Spin Angular Momentum Of Light Research Articles

Helicity-Sensitive Terahertz Detection in Monolayer 1T'-WTe2.

Valleytronics, identified as electronic properties of the energy band extrema in momentum space, has been intensively revived following the emergence of two-dimensional transition metal dichalcogenides (TMDCs) as their valley information can be controlled and probed through the spin angular momentum of light. Previous optical investigations of valleytronics have been limited to the visible/near-infrared spectral regime through which the carriers of most TMDCs can be excited. Monolayer 1T'-WTe2 with broken time-reversal symmetry provides a fertile platform to study the long-wavelength photonic properties in different valleys. Here, we employed a circularly polarized terahertz (THz) laser to selectively excite the valley of monolayer 1T'-WTe2 and demonstrate that the helicity-dependent photoresponse is generated via the photogalvanic effect (PGE). We also observed that the photocurrent is controlled by circular polarization and the external electric field. Because of the tunable Berry curvature dipole derived from the nontrivial wave functions near the inverted gap edge in monolayer WTe2, the bandgap can be tuned efficiently. Our results provide a versatile venue for controlling, detecting, and processing valleytronics and applications in on-chip THz imaging and quantum information processing.

Multi-band spin-selective transmission and reflection in bi-layer T-shaped metamaterial

The control of the spin angular momentum of light plays an irreplaceable role in modern photon applications. In this work, we proposed a chiral metamaterial based on a bi-layer T-shaped structure. When circularly polarized light is incident along the normal direction (-z), the spin-selective transmissions and reflections occur in the mid-infrared frequency, which results in different transmittance and reflectance in quad-band frequencies. Here, the difference in transmittance (ΔT) values are 0.5, 0.4, 0.5, and 0.7, respectively. And the difference in reflectance (ΔR) reaches up to 0.4, 0.3, 0.4, and 0.5, respectively. Due to the chirality of the structure, the resulting circular dichroism (CD) is close to 0.2. In addition, the frequency and amplitude of the quad-band spin selections can be easily manipulated by rotating the rotation angle θ. Finally, we found that the similar spin-selective transmission and reflection can be achieved in Gigahertz (GHz) and Terahertz (THz) wavebands. The metamaterial might promote the application and development of versatile photonic devices.

Insights into propagating surface plasmons in Ag–Cu alloy thin films: Enhancement of spin angular momentum of light

Surface plasmon polaritons (SPPs) can be supported by metal–dielectric interfaces and have been exploited for various applications. Typically, most studies deal with plasmons excited in pure metallic films or homogenous alloy thin films and the understanding of plasmon behavior in films with complex microstructures is limited. In this work, we numerically study the surface plasmons that are supported at the interface of an Ag–Cu alloy film that undergoes spinodal decomposition to produce a two-phase microstructure, when an initially compositionally homogenous alloy film (with composition within spinodal limits) is processed within the miscibility gap. We use phase-field simulated spinodally decomposed microstructures for our optical simulations to study the effect of microstructure on propagating surface plasmons in Ag–Cu alloy films. We demonstrate that the far-field response is governed principally by the composition of the alloy film and is not affected by the microstructural feature size. On the contrary, near-fields are strongly dependent on the microstructure and composition of the films. The origin of inhomogenous fields is demonstrated to be the result of constructive and destructive interference of SPPs. Finally, we demonstrate the enhancement of both transverse and longitudinal components of spin angular momentum in these phase-separated alloy films. The longitudinal components can be enhanced by more than a hundred times in the alloy films as compared to the pure metal films. This study paves the way for exploiting multi-phase alloy thin films for applications in sensing, nanomanipulation, and light modulation.

An opto-thermal approach for rotating a trapped core-shell magnetic microparticle with patchy shell.

The ability to trap and rotate magnetic particles has important applications in biophysical research and optical micromachines. However, it is difficult to achieve the spin rotation of magnetic particles with optical tweezers due to the limit in transferring spin angular momentum of light. Here, we propose a method to obtain controlled spin rotation of a magnetic microparticle by the phoretic torque, which is originated from inhomogeneous heating of the microparticle's surface. The microparticle is trapped and rotated nearby the laser focus center. The rotation frequency is several Hertz and can be controlled by adjusting the laser power. Our work provides a method to the study of optical rotation of microscopic magnetic particles, which will push toward both translational and rotational manipulation of the microparticles simultaneously in a single optical trap.

Strong Angular-Momentum Optomechanical Coupling for Macroscopic Quantum Control

Optomechanical systems offer unique opportunities to explore macroscopic quantum state and related fundamental problems in quantum mechanics. Here, we propose a quantum optomechanical system involving exchange interaction between spin angular momentum of light and a torsional oscillator. We demonstrate that this system allows coherent control of the torsional quantum state of a torsional oscillator on the single-photon level, which facilitates efficient cooling and squeezing of the torsional oscillator. Furthermore, the torsional oscillator with a macroscopic length scale can be prepared in Schr\"odinger catlike state. Our work provides a platform to verify the validity of quantum mechanics in macroscopic systems on the micrometer and even centimeter scale.

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.

Highly Selective High‐Speed Circularly Polarized Photodiodes Based on π‐Conjugated Polymers

AbstractChiral π‐conjugated molecular systems that are intrinsically sensitive to the handedness of circularly polarized (CP) light potentially allow for miniaturized, low‐cost CP detection devices. Such devices promise to transform several technologies, including biosensing, quantum optics, and communication of data encrypted by exploiting the spin angular momentum of light. Here a simple, bilayer organic photodiode (CP OPD) comprising an achiral π‐conjugated polymer–chiral additive blend as the electron donor layer and an achiral C60 electron acceptor layer is realized. These devices exhibit considerable photocurrent dissymmetry gph, with absolute values as high as 0.85 and dark currents as low as 10 pA. Impressively, they showcase a linear dynamic range of 80 dB, and rise and fall times of ≈7 µs, which significantly outperforms all previously reported CP selective photodetectors. Mechanistically, it is shown that the gph is sensitive to the thickness of both the chiral donor and achiral acceptor layers and that a trade‐off exists between the external quantum efficiency and gph. The fast‐switching speeds of these devices, coupled with their large dynamic range and highly selective response to CP light, opens up the possibility of their direct application in CP sensing and optical communications.

Optical spin sorting chain.

Transverse spin angular momentum of light is a key concept in recent nanophotonics to realize unidirectional light transport in waveguides by spin-momentum locking. Herein we theoretically propose subwavelength nanoparticle chain waveguides that efficiently sort optical spins with engineerable spin density distributions. By arranging high-refractive-index nanospheres or nanodisks of different sizes in a zigzag manner, directional optical spin propagation is realized. The origin of efficient spin transport is revealed by analyzing the dispersion relation and spin angular momentum density distributions, being attributed to guided modes that possess transverse spin angular momenta. In contrast to conventional waveguides, the proposed asymmetric waveguide can spatially separate up- and down-spins and locate one parity inside and the other outside the structure. Moreover, robustness against bending the waveguide and its application as an optical spin sorter are presented. Compared to previous reports on spatial engineering of local spins in photonic crystal waveguides, we achieved miniaturization of the entire footprint down to the subwavelength scale.

Optimal control over high-order-harmonic ellipticity in two-color cross-linearly-polarized laser fields

We investigate the generation of high-order harmonics with arbitrary ellipticity using synthesized two-color (800-nm + 400-nm) cross-linearly-polarized laser fields. It is shown that one can realize control over harmonic ellipticity by finely adjusting the relative phase and the crossing angle of such two-color fields. Through analyzing the time-frequency distributions and the quantum orbits of harmonic radiations, we show that the harmonic helicity is originated from the unsymmetrical contribution between different quantum orbits released in one 800-nm cycle. And, we further reveal that the controllability of harmonic ellipticity in such two-color fields is closely related to the modulation of crossing angle and relative phase on the driving light's spin angular momentum. Finally, we show the probability of producing and modulating isolated helical attosecond pulses with few-cycle two-color cross-linearly-polarized laser fields.

Light Spin Angular Momentum Spatial Mode Converter Based on Dielectric Metasurface

In recent years, the concept of metasurface has brought impact on the way of manipulating light using conventional wavefront shaping components by both reducing the device size and increasing the design flexibility. In this work, a spatial mode converter based on dielectric metasurfaces for spin angular momentum is proposed. The structure parameters of dielectric nanobricks are designed by spatial phase distribution derivation based on the geometric phase and the propagation phase. It is shown that the mode converter can transfer the fundamental spatial mode of spin angular momentum including circular or elliptical polarizations into higher order spatial modes. The generated higher order modes of spin angular momentum contribute a new degree of freedom for light modulation besides the handedness of polarization. The metasurface device can reduce the complexity of the existing mode-division multiplexing systems for spin angular momentum of light, and meanwhile exhibit a high efficiency, which may find applications in optical communication, complex structured beam, and quantum optics.

Gigantic vortical differential scattering as a monochromatic probe for multiscale chiral structures

Spin angular momentum of light is vital to investigate enantiomers characterized by circular dichroism (CD), widely adopted in biology, chemistry, and material science. However, to discriminate chiral materials with multiscale features, CD spectroscopy normally requires wavelength-swept laser sources as well as wavelength-specific optical accessories. Here, we experimentally demonstrate an orbital-angular-momentum-assisted approach to yield chiroptical signals with monochromatic light. The gigantic vortical differential scattering (VDS) of ∼120% is achieved on intrinsically chiral microstructures fabricated by femtosecond laser. The VDS measurements can robustly generate chiroptical properties on microstructures with varying geometric features (e.g., diameters and helical pitches) and detect chiral molecules with high sensitivity. This VDS scheme lays a paradigm-shift pavement toward efficiently chiroptical discrimination of multiscale chiral structures with photonic orbital angular momentum. It simplifies and complements the conventional CD spectroscopy, opening possibilities for measuring weak optical chirality, especially on mesoscale chiral architectures and macromolecules.

Transverse spinning of unpolarized light

It is well known that spin angular momentum of light, and therefore that of photons, is directly related to their circular polarization. Naturally, for totally unpolarized light, polarization is undefined and the spin vanishes. However, for nonparaxial light, the recently discovered transverse spin component, orthogonal to the main propagation direction, is largely independent of the polarization state of the wave. Here we demonstrate, both theoretically and experimentally, that this transverse spin survives even in nonparaxial fields (e.g., tightly focused or evanescent) generated from a totally unpolarized light source. This counterintuitive phenomenon is closely related to the fundamental difference between the degrees of polarization for 2D paraxial and 3D nonparaxial fields. Our results open an avenue for studies of spin-related phenomena and optical manipulation using unpolarized light.

Optical field tuning of localized plasmon modes in Ag microcrystals at the nanofemto scale.

Nanoscale plasmonic field enhancement at sub-wavelength metallic particles is crucial for surface sensitive spectroscopy, ultrafast microscopy, and nanoscale energy transduction. Here, we demonstrate control of the spatial distribution of localized surface plasmon modes at sub-optical-wavelength crystalline silver (Ag) micropyramids grown on a Si(001) surface. We employ multiphoton photoemission electron microscopy (mP-PEEM) to image how the plasmonic field distributions vary with the photon energy, light polarization, and phase in coherent two-pulse excitation. For photon energy hυ > 2.0 eV, the mP-PEEM images show single photoemission locus, which splits into a dipolar pattern that straddles the Ag crystal at a lower energy. We attribute the variation to the migration of plasmon resonances from the Ag/vacuum to the Ag/Si interfaces by choice of the photon energy. Furthermore, the dipolar response of the Ag/Si interface follows the polarization state of light: for linearly polarized excitations, the plasmon dipole follows the in-plane electric field vector, while for circularly polarized excitations, it tilts in the direction of the handedness due to the conversion of spin angular momentum of light into orbital angular momentum of the plasmons excited in the sample. Finally, we show the coherent control of the spatial plasmon distribution by exciting the sample with two identical circularly polarized light pulses with delay defined with attosecond precision. The near field distribution wobbles at the pyramid base as the pump-probe delay is advanced due to interferences among the contributing fields. We illustrate how the frequency, polarization, and pulse structure can be used to design and control plasmon fields on the nanofemto scale for applications in chemistry and physics.

Dual-Helicity Decoupled Coding Metasurface for Independent Spin-to-Orbital Angular Momentum Conversion

Controlling the conversion of light's spin angular momentum to orbital angular momentum (OAM) is crucial for applications, including optical systems and wireless communication. In this regard, metasurfaces are often limited by the difficulty of producing independent spin-to-OAM conversions. This study uses a reflective, dual-helicity decoupled coding metasurface to realize completely independent control of OAM vortices for two orthogonal helicities, achieving a free combination of distinctive OAM topological charges, arbitrary helicity, anomalous scattering, and complex spatial beam editing. These results could help to integrate versatile functionalities for advanced compact systems.

Spin-Selective Transmission in Chiral Folded Metasurfaces.

Controlling the spin angular momentum of light (or circular polarization state) plays a crucial role in the modern photonic applications such as optical communication, circular dichroism spectroscopy, and quantum information processing. However, the conventional approaches to manipulate the spin of light require naturally occurring chiral or birefringent materials of bulky sizes due to the weak light-matter interactions. Here we experimentally demonstrate an approach to implement spin-selective transmission in the infrared region based on chiral folded metasurfaces that are capable of transmitting one spin state of light while largely prohibiting the other. Due to the intrinsic chirality of the folded metasurface, a remarkable circular dichroism as large as 0.7 with the maximum transmittance exceeding 92% is experimentally demonstrated. The giant circular dichroism is interpreted within the framework of charge-current multipole expansion. Moreover, the intrinsic chirality can be readily controlled by manipulating the folding angle of the metasurface with respect to the cardinal plane. Benefiting from its strong chirality and spin-dependent transmission characteristics, the proposed folded metasurface may be applied to a range of novel photon-spin selective devices for optical communication technologies and biophotonics.

GHz Rotation of an Optically Trapped Nanoparticle in Vacuum.

We report on rotating an optically trapped silica nanoparticle in vacuum by transferring spin angular momentum of light to the particle's mechanical angular momentum. At sufficiently low damping, realized at pressures below 10^{-5} mbar, we observe rotation frequencies of single 100nm particles exceeding 1GHz. We find that the steady-state rotation frequency scales linearly with the optical trapping power and inversely with pressure, consistent with theoretical considerations based on conservation of angular momentum. Rapidly changing the polarization of the trapping light allows us to extract the pressure-dependent response time of the particle's rotational degree of freedom.

Parallel sorting of orbital and spin angular momenta of light in a record large number of channels.

Parallel sorting of orbital and spin angular momentum components of structured optical beams is demonstrated. Both spin channels are multiplexed within the novel orbital angular momentum (OAM) sorter, reducing the size, weight, and number of elements. The sorted states are linearly spaced over 70 topological charge values. We experimentally and theoretically evaluate the operational range and crosstalk between neighboring channels and find that 30 orbital angular momentum states are available per spin channel for quantum communication or cryptography. This is achieved using an angular momentum sorter that we developed based on geometric phase optical elements. We present two devices consisting of liquid crystal polymer films photoaligned with complex two-dimensional patterns.

Spin and Geometric Phase Control Four‐Wave Mixing from Metasurfaces

AbstractThe spin angular momentum of light plays an important role in nonlinear interactions in optical systems with rotational symmetries. Here, the existence of the nonlinear geometric Berry phase is demonstrated in the four‐wave mixing process and applied to spin‐controlled nonlinear light generation from plasmonic metasurfaces. The polarization state of four‐wave mixing from the ultrathin metasurfaces, comprising gold meta‐atoms with four‐fold rotational symmetry, can be controlled by manipulating the spin of the excitation beams. The mutual orientation of the meta‐atoms in the metasurface influences the intensity of four‐wave mixing via the geometric phase effects. These findings provide novel solutions for designing metasurfaces for spin‐controlled nonlinear optical processes with inbuilt all‐optical switching.

The spin Hall effect of light in moving medium

In this paper, we investigate the spin Hall effect of light in moving inhomogeneous medium using the Gordon metric and the Maxwell’s equations in the gravitational field. Light experiences a moving medium as a gravitational field by means of the Gordon metric. It is shown that the spin Hall effect of light is modified by the motion of medium, and the deflection of the ray trajectory is dependent on the polarization and the motion of the medium. It is interesting that there is no coupling of the spin angular momentum of light and the effective gravitational field when the medium is moving along the direction of the gradient [Formula: see text]. The results provide a potential method for controlling the spin Hall effect of light in medium.

Harmonic angular Doppler effect

By making use of the spin angular momentum of light, rotational frequency shifts of harmonic waves generated by spinning nonlinear media have been observed.