more potential development in filters, optical switches, optical communication systems, and semiconductor technology.

Second harmonic generation enhancement and directional emission from topological corner state based on the quantum spin Hall effect.

Topological corner state has attracted much research interests since it does not obey the conventional bulk-edge correspondence and enables tightly confined light within small volumes. In this work, we demonstrate an enhanced second harmonic generation (SHG) from a topological corner state and its directional emission. To this end, we design an all-dielectric topological photonic crystal based on optical quantum spin Hall effect. In this framework, pseudospin states of photons, topological phase, and topological corner state are subsequently constructed by engineering the structures. It is shown that a high Q-factor of 3.66×1011 can be obtained at the corner state, showing strong confinement of light at the corner. Consequently, SHG is significantly boosted and manifests directional out-of-plane emission. More importantly, the enhanced SHG has robustness against a broad class of defects. These demonstrated properties offer practical advantages for integrated optical circuits.

Magnetization-induced optical rectification and inverse spin Hall effect for interfacial terahertz generation in metallic heterostructures

Metallic heterostructure as promising terahertz (THz) spintronic emitters has promoted the development of both spintronics and THz science. However, the underlying physics at the metallic interface, such as the nonlinear optical effect, remains unclear. Herein, we present interface magnetization induced THz generation from metallic heterostructure consisting of Heusler alloy CoFeMnSi (CFMS) and Pd thin films. THz generation is ascribed to 35% contribution from the magnetization-induced optical rectification (MOR) and 65% contribution from inverse spin Hall effect (ISHE) based on the pump polarization and sample azimuthal angle dependent measurement. Furthermore, the contribution ratio of the MOR decreases to 12% via lowering the CFMS grown temperature, which is due to the reduced crystalline quality and possible metal to semiconductor transformation in CFMS. Our results not only clarify MOR and ISHE in metallic heterostructure for the scientific field, but they also benefit THz source optimization for the technology field.

Selectively steering photon spin angular momentum via electron-induced optical spin Hall effect.

The development of the optical spin Hall effect (OSHE) realizes the splitting of different spin components, contributing to the manipulation of photon spin angular momentum that acts as the information carrier for quantum technology. However, OSHE with optical excitation lacks active control of photon angular momentum at deep subwavelength scale because of the optical diffraction limit. Here, we experimentally demonstrate a selective manipulation of photon spin angular momentum at a deep subwavelength scale via electron-induced OSHE in Au nanoantennas. The inversion of the OSHE radiation pattern is observed by angle-resolved cathodoluminescence polarimetry with the electron impact position shifting within 80 nm in a single antenna unit. By this selective steering of photon spin, we propose an information encoding with robustness, privacy, and high level of integration at a deep subwavelength scale for the future quantum applications.

Optical Spin Hall Effect in Closed Elliptical Plasmonic Nanoslit with Noncircular Symmetry.

We investigated the optical spin Hall effect (OSHE) of the light field from a closed elliptical metallic curvilinear nanoslit instead of the usual truncated curvilinear nanoslit. By making use of the characteristic bright spots in the light field formed by the noncircular symmetry of the elliptical slit and by introducing a method to separate the incident spin component (ISC) and converted spin component (CSC) of the output field, the OSHE manifested in the spot shifts in the CSC was more clearly observable and easily measurable. The slope of the elliptical slit, which was inverse along the principal axes, provided a geometric phase gradient to yield the opposite shifts of the characteristic spots in centrosymmetry, with a double shift achieved between the spots. Regarding the mechanism of this phenomenon, the flip of the spin angular momentum (SAM) of CSC gave rise to an extrinsic orbital angular momentum corresponding to the shifts of the wavelet profiles of slit elements in the same rotational direction to satisfy the conservation law. The analytical calculation and simulation of finite-difference time domain were performed for both the slit element and the whole slit ellipse, and the evolutions of the spot shifts as well as the underlying OSHE with the parameters of the ellipse were achieved. Experimental demonstrations were conducted and had consistent results. This study could be of great significance for subjects related to the applications of the OSHE.

Revisiting the anomalous spin-Hall effect of light near the Brewster angle

Optical spin-Hall effect (SHE) exhibits many intriguing features as a linearly polarized (LP) light beam strikes an interface at incident angles around the Brewster angle, but the underlying physics remains obscure. Here, we elucidate the physics through reanalyzing this problem employing rigorous calculations and the Berry phase concept. As a circularly polarized (CP) light beam strikes an optical interface, the reflected light beam contains two components, a spin-flipped abnormal mode acquiring geometric phases (thus exhibiting a spin-Hall shift) and a spin-maintained normal mode without such phases. Strengths of these two modes are determined by the incident angle and the optical properties of the interface. Under the LP incidence, however, a spin component inside the reflected light beam must be the sum of normal and abnormal components of reflected light beams corresponding to CP incidences with different helicity, which thus sensitively depends on the incident angle. In particular, at incident angles near the Brewster one, reflection coefficients for two CP components exhibit opposite signs, leading to significant destructive interferences between normal and abnormal modes, finally generating highly deformed reflected light patterns with anomalously enhanced spin-Hall shifts. These findings can be extended to both reflected and transmitted cases with Brewster-like behaviors. Our analyses reinterpret previously discovered effects, providing an alternative understanding on the SHE of light.

Focused linearly-polarized-light scattering from a silver nanowire: Experimental characterization of the optical spin-Hall effect

Spin-orbit interactions (SOI) are a set of sub-wavelength optical phenomenon in which spin and spatial degrees of freedom of light are intrinsically coupled. One of the unique example of SOI, spin-Hall effect of light (SHEL) has been an area of extensive research with potential applications in spin controlled photonic devices as well as emerging fields of spinoptics and spintronics. Here, we report our experimental study on SHEL due to forward scattering of focused linearly polarized Gaussian and Hermite-Gaussian ($\textrm{HG}_{10}$) beams from a silver nanowire (AgNW). Spin dependent anti-symmetric intensity patterns are obtained when the polarization of the scattered light is analysed. The corresponding spin-Hall signal is obtained by computing the far-field longitudinal spin density ($s_3$). Furthermore, by comparing the $s_3$ distributions, significant enhancement of the spin-Hall signal is found for $\textrm{HG}_{10}$ beam compared to Gaussian beam. The investigation of the optical fields at the focal plane of the objective lens reveals the generation of longitudinally spinning fields as the primary reason for the effects. The experimental results are corroborated by 3-dimensional numerical simulations. The results lead to better understanding of SOI and can have direct implications on chip-scale spin assisted photonic devices.

Optical analogue of Dresselhaus spin–orbit interaction in photonic graphene

The concept of gauge fields plays a significant role in many areas of physics from particle physics and cosmology to condensed matter systems, where gauge potentials are a natural consequence of electromagnetic fields acting on charged particles and are of central importance in topological states of matter. Here, we report on the experimental realization of a synthetic non-Abelian gauge field for photons in a honeycomb microcavity lattice. We show that the effective magnetic field associated with TE-TM splitting has the symmetry of Dresselhaus spin-orbit interaction around Dirac points in the dispersion, and can be regarded as an SU(2) gauge field. The symmetry of the field is revealed in the optical spin Hall effect (OSHE), where under resonant excitation of the Dirac points precession of the photon pseudospin around the field direction leads to the formation of two spin domains. Furthermore, we observe that the Dresselhaus field changes its sign in the same Dirac valley on switching from s to p bands in good agreement with the tight binding modelling. Our work demonstrating a non-Abelian gauge field for light on the microscale paves the way towards manipulation of photons via spin on a chip.

Diffraction-induced enhancement of optical spin Hall effect in a dielectric grating

The enhancement of the optical spin Hall effect (OSHE) of a transmitted beam has been achieved in a small incident angle of a sub-degree scale. The OSHE at a large incident angle can be beneficial in optical applications, such as polarization-dependent sensors and filters, but studies to increase the OSHE at a large incident angle have been elusive in transmission mode. We propose a dielectric grating on a metal layer to achieve the grating-induced increase of the OSHE. The polarization-dependent transmission and OSHE that reaches several wavelengths are numerically demonstrated. We also show the tunability of the operating wavelength and incident angle of the OSHE by changing materials and geometrical parameters.

Selective magnetic responses of silicon nanoparticles modulated by waveguide structures.

High-refractive-index nanoparticles (NPs), such as silicon NPs, were considered as effective carriers in their response to a magnetic field at optical frequencies. Such NPs play an important role in many state-of-the-art technologies in nano-optics. Although the resonance properties of these NPs when varying their structural parameters have been studied intensely in the past few years, their interaction with the underlying substrate has seldom been discussed, in particular, when the substrate is a waveguide structure that significantly modulates the optical responses of the NPs. We proposed and studied a selective magnetic coupling system comprising a Si-NP on a metal-dielectric waveguide (MDW). The MDW structure supports either a transverse electric (TE) or a transverse magnetic (TM) mode that induces a large polarization dependence in the magnetic resonance. A new manifestation of the optical spin Hall effect was demonstrated in which a vertical rotating magnetic dipole excites a TE-type waveguide mode with a specific unidirectional emission. Making use of this polarization response, we developed a scanning imaging system that can selectively map the transverse or longitudinal magnetic field component of a focused beam depending on the type of MDW used in the system. This selective magnetic resonance coupling system is expected to be valuable for studying the fundamental interactions between the magnetic field and matter and for developing related nano-applications.

Magnetic control of polariton spin transport

Polaritons are hybrid light–matter quasiparticles arising from the strong coupling of excitons and photons. Owing to the spin degree-of-freedom, polaritons form spinor fluids able to propagate in the cavity plane over long distances with promising properties for spintronics applications. Here we demonstrate experimentally the full control of the polarization dynamics of a propagating exciton–polariton condensate in a planar microcavity by using a magnetic field applied in the Voigt geometry. We show the change of the spin-beat frequency, the suppression of the optical spin Hall effect, and the rotation of the polarization pattern by the magnetic field. The observed effects are theoretically reproduced by a phenomenological model based on microscopic consideration of exciton–photon coupling in a microcavity accounting for the magneto-induced mixing of exciton–polariton and dark, spin-forbidden exciton states.

Observation of Enhanced Optical Spin Hall Effect in a Vertical Hyperbolic Metamaterial

Hyperbolic metamaterials, horizontally stacked metal and dielectric multilayer, have recently been studied as a platform to observe optical spin Hall effect. However, the large optical spin Hall effect in the horizontal hyperbolic metamaterials accompanies extremely low transmission, which obstructs its practical applications. Reducing the sample thickness to augment the transmission causes diminishment of the shift. In this letter, we demonstrate that a vertical hyperbolic metamaterial can enhance the shift by several orders of magnitude in comparison to the shift of its horizontal counterpart. Under the same conditions of material combinations and total thickness, the shift enhancement, which is incident angle-dependent, can be higher than 800-fold when the incident angle is 5 degree, and 5000-fold when the incident angle is 1 degree. As a proof of concept, we fabricate a large-scale gold nano-grating by nanoimprint lithography and measure the helicity-dependent shift by Stokes polarimetry setup, which agrees well with the simulated result. The gigantic optical spin Hall effect in a vertical hyperbolic metamaterial will enable helicity-dependent control of optical devices including filters, sensors, switches and beam splitters.

Oscillations of the Degree of Circular Polarization in the Optical Spin Hall Effect

The optical spin Hall effect appears when elastically scattered exciton polaritons couple to an effective magnetic field inside of quantum wells in semiconductor microcavities. Theory predicts an oscillation of the pseudospin of the exciton polaritons in time. Here, we present a detailed analysis of momentum space dynamics of the exciton polariton pseudospin. Compared to what is predicted by theory, we find a higher modulation of the temporal oscillations of the pseudospin. We attribute the higher modulation to additional components of the effective magnetic field which have been neglected in the foundational theory of the optical spin Hall effect. Adjusting the model by adding non-linear polariton-polariton interactions, we find a good agreement in between the experimental results and simulations.

Revisitation of Extraordinary Young’s Interference: from Catenary Optical Fields to Spin–Orbit Interaction in Metasurfaces

Extraordinary Young’s interference is an anomalous phenomenon observed at metallic surface in 2004. The shrinkage of slit size into deep subwavelength scale introduces many novel effects that cannot be described by classic interference theory. In this paper, the Young’s interference excited by subwavelength slits is investigated via rigorous Maxwell’s equations. We reveal a universal spin–orbit interaction mechanism associated with the catenary optical fields in 2D isotropic metal-dielectric structures for the generation of optical vortex array and spin Hall effect. Based on the interference of vectorial vortex arrays with opposite topological charges, a simple yet powerful method for the fabrication of space-variant metasurfaces is proposed for the first time, which breaks the fundamental limit of traditional interference lithography. These results may provide numerous new perspectives on both the design of catenary metasurfaces and optical spin-controlled devices.

Oscillations of the degree of circular polarization in the optical spin Hall effect

AbstractThe optical spin Hall effect appears when elastically scattered exciton polaritons couple to an effective magnetic field inside of quantum wells in semiconductor microcavities. Theory predicts an oscillation of the pseudospin of the exciton polaritons in time. Here, we present a detailed analysis of momentum space dynamics of the exciton polariton pseudospin. Compared to what is predicted by theory, we find a higher modulation of the temporal oscillations of the pseudospin. We attribute the higher modulation to additional components of the effective magnetic field which have been neglected in the foundational theory of the optical spin Hall effect. Adjusting the model by adding non-linear polariton-polariton interactions, we find a good agreement in between the experimental results and simulations.

Theory of optically controlled anisotropic polariton transport in semiconductor double microcavities

Exciton polaritons in semiconductor microcavities exhibit many fundamental physical effects, with some of them amenable to being controlled by external fields. The polariton transport is affected by the polaritonic spin-orbit interaction, which is caused by the splitting of transverse-electric and transverse-magnetic (TE-TM) modes. This is the basis for a polaritonic Hall effect, called optical spin Hall effect (OSHE), which is related to the formation of spin/polarization textures in momentum space, determining anisotropic ballistic transport, as well as related textures in real space. Owing to Coulombic interactions between the excitonic components of the polaritons, optical excitation of polaritons can affect the OSHE. We present a theoretical analysis of the OSHE and its optical control in semiconductor double microcavities, i.e. two optically coupled cavities, which are particularly well suited for the creation of polaritonic reservoirs that affect the spin-texture-forming polaritons. The theory is formulated in terms of a set of double-cavity spinor-polariton Gross-Pitaevskii equations. Numerical solutions feature, among other things, a controlled rotation of the spin texture in momentum space. The theory also allows for an identification of the effective magnetic field component that determines the optical control in phenomenological pseudo-spin models in terms of exciton interactions and the polariton density in the second lower polariton branch.

Dynamics of the optical spin Hall effect

We study the time evolution of the Optical Spin Hall Effect (OSHE), which occurs when exciton-polaritons undergo resonant Rayleigh scattering. The resulting spin pattern in momentum space is quantified by calculating the degree of circular polarization of the momentum space image for each point in time. We find the degree of circular polarization performing oscillations, which can be described within the framework of the pseudospin model by Kavokin et al. (Ref. 1).

Exceptional points in anisotropic planar microcavities

Planar microcavities allow the control and manipulation of spin-polarization, manifested in phenomena like the optical spin Hall effect due to the intrinsic polarization mode splitting. Here, we study a transparent microcavity with broken rotational symmetry, realized by aligning the optical axis of a uniaxial cavity material in the cavity plane. We demonstrate that the in-plane optical anisotropy gives rise to exceptional points in the dispersion relation, which occur pair-wise, are circularly polarized, and are cores of polarization vortices. These exceptional points are a result of the non-Hermitian character of the system, and are in close relationship to singular optical axes in absorptive biaxial systems.