Carrying Spin Angular Momentum Research Articles

Rapid modulation of left- and right-handed optical vortices for precise measurements of helical dichroism.

Helical dichroism (HD), which is defined as the difference in optical absorption between chiral pairs of lights involving left-handed (LH) and right-handed (RH) optical vortices (OVs) carrying orbital angular momentum (OAM), is a promising way to characterize chiral materials. In the current major methods of OV generation using spatial light modulators (SLMs), the speed of OAM switching is typically as slow as 100Hz, which is comparable to low-frequency noise, making precise chiral detection difficult. Here, we theoretically propose and experimentally demonstrate a rapid modulation of the LH and RH OVs at around 50kHz. This modulation is achieved through a rapid modulation of circularly polarized lights carrying spin angular momentum (SAM), combined with a SAM-OAM conversion technique. We establish a theory not only for rapid OV modulation but also for HD measurements using the modulated OVs. We experimentally verify the theory using helical phase holograms drawn on a SLM as a pseudo-HD active sample. Our work addresses the limitations of current methods and offers a new avenue for precise HD measurements, paving the way for the development of sensitive chiral-optical spectroscopy techniques.

Optical dipole potential energy of a two-level atom interacting with a tightly focused truncated optical Bessel beam

Truncated optical Bessel (TOB) beams are an orthonormal set of optical vortex beams that possess orbital angular momentum (OAM). It is characterized by a significant longitudinal electric field component at the sub-wavelength scales. The photons of a circularly polarized TOB beam also carry spin angular momentum (SAM) and can interact with a two-level atom, in the far-off resonance regime, giving rise to an optical dipole potential energy that involves a spin–orbit term that stems from the inclusion of the longitudinal field component. When a two-level atom is simultaneously irradiated by two TOB beams, the optical dipole potential energy, in some special cases, is entirely dependent on the OAM–SAM coupling. We explore the cold-atom trapping options enabled by the use of TOB beams, either single ones or combinations thereof. We put emphasis on case in which the OAM–SAM coupling is the dominant contribution. The similarities and differences between the results obtained for the TOB beams and those for the Laguerre–Gaussian beams are also discussed.

Alignment‐Free Angular Momentum Detection via Spin‐Independent Astigmatic Transformation

AbstractLight beam carrying spin angular momentum (SAM) and orbital angular momentum (OAM) have attracted great interest due to their great potential for high‐capacity optical communication, optical tweezers, and quantum information. However, the current detection methods suffer from the lack of robustness to misalignments in the optical system, which limits the detection accuracy and the application for off‐axis vortex array detection. Here, a high‐efficiency, broadband, and single‐layer metasurface design is proposed for alignment‐free angular momentum detection via spin‐independent astigmatic transformation (SIAT). The SAM and OAM states can be recognized from the diffraction direction and patterns, respectively. Proof‐of‐concept demonstrations are experimentally carried out on different incident beams including circular‐polarization, linear‐polarization, and vector vortex beams, as well as vortex beam arrays, showing a high diffraction efficiency of ≈81% at 1064 nm while operating within the broadband region of 900–1300 nm. The alignment‐free merit for the incident position, incident angle, and detector distance grants the device great potential for integrated quantum systems and multiparticle micromanipulation with optical tweezers.

Poincaré Beams for Magnetic Field Sensing

Poincaré beam (PB), carrying spin angular momentum (SAM) and orbital angular momentum (OAM), plays a significant role in a wide range of optical applications. The inhomogeneous polarization distribution of PB can be represented as a point on the surface of hybrid-order Poincaré sphere (HyOPS). Here, we have investigated the properties of PB propagation in a magnetic fluid under an applied magnetic field. The PB consists of a Gaussian and a vortex beam, which are in orthogonally linear polarization. The Stokes parameters of PB are measured in the experiment to build the Stokes phase profile. Besides, one of the phase contours is marked to analyze the changes of the phase profile. The results show that the phase contours rotate, and the rotation angle is well linearly to magnetic field strength with sensitivity of 0.934°/mT in the range of 2.42 to 27.79 mT, which is consistent with the expectation of the theoretical formula. Our results may contribute to the effort of exploring potential optical sensing at the onset of PB.

Spin-locked scattering of unpolarized light by magnetoelectric coupling

Unpolarized light is ubiquitous in nature. It does not carry spin angular momentum but it can be converted to partially polarized light with spin angular momentum after scattering. In this work, we demonstrate that the unpolarized light not only possesses a definite spin state after scattering but also induces lateral separation of scattered light with opposite chiralities. Such spin-locked scattering of unpolarized light stems from the inherent property of magnetoelectric coupling of the scatterer. Moreover, the spin-locked scattering for arbitrary linearly polarized light is also investigated and compared with the case of unpolarized light. Our findings can broaden the application range of unpolarized light and open other avenues for spin-related phenomena induced by unpolarized light.

Spin Hall effect of transversely spinning light.

Light carries spin angular momentum, which, in the free space, is aligned to the direction of propagation and leads to intriguing spin Hall phenomena at an interface. Recently, it was shown that a transverse-spin (T-spin) state could exist for surface waves at an interface or for bulk waves inside a judiciously engineered metamaterial, with the spin oriented perpendicular to the propagation direction. Here, we demonstrate the spin Hall effect for transversely spinning light—a T-spin–induced beam shift at the interface of a metamaterial. It is found that the beam shift takes place in the plane of incidence, in contrast to the well-known Imbert-Fedorov shifts. The observed T-spin–induced beam shift is of geometrodynamical nature, which can be rendered positive or negative controlled by the orientation of T-spin of the photons. The unconventional spin Hall effect of light found here provides a previously unexplored mechanism for manipulating light-matter interactions at interfaces.

Optical manipulation of optical vortex beam on gold nanoparticle

ABSTRACT The optomechanical manipulation of an optical vortex beam (Bessel beam) upon a single gold nanoparticle (GNP) was studied theoretically. This structured light carries spin angular momentum (SAM) and orbital angular momentum (OAM) simultaneously via the spin-orbit interaction (SOI) of light. Due to the plasmon-enhanced light absorption and scattering of GNP, the SAM and OAM of photons are transferred to GNP, exhibited by the spin and orbital motions of GNP. The optical force and torque upon GNP are analyzed through the surface integrals of Maxwell's stress tensor. We found that there are specific stable circular orbits of GNP through the simulation of GNP's equation of motion; the revolution direction depends on the sign of the order of Bessel beam. If the order of Bessel beam is a positive integer, the GNP performs an orbital motion with a counterclockwise revolution. On the contrary, for a Bessel beam of a negative order, a clockwise revolution of GNP is induced. Additionally, the spin direction of GNP depends on the handedness of light: right-hand or left-hand circular polarization.

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.

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.

Controlling the polarization of chiral dipolar emission with a spherical dielectric nanoantenna.

Circularly polarized light (CPL) carrying spin angular momentum is crucial to many applications, such as quantum computing, optical communication, novel displays, and biosensing. Nonetheless, the emission from chiral molecules contains comparable CPL components with opposite handedness, resulting in low levels of CPL overall with a small dissymmetry factor and fixed handedness consistent with the handedness of the molecules. Nanoantennas have proved to be useful tools for controlling the emission properties of quantum emitters. In particular, dielectric resonators support electric and magnetic modes, which implies unparalleled opportunities to interact with chiral molecules whose emission originates from both electric and magnetic dipole transitions. In this work, we theoretically study the effects of a spherical dielectric nanoantenna on the directionality and polarization of emission from a chiral molecule. With exact analytical solutions based on generalized Mie theory, we show that directional chiral light emission and nontrivial polarization modulation, such as handedness reversal or chirality enhancement, can be achieved simultaneously for a chiral dipole tangentially coupled with a silicon nanosphere. The influence of the relative strength and orientation between the electric and magnetic dipole moments is also discussed. Our results suggest a new approach to controlling chiral dipolar emission and could benefit the development of chiral light sources.

Multiple Longitudinal Polarization Vortices Generated via All‐Silicon Metasurface

AbstractPhotons carry spin angular momentum and orbital angular momentum, vortex beams with helical phase fronts provide a new understanding of optical and physical phenomena. Here, the creation of four longitudinal polarized vortices with arbitrary topological charges is demonstrated using all‐silicon metasurfaces in terahertz band, integrating propagation phase and Pancharatnam–Berry phase. This efficient scheme allows the authors to flexibly control the longitudinal polarized electric field by designing the field distribution of the transverse polarization. As a proof of concept, two metasurfaces are designed to demonstrate the flexibility of such approach. One is used to generate dual longitudinal polarized superpositions of orbital angular momentum states with arbitrary topological charges, and the other is designed to generate four discrete longitudinal vortices. This method may find applications in manipulation of electron bunches and new imaging technology.

Towards New Chiroptical Transitions Based on Thought Experiments and Hypothesis

We studied supramolecular chirality induced by circularly polarized light. Photoresponsive azopolymers form a helical intermolecular network. Furthermore, studies on photochemical materials using optical vortex light will also attract attention in the future. In contrast to circularly polarized light carrying spin angular momentum, an optical vortex with a spiral wave front and carrying orbital angular momentum may impart torque upon irradiated materials. In this review, we summarize a few examples, and then theoretically and computationally deduce the differences in spin angular momentum and orbital angular momentum depending on molecular orientation not on, but in, polymer films. UV-vis absorption and circular dichroism (CD) spectra are consequences of electric dipole transition and magnetic dipole transition, respectively. However, the basic effect of vortex light is postulated to originate from quadrupole transition. Therefore, we explored the simulated CD spectra of azo dyes with the aid of conventional density functional theory (DFT) calculations and preliminary theoretical discussions of the transition of CD. Either linearly or circularly polarized UV light causes the trans–cis photoisomerization of azo dyes, leading to anisotropic and/or helically organized methyl orange, respectively, which may be detectable by CD spectroscopy after some technical treatments. Our preliminary theoretical results may be useful for future experiments on the irradiation of UV light under vortex.

Broadband generation of perfect Poincar\xe9 beams via dielectric spin-multiplexed metasurface

The term Poincaré beam, which describes the space-variant polarization of a light beam carrying spin angular momentum (SAM) and orbital angular momentum (OAM), plays an important role in various optical applications. Since the radius of a Poincaré beam conventionally depends on the topological charge number, it is difficult to generate a stable and high-quality Poincaré beam by two optical vortices with different topological charge numbers, as the Poincaré beam formed in this way collapses upon propagation. Here, based on an all-dielectric metasurface platform, we experimentally demonstrate broadband generation of a generalized perfect Poincaré beam (PPB), whose radius is independent of the topological charge number. By utilizing a phase-only modulation approach, a single-layer spin-multiplexed metasurface is shown to achieve all the states of PPBs on the hybrid-order Poincaré Sphere for visible light. Furthermore, as a proof-of-concept demonstration, a metasurface encoding multidimensional SAM and OAM states in the parallel channels of elliptical and circular PPBs is implemented for optical information encryption. We envision that this work will provide a compact and efficient platform for generation of PPBs for visible light, and may promote their applications in optical communications, information encryption, optical data storage and quantum information sciences.

Achieving Broadband Spin‐Correlated Asymmetric Reflection Using a Circular Dichroitic Meta‐Mirror

AbstractGuiding electromagnetic waves carrying spin angular momentum is an important issue in both optical and microwave communities that thus has intrigued enormous interest in the past few decades. In particular, asymmetrical propagation is seriously considered as a possibility to push the development of encryption communication and electromagnetic circulators, which is a hot area but still a challenge. Circular dichroism, referring to differential absorption of distinct polarization states, provides an alternative scheme to achieve asymmetric propagation in a spin‐correlated way. Here, a paradigm of obtaining broadband asymmetric reflection to push the exploration in this aspect is proposed. With this aim, an N‐shaped resonator loaded with two lumped resistors is designed as a meta‐atom to reveal the amplitude flipping of nearly 0 and 1 for distinct polarization states. Combining with Pancharatnam–Berry phases, the arbitrary phase profile of a designated spin state can be achieved. As a proof‐of‐principle, a robust meta‐mirror is fabricated to acquire broadband electromagnetic focusing under the illumination of left‐handed circularly polarized (LHCP) wave. On the other hand, the reflection of right‐handed circularly polarized (RHCP) wave is substantially suppressed. Both numerical simulation and experimental measurement are carried out to demonstrate the feasibility of this system.

Broadband detection of multiple spin and orbital angular momenta via dielectric metasurface.

Light beams carrying spin angular momentum (SAM) and orbital angular momentum (OAM) have created novel opportunities in the areas of optical communications, imaging, micromanipulation and quantum optics. However, complex optical setups are required to simultaneously manipulate, measure and analyze these states, which significantly limits system integration. Here, we introduce a novel detection approach for measuring multiple SAM and OAM modes simultaneously through a planar nanophotonic demultiplexer based on an all-dielectric metasurface. Coaxial light beams carrying multiple SAM and OAM states of light upon transmission through the demultiplexer are spatially separated into a range of vortex beams with different topological charge, each propagating along a specific wavevector. The broadband response, material dispersion and momentum conservation further enable the demultiplexer to achieve wavelength demultiplexing. We envision the ultracompact multifunctional architecture to enable simultaneous manipulation and measurement of polarization and spin encoded photon states with applications in integrated quantum optics and optical communications.

Metasurface-Enabled Generation of Circularly Polarized Single Photons.

Single photons carrying spin angular momentum (SAM), i.e., circularly polarized single photons generated typically by subjecting a quantum emitter (QE) to a strong magnetic field at low temperatures, are at the core of chiral quantum optics enabling nonreciprocal single-photon configurations and deterministic spin-photon interfaces. Here, a conceptually new approach to the room-temperature generation of SAM-coded single photons (SSPs) is described, which entails QE nonradiative coupling to surface plasmons being transformed, by interacting with an optical metasurface, into a collimated stream of SSPs with the designed handedness. Design, fabrication, and characterization of SSP sources, consisting of dielectric circular nanoridges with azimuthally varying widths deterministically fabricated on a dielectric-protected silver film around a nanodiamond containing a nitrogen-vacancy center, are reported. With properly engineered phases of QE-originated fields scattered by nanoridges, the outcoupled photons are characterized by a well-defined SAM (with the chirality >0.8) and high directionality (collection efficiency up to 92%).

Magnetic Logic Gate Based on Polarized Spin Waves

Spin wave, the precession of magnetic order in magnetic materials, is a collective excitation that carries spin angular momentum. Similar to the acoustic or optical waves, the spin wave also possesses the polarization degree of freedom. Although such polarization degrees of freedom are frozen in ferromagnets, they are fully unlocked in antiferromagnets or ferrimagnets. Here we introduce the concept of magnetic gating and demonstrate a spin wave analog of the Datta-Das spin transistor in antiferromagnet. Utilizing the interplay between polarized spin wave and the antiferromagnetic domain walls, we propose a universal logic gate of pure magnetic nature, which realizes all Boolean operations in one single magnetic structure. We further construct a full functional 4-bit Arithmetic Logic Unit using only sixteen spin wave universal logic gates, operating in a weaving fashion as a Jacquard loom machine. The spin wave-based architecture proposed here also sets a model for the future energy efficient non-volatile computing, the distributed processing-in-memory computing, and the evolvable neuromorphic computing.

Magnetization switching by magnon-mediated spin torque through an antiferromagnetic insulator.

Widespread applications of magnetic devices require an efficient means to manipulate the local magnetization. One mechanism is the electrical spin-transfer torque associated with electron-mediated spin currents; however, this suffers from substantial energy dissipation caused by Joule heating. We experimentally demonstrated an alternative approach based on magnon currents and achieved magnon-torque-induced magnetization switching in Bi2Se3/antiferromagnetic insulator NiO/ferromagnet devices at room temperature. The magnon currents carry spin angular momentum efficiently without involving moving electrons through a 25-nanometer-thick NiO layer. The magnon torque is sufficient to control the magnetization, which is comparable with previously observed electrical spin torque ratios. This research, which is relevant to the energy-efficient control of spintronic devices, will invigorate magnon-based memory and logic devices.

Cascaded rotational Doppler effect.

We propose and substantiate experimentally the cascaded rotational Doppler effect for interactions of spinning objects with light carrying spin angular momentum. Based on mirror symmetry for electromagnetic interactions, we reveal that the frequency shift can be doubled through cascading two rotational Doppler processes that are mirror-imaged to each other. This effect is further experimentally verified with a rotating half-wave plate, and the mirror-imaging process is achieved by reflecting the frequency-shifted circularly polarized wave upon a mirror with a quarter-wave plate in front of it. The mirror-symmetry-protected frequency shift enhancement could be applied for detection of rotating systems ranging from molecules to macroscopic bodies with high precision and sensitivity.

Polarization properties of fiber-based orbital angular momentum modes

Abstract Generally, orbital angular momentum (OAM) can be generated by combining two vector modes (VMs) or combining two linearly polarized modes (LPMs). The vortices generated by the first method are circularly polarized and carry spin angular momentum (SAM). While, the vortices generated by the second method are linearly polarized with no SAM. Although these two methods are closely related, they are essentially different. The difference and relation between the two methods are theoretically clarified in this paper. Two different kinds of OAM modes are experimentally demonstrated and validated in a two-mode fiber.