Spin Angular Momentum Research Articles

All-Optical Polarization Manipulation Through Orbital Polarization Holography

Spin-orbital optical phenomena are closely related with light-matter interactions and have been of great interest in the last few years. Here, the effect of optical orbital angular momentum (OAM) on polarized waves carrying spin angular momentum (SAM) has been investigated experimentally by means of orbital polarization holography and analyzed with Jones matrices theoretically. We report that all-optical OAM-to-polarization manipulation can be realized with a controllable holographic grating recorded through the interference of orthogonally polarized beams in various helical modes in a kind of photo-alignment supermolecular liquid-crystalline films. The polarization states of diffraction beams can be controlled through adjusting the spatial degree of freedom of the recording light field. The OAM-controlled polarization manipulation is discussed in terms of Jones matrices and photoinduced birefringence. Because of the realization of OAM-to-SAM conversion, this work may find applications in a variety of devices.

γγ decay as a probe of neutrinoless ββ decay nuclear matrix elements

We study double gamma (γγ) decay nuclear matrix elements (NMEs) for a wide range of nuclei from titanium to xenon, and explore their relation to neutrinoless double-beta (0νββ) NMEs. To favor the comparison, we focus on double-magnetic dipole transitions in the final ββ nuclei, in particular the γγ decay of the double isobaric analog of the initial ββ state into the ground state. For the decay with equal-energy photons, our large-scale nuclear shell model results show a good linear correlation between the γγ and 0νββ NMEs. Our analysis reveals that the correlation holds for γγ transitions driven by the spin or orbital angular momentum due to the dominance of zero-coupled nucleon pairs, a feature common to 0νββ decay. Our shell-model findings point out the potential of future γγ decay measurements to constrain 0νββ NMEs, which are key to answer fundamental physics questions based on 0νββ experiments.

Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles.

Circularly polarized light carries spin angular momentum, so it can exert an optical torque on the polarization-anisotropic particle by the spin momentum transfer. Here, we show that giant positive and negative optical torques on Mie-resonant (gain) particles arise from the emergence of superhybrid modes with magnetic multipoles and electric toroidal moments, excited by linearly polarized beams. Anomalous positive and negative torques on particles (doped with judicious amount of dye molecules) are over 800 and 200 times larger than the ordinary lossy counterparts, respectively. Meanwhile, a rotational motor can be configured by switching the s- and p-polarized beams, exhibiting opposite optical torques. These giant and reversed optical torques are unveiled for the first time in the scattering spectrum, paving another avenue toward exploring unprecedented physics of hybrid and superhybrid multipoles in metaoptics and optical manipulations.

Experimental demonstration of optical trapping and manipulation with multifunctional metasurface.

Chip-scale optical tweezers, which are usually implemented in a planar format without using bulky diffractive optical elements, are recognized as a promising candidate to be integrated with a lab-on-a-chip system. However, traditional chip-scale optical tweezers are often static and allow for only one type of manipulation functionality since the geometrical parameters of the tweezers are fixed. Herein, we introduce a new, to the best of our knowledge, class of on-chip optical tweezers for diverse types of manipulation of micro-particles. Utilizing both the propagation phase and Pancharatnam-Berry phase, we experimentally demonstrate the spin-dependent trapping, moving, and circling of micro-particles with the transfer of optical gradient force and orbital angular momentum to particles. We further show that the spin angular momentum of the output beam provides an additional degree of freedom to control the spinning rotation of particles. This new type of optical tweezers paves the way for multifunctional and dynamical trapping and manipulation of particles with a lab-on-a-chip system.

Spin-to-orbital angular momentum conversion through a coplanar converter

Spin angular momentum (SAM) and orbital angular momentum (OAM), associated with the circular polarization and helical phase front of an electromagnetic wave, respectively, have been gaining increasing interest in various fields. In this paper, a new method of SAM to OAM conversion in the microwave band is proposed. The designed coplanar structure converses the SAM to OAM, whose three electric components, , , and , carry same-order OAM. The paper explains the principle of SAM to OAM conversion and provides an experimental demonstration. Both simulation and measurement results demonstrate that this conversion method from SAM to OAM is feasible. Our work could provide a promising platform to study photonic spin-orbital interaction and find applications in the near-field vortex generators.

Circularly Polarized Light Can Override and Amplify Asymmetry in Supramolecular Helices.

Circularly polarized light (CPL) is an inherently chiral entity and is considered one of the possible deterministic signals that led to the evolution of homochirality. While accumulating examples indicate that chirality beyond the molecular level can be induced by CPL, not much is yet known about circumstances where the spin angular momentum of light competes with existing molecular chiral information during the chirality induction and amplification processes. Here we present a light-triggered supramolecular polymerization system where chiral information can both be transmitted and nonlinearly amplified in a "sergeants-and-soldiers" manner. While matching handedness with CPL resulted in further amplification, we determined that opposite handedness could override molecular information at the supramolecular level when the enantiomeric excess was low. The presence of a critical chiral bias suggests a bifurcation point in the homochirality evolution under random external chiral perturbation. Our results also highlight opportunities for the orthogonal control of supramolecular chirality decoupled from molecular chirality preexisting in the system.

Circular Polarization Conversion in Single Plasmonic Spherical Particles

Temporal and spectral behaviors of plasmons determine their ability to enhance the characteristics of metamaterials tailored to a wide range of applications, including electric-field enhancement, hot-electron injection, sensing, as well as polarization and angular momentum manipulation. We report a dark-field (DF) polarimetry experiment on single particles with incident circularly polarized light in which gold nanoparticles scatter with opposite handedness at visible wavelengths. Remarkably, for silvered nanoporous silica microparticles, the handedness conversion occurs at longer visible wavelengths, only after adsorption of molecules on the silver. Finite element analysis (FEA) allows matching the circular polarization (CP) conversion to dominant quadrupolar contributions, determined by the specimen size and complex susceptibility. We hypothesize that the damping accompanying the adsorption of molecules on the nanostructured silver facilitates the CP conversion. These results offer new perspectives in molecule sensing and materials tunability for light polarization conversion and control of light spin angular momentum at submicroscopic scale.

Understanding of transverse spin angular momentum in tightly focused linearly polarized vortex beams.

Spin angular momentum (SAM) is widely used in spin-dependent unidirectional optical interfaces, optical manipulation, integrated optical signal processing, laser structuring and other fields, but its physical mechanism has not been fully understood so far. In this work, we investigate the three-dimensional (3D) SAM in tightly focused x-polarized first-order vortex beams from the perspectives of light field itself, phase distribution, and focusing propagation. It is shown that the distribution of three orthogonal components of SAM at the focal plane has pseudo two-fold rotational symmetry, because the cycloidal rotation of the electric field of the tightly focused vortex beam is opposite. The 3D SAM distribution in the focal region is visualized by mapping the 3D distribution of state of polarization (SoP). In addition, a principle experimental method for identifying the transverse SAM by using the direction of particle's rotation axis in optical tweezers is proposed.

Quark spin-orbit correlations in the pion in a light-cone quark model

We study the correlation between the quark spin and orbital angular momentum inside the pion meson. Similar to the case inside the nucleon, the longitudinal spin-orbit correlation ${C}_{z}^{q/\ensuremath{\pi}}$ in the pion meson can be expressed in terms of the corresponding generalized parton distributions (GPDs) and generalized transverse momentum distributions (GTMDs). This provides new information about the spin structure of the pion. Using the wave functions of the pion in the light-cone quark model and the overlap representation for GPDs and GTMDs, we present the analytical results for the quark longitudinal spin-orbit correlation. We find that the GPD approach and the GTMD approach lead to the same results. The numerical results are also obtained, showing that the correlation in the pion is antialigned. In addition, we compare ${C}_{z}^{q/\ensuremath{\pi}}$ from the GPD approach and the GTMD approach, with $x$ and the transverse momentum ${k}_{T}$ unintegrated.

The Effects of Tilt on the Time Variability of Millimeter and Infrared Emission from Sagittarius A*

Abstract Using a combination of general-relativistic magnetohydrodynamics simulations and ray tracing of synchrotron emission, we study the effect of modest (24°) misalignment between the black hole spin and plasma angular momentum, focusing on the variability of total flux, image centroids, and image sizes. We consider both millimeter and infrared (IR) observables motivated by Sagittarius A* (Sgr A*), though our results apply more generally to optically thin flows. For most quantities, tilted accretion is more variable, primarily due to a significantly hotter and denser coronal region well off the disk midplane. We find (1) a 150% increase in millimeter light-curve variability when adding tilt to the flow; (2) the tilted image centroid in the millimeter shifts on a scale of 3.7 μas over 28 hr (5000 gravitational times) for some electron temperature models; (3) tilted disk image diameters in the millimeter can be 10% larger (52 versus 47 μas) than those of aligned disks at certain viewing angles; (4) the tilted models produce significant IR flux, similar to that seen in Sgr A*, with comparable or even greater variability than observed; and (5) for some electron models, the tilted IR centroid moves by more than 50 μas over several hours, in a similar fashion to the centroid motion detected by the GRAVITY interferometer.

Subdiffraction Focusing Metalens Based on the Depletion of Bessel Beams

Superresolution microscopy is of great interests in modern optics and photonics applications. Subdiffraction focusing lens, that has superresolution focal spot with local spatial frequency greater than the maximum Fourier component, has attracted intensive attentions in past decade, because of the significant capability of unlabeled far-field superresolution. Here, we propose a dielectric subdiffraction focusing metalens based on the depletion of Bessel beams. Different from traditional binary wavefront modulation elements, this metalens is designed according to the reverse engineering of subdiffraction focal field, which is constructed from the depletion of a fundamental Bessel beam. With the independent amplitude and phase control ability of dielectric metasurface, this metalens enables breaking the diffraction limit and can produce a focal spot with full width at half maximum of 0.3<i>&#x03BB;</i>&#x002F;NA. This work may provide a platform for the design of integrated and compact photonic devices to control the light propagation dynamics and spin angular momentum associated with circular polarization.

Verification of Kochen-Specker-type quantum contextuality with a single photon

Contextuality provides one of the fundamental characterizations of quantum phenomena, and can be used as a resource in lots of quantum information processing. In this paper, we summarize and derive some equivalent noncontextual inequalities from different noncontextual models of the proofs for the Kochen-Specker theorem based on Greenberger-Horne-Zeilinger states. These noncontextual inequalities are equivalent up to some correlation items which hold both for noncontextual hidden variable theories and quantum mechanics. Therefore, using single-photon hyperentangled Greenberger-Horne-Zeilinger states encoded by spin, path, and orbital angular momentum, we experimentally verify several state-dependent noncontextual models of the proofs for the Kochen-Specker theorem by testing an extreme simplest Mermin-like inequality.

Generation of topologically complex three-dimensional electron beams in a plasma photocathode

Laser-triggered ionization injection is a promising way of generating controllable high-quality electrons in plasma-based acceleration. We show that ionization injection of electrons into a fully nonlinear plasma wave wake using a laser pulse comprising of one or more Laguerre-Gaussian modes with combinations of spin and orbital angular momentum can generate exotic three-dimensional spatial distributions of high-quality relativistic electrons. The phase dependent residual momenta and initial positions of the ionized electrons are encoded into their final phase space distributions, leading to complex spatiotemporal structures. The structures are formed as a result of the transverse (betatron) and longitudinal (phase slippage and energy gain) dynamics of the electrons in the wake immediately after the electrons are injected. Theoretical analysis and three-dimensional simulations verify this mapping process leads to the generation of these complex topological beams. These beams may trigger novel beam-plasma interactions as well as produce coherent radiation with orbital angular momentum when sent through a resonant undulator.

Ultrafast Spin‐to‐Orbit and Orbit‐to‐Local‐Spin Conversions of Tightly Focused Hybridly Polarized Light Pulses

Spin–orbit interaction (SOI) has provided a new viable roadmap for the development of spin‐based photonics devices. However, existing strategies to control the SOI focus commonly on tailoring the spatial dimension of light fields yet neglecting the inherent temporal one. Herein, ultrafast temporal effects on both spin‐to‐orbit and orbit‐to‐local‐spin conversions based on the time‐assistant vectorial diffractive theory and the fast Fourier transformation are first presented. Such interconversions depend upon whether the incident hybridly vectorial light pulse carries vortex phase or not in a single high numerical aperture geometry. For the case of the absence of vortex phase, it is found that it enables orbit angular momentum‐carrying transverse component fields, and the resultant orbit angular momentum embedded within focused light fields remains constantly revolving as time elapses, which indicates that the controllable spin‐to‐orbit conversion occurs. By contrast, it is revealed that hybridly polarized vectorial‐vortex light pulses allow access to the locally excited circular polarization at the focus, and this induced local circular polarization is independent of ultrafast varying time, whereas the resulting spin angular momentum density components experience the alternation between the appearance and annihilation over time, thus giving rise to the tunable orbit‐to‐local‐spin conversion. These exotic ultrafast interconversions not only breathe a new life into the area of ultrafast photonics, but refresh our understanding on the paradigm of photonic SOI.

Spin-affected reflexive and stretching separation of off-center droplet collision

A volume-of-fluid (VOF) based numerical study on the spin-affected droplet separation of off-center collisions is presented. The roles of orbital angular momentum ${L}_{o}$ and spin angular momentum ${L}_{s}$ in affecting the droplet separation are illustrated by changing the chirality of droplet spin, showing that an augmented total angular momentum can promote the stretching separation and suppress the reflexive separation. A unified theoretical model based on conservation laws is proposed to qualitatively describe the boundaries of the coalescence-separation transition influenced by droplet self-spin motion.

Generalized approach to plasmonic phase modulation in topological bi-gratings

We study the role of optical momenta in surface plasmon excitation by topological bi-gratings. We analyze plasmonic structures composed of rectangular apertures rotating in 2D space with given rates, which couple to specific plasmon trajectories. We experimentally demonstrate the control of plasmonic propagation through premeditated design of the gratings, which interact with linear, spin, and orbital angular momentum of light. We show that these trajectories can be selected purely through momenta modulation with numerous degrees of freedom for a given structure geometry. Moreover, it is shown that the symmetry of the final plasmonic distribution is defined by selecting specific rotation rates, and that it can be different from the original grating symmetry.

The Cosmic Microwave Background Radiation as Viewed in the Planck Vacuum Theory

&#x0D; &#x0D; &#x0D; The invisible Planck vacuum (PV) state is a degenerate collection of Planck particle (PP) cores that fills a sevendimensional spacetime. The degenerate nature of this PV state assures that the only motion available to the separate PP cores is a spin angular momentum and a local random PP motion. This spherical confinement of each PP core in the vacuum state leads to the cosmic-microwave-background-radiation (CMBR) spectrum seen in the free-space night sky.The CMBR is the experimental proof for the PV theory.&#x0D; &#x0D; &#x0D;

Photon spin angular momentum driven magnetization dynamics in ferromagnet/heavy metal bilayers

Thin-film magnetization controlled by optical helicity has been recently reported. Although circularly polarized light has spin angular momentum, helicity-dependent all-optical magnetization switching is mediated by the stochastic thermal process, such as magnetic circular dichroism, and the effect of photon spin angular momentum is considered to be a secondary role. Conversely, the inverse Faraday effect in ferromagnetic thin films and photon spin angular momentum injection into heavy metal thin films have been observed, which can induce torque on metallic thin-film magnets. In this study, we show photon spin angular momentum driven magnetization dynamics in bilayers of Co/(Pt, Au) thin films with various thicknesses. The heavy metal Pt, Au, and ferromagnetic Co layer thickness dependencies of photon spin angular momentum driven torques are discussed in terms of field-like torque owing to the inverse Faraday effect and spin-transfer torque caused by photon spin angular momentum injection into the heavy metal layer with details of optical and magnetic properties. This study provides a better understanding of photon spin angular momentum induced magnetization dynamics in metallic thin-film heterostructures for efficient photon-driven magnetization manipulation.

Extreme-ultraviolet vector-vortex beams from high harmonic generation

Structured light in the short-wavelength regime opens exciting avenues for the study of ultrafast spin and electronic dynamics. Here, we demonstrate theoretically and experimentally the generation of vector-vortex beams (VVB) in the extreme ultraviolet through high-order harmonic generation (HHG). The up-conversion of VVB, which are spatially tailored in their spin and orbital angular momentum, is ruled by the conservation of the topological Pancharatnam charge in HHG. Despite the complex propagation of the driving beam, high-harmonic VVB are robustly generated with smooth propagation properties. Remarkably, we find out that the conversion efficiency of high-harmonic VVB increases with the driving topological charge. Our work opens the possibility to synthesize attosecond helical structures with spatially varying polarization, a unique tool to probe spatiotemporal dynamics in inhomogeneous media or polarization-dependent systems.

Inverse Faraday effect from the orbital angular momentum of light

It is usually admitted that the inverse Faraday effect (IFE) originates from the spin angular momentum (SAM) of light. In this paper, we show that part of the IFE in a metal is induced by the orbital angular momentum (OAM) of light. On the basis of a hydrodynamic model of the conduction electron gas, we describe the dependence of the IFE on the spin and orbital angular momenta as well as spin-orbit interaction in a nonparaxial light beam. We also numerically quantify the relative contributions of the SAM and OAM of light to the IFE in a thin gold film illuminated by different focused beams carrying SAM and/or OAM. The OAM of light provides an additional degree of freedom in the control of the IFE and resulting optomagnetic field, thus potentially impacting various research fields, including all-optical magnetization switching and spin-wave excitation.