Opposite Helicity Research Articles

Terahertz pulse generation by two-color laser fields with circular polarization

We study the influence of the polarization states of ionizing femtosecond two-color pulses on the emitted terahertz radiation in gases. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and a (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent of the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated with longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly-polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors.

Helicity‐resolved resonant Raman spectroscopy of layered WS2

AbstractWhen excited by circularly polarized light, the Raman scattered light may have the same or opposite helicity as the incident light determined by the helicity selection rule. For two‐dimensional (2D) transition metal dichalcogonides (TMDCs), the helicity selection rule can be broken down due to the strong Fröhlich exciton–phonon interaction. However, how the helicity selection rule changes with excitation energy has not been reported yet. Here, we study the helicity‐resolved Raman scattering of layered WS2 excited by circularly polarized light for excitations off‐excitonic resonance and near resonance to different excitons. We find that for off‐resonance excitation, the helicity of mode of WS2 obeys the helicity selection rule determined by the symmetry of crystal structure and vibration modes. When excited near resonance to the B exciton, the breakdown of the helicity selection rule is observed, which is attributed to the appearance of nonzero diagonal elements of the Raman tensor resulted from the Fröhlich exciton–phonon interaction. The layer number dependence of the helicity polarization ratio of the mode shows that the proportion of helicity‐conserved component decreases with the increase of layer number. When the excitation energy is near resonance to the A exciton, the different helicity polarization ratio is found and remains unchanged with increasing layer number, which may be attributed to the different coupling strengths of exciton–phonon interactions.

Compact and broad wavelength range tunable orbital angular momentum mode generator based on cascaded helical photonic crystal fibers.

A compact and broad wavelength range tunable orbital angular momentum (OAM) generator was experimentally demonstrated by cascading two helical photonic crystal fibers (HPCFs) with opposite helicity, i.e., clockwise-twisted + anticlockwise-twisted HPCF. Such an OAM generator exhibited a length of approximately 9 mm and generated a high-quality OAM mode with a wavelength range of 35 nm. Moreover, the wavelength range is expected to be tuned from 17.9-51.3 nm by applying mechanical torsion.

Photonic Rashba effect from quantum emitters mediated by a Berry-phase defective photonic crystal.

Heterostructures combining a thin layer of quantum emitters and planar nanostructures enable custom-tailored photoluminescence in an integrated fashion. Here, we demonstrate a photonic Rashba effect from valley excitons in a WSe2 monolayer, which is incorporated into a photonic crystal slab with geometric phase defects, that is, into a Berry-phase defective photonic crystal. This phenomenon of spin-split dispersion in momentum space arises from a coherent geometric phase pickup assisted by the Berry-phase defect mode. The valley excitons effectively interact with the defects for site-controlled excitation, photoluminescence enhancement and spin-dependent manipulation. Specifically, the spin-dependent branches of photoluminescence in momentum space originate from valley excitons with opposite helicities and evidence the valley separation at room temperature. To further demonstrate the versatility of the Berry-phase defective photonic crystals, we use this concept to separate opposite spin states of quantum dot emission. This spin-enabled manipulation of quantum emitters may enable highly efficient metasurfaces for customized planar sources with spin-polarized directional emission.

Helicity-delinked manipulations on surface waves and propagating waves by metasurfaces

Abstract Although many approaches have been proposed to manipulate propagating waves (PWs) and surface waves (SWs), usually each operation needs a separate meta-device, being unfavorable for optical integrations. Here, we propose a scheme to design a single meta-device that can efficiently generate SWs and/or PWs with pre-designed wavefronts, under the excitations of circularly polarized (CP) PWs with different helicity. As a proof of concept, we design and fabricate a microwave meta-device and experimentally demonstrate that it can convert incident CP waves of opposite helicity to SWs possessing different wavefronts and traveling to opposite directions, both exhibiting very high efficiencies. We further generalize our scheme to design a meta-device and numerically demonstrate that it can either excite a SW beam with tailored wavefront or generate a far-field PW with pre-designed wavefront, as shined by CP waves with different helicity. Our work opens the door to achieving simultaneous controls on far- and near-field electromagnetic environments based on a single ultra-compact platform.

Polarization control of a free-electron laser oscillator using helical undulators of opposite helicities

Polarized photon beams provide a unique experimental tool for the study of various polarization-dependent physical processes. Here, we report the experimental demonstration of full polarization control of an oscillator free-electron laser (FEL) using helical undulators of opposite helicities. Using two helical undulator magnets of opposite helicities and a buncher magnet in between, we have generated a linearly polarized FEL beam with any desirable polarization direction. With the development of a high-precision FEL polarimeter, we are able to optimize the highly polarized FEL beams in visible wavelengths and measure the polarization with high accuracy, demonstrating linear polarization ${P}_{\mathrm{lin}}g0.99$ on the routine basis and with the maximum polarization reaching ${P}_{\mathrm{lin}}=0.998$. In this paper, we describe the FEL configuration, experimental setup, and related beam diagnostics, including the newly developed high-precision FEL polarimeter. We report our experimental approaches to generate, tune up, and characterize the polarization controllable FEL beams and share a new insight into how high-degree polarization is realized based upon our investigation of the temporal structure of the FEL beam. This FEL polarization control technique has been used successfully to generate a polarization controllable Compton $\ensuremath{\gamma}$-ray beam for nuclear physics experiments.

Spontaneous magnetic bubbles and large topological Hall effect in Mn3-xFexSn compound

Magnetic and transport properties as well as magnetic domain structures have been investigated in a hexagonal polycrystalline Mn2.1Fe0.9Sn compound. The in-situ observation of Lorentz transmission electron microscopy shows that spontaneous magnetic bubbles owing a pair of Bloch lines and biskyrmions consisting of two skyrmions of opposite spin helicities coexist in this compound at low temperature. A large topological Hall resistivity up to about 9 µΩ·cm at 50 K has been found owing to the formation of noncoplanar spin textures deriving from the competition among magnetocrystalline anisotropy, antiferromagnetic coupling, and ferromagnetic interaction.

The Bisognano\u2013Wichmann Property for Asymptotically Complete Massless QFT

We prove the Bisognano–Wichmann property for asymptotically complete Haag–Kastler theories of massless particles. These particles should either be scalar or appear as a direct sum of two opposite integer helicities, thus, e.g., photons are covered. The argument relies on a modularity condition formulated recently by one of us (VM) and on the Buchholz’ scattering theory of massless particles.

Ellipticity control of high-order harmonic generation with nearly orthogonal two-color laser fields

Elliptically polarized high-order harmonics opens a path toward the study of ultrafast dynamics in circular dichroism of magnetic materials, chiral molecules, and electronic spin motion. Recently, efficient high-order harmonic generation with controllable ellipticity has attracted considerable attention in experiment and theory. Here, we experimentally demonstrate a scenario using cross-linearly-polarized two-color fields consisting of a fundamental field and its second harmonic (SH). Through adjusting the relative phase, crossing angle, and intensity ratio of the cross-linearly-polarized two-color field, we obtain ellipticity-tunable high-order harmonics, with ellipticity up to $\ensuremath{\varepsilon}=0.73$. Meanwhile, the elliptically polarized high-order harmonics preserve high efficiency. Furthermore, when the intensity of the SH field is stronger than that of the fundamental field, the even order harmonics will become several times stronger than the odd order harmonics with opposite helicity.

High-purity orbital angular momentum states from a visible metasurface laser

Orbital angular momentum (OAM) from lasers holds promise for compact, at-source solutions for applications ranging from imaging to communications. However, conjugate symmetry between circular spin and opposite helicity OAM states (±l) from conventional spin–orbit approaches has meant that complete control of light’s angular momentum from lasers has remained elusive. Here, we report a metasurface-enhanced laser that overcomes this limitation. We demonstrate new high-purity OAM states with quantum numbers reaching l = 100 and non-symmetric vector vortex beams that lase simultaneously on independent OAM states as much as Δl = 90 apart, an extreme violation of previous symmetric spin–orbit lasing devices. Our laser conveniently outputs in the visible, producing new OAM states of light as well as all previously reported OAM modes from lasers, offering a compact and power-scalable source that harnesses intracavity structured matter for the creation of arbitrary chiral states of structured light. A metasurface laser generates orbital angular momentum states with quantum numbers reaching l = 100. Simultaneous output vortex beams, with Δl as great as 90, are demonstrated in the visible regime.

Bragg solitons in topological Floquet insulators.

We consider a topological Floquet insulator consisting of two honeycomb arrays of identical waveguides having opposite helicities. The interface between the arrays supports two distinct topological edge states, which can be resonantly coupled by additional weak longitudinal refractive index modulation with a period larger than the helix period. In the presence of Kerr nonlinearity, such coupled edge states enable topological Bragg solitons. Theory and examples of such solitons are presented.

Flipping the helicity of X-rays from an undulator at unprecedented speed

X-ray circular dichroism (XMCD), one of the main tools to study magnetism, benefits enormously from the capability of a fast alterable helicity of circularly polarized X-ray photons. Here we present a method for boosting the alternating frequency between right- and left-handed photons to the MHz regime, more than three orders of magnitude faster than state-of-the-art technologies. The method is based on a twin elliptical undulator installed in an electron storage ring being operated in a novel mode where the electron optics is tuned close to a resonance with electrons captured in transverse resonance island buckets. Propagating through the twin undulator, electrons from different islands emit photons of the same wavelength but of opposite helicity. These two helicity components can be alternated as fast as 2 ns. In a proof-of-principle experiment at BESSY II, we demonstrate XMCD at the L2,3 absorption edges of Ni with an 800 ns helicity flip.

Strategies for Probing P-Parity Violation in Nuclear Collisions at the NICA Accelerator Facility

Possible approaches to experimentally detecting P-parity violation in polarized proton or deuteron interactions with an unpolarized target are discussed. Within the first approach, a polarized proton or deuteron beam is scattered on a thick internal target deployed in one of rings of the NICA collider, and the spin state of the circulating beam is transformed to the mode of precession in the horizontal plane using an RF spin flipper. In this case, parity-violating effects can be probed by measuring the correlation between the interaction cross section and the particle spin direction. In an alternative experimental approach, beam-particle spins are rotated into the horizontal plane using a spin flipper, and then the beam is extracted to a beamline at a definite stage of spin precession. In this experimental scheme, which can be viewed as more traditional, one then collides the extracted beam with a dense external target and compares the measured cross sections for particles with opposite helicities passing through the target.

Spin-preserving chiral photonic crystal mirror

Chirality refers to a geometric phenomenon in which objects are not superimposable on their mirror image. Structures made of nanoscale chiral elements can exhibit chiroptical effects, such as dichroism for left- and right-handed circularly polarized light, which makes these structures highly suitable for applications ranging from quantum information processing and quantum optics to circular dichroism spectroscopy and molecular recognition. At the same time, strong chiroptical effects have been challenging to achieve even in synthetic optical media, and chiroptical effects for light with normal incidence have been speculated to be prohibited in thin, lossless quasi-two-dimensional structures. Here, we report an experimental realization of a giant chiroptical effect in a thin monolithic photonic crystal mirror. Unlike conventional mirrors, our mirror selectively reflects only one spin state of light while preserving its handedness, with a near-unity level of circular dichroism. The operational principle of the photonic crystal mirror relies on guided-mode resonance (GMR) with a simultaneous excitation of leaky transverse electric (TE-like) and transverse magnetic (TM-like) Bloch modes in the photonic crystal slab. Such modes are not reliant on the suppression of radiative losses through long-range destructive interference, and even small areas of the photonic crystal exhibit robust circular dichroism. Despite its simplicity, the mirror strongly outperforms earlier reported structures and, contrary to a prevailing notion, demonstrates that near-unity reflectivity contrast for opposite helicities is achievable in a quasi-two-dimensional structure.

Interference effect in high-order harmonic generation from degenerate current-carrying orbitals of polyatomic molecules

When the molecules with degenerate current-carrying orbitals are exposed to the bichromatic counterrotating circularly polarized (BCCP) laser fields, the destructive interference between high harmonics from these orbitals results in a strong suppression of harmonic emission with the selected helicity. As a result, there are large intensity differences between the high harmonics with opposite helicities in a wide spectral range. This interference effect is related to the noncylindrical symmetry of the molecular current-carrying orbitals and therefore depends on the relative orientation between the molecule and the driving laser fields. Furthermore, our results show that this interference can be controlled by adjusting the relative intensity ratio of the BCCP field. Finally, we show that highly elliptically polarized attosecond pulses can be generated by using this interference and their ellipticities can be tuned by controlling the molecular orientation.

Chiral Reaction Field with Thermally Invertible Helical Sense that Controls the Helicities of Conjugated Polymers.

A chiral reaction field with thermally invertible helical sense enables control of the helicity of the reaction product, which is a central challenge in asymmetric synthesis that has yet to be overcome. A novel chiral compound comprising two types of chiral moieties with opposite helicities and temperature dependences is synthesized; this compound is added as a chiral dopant to a mixture of nematic liquid crystals to prepare a chiral nematic liquid crystal (N*-LC). The N*-LC containing the chiral dopant exhibits thermally invertible helicity to yield left- and right-handed helical senses at low and high temperatures, respectively. Interfacial polymerization of acetylene is achieved in the N*-LC by modulating the temperature. Helical polyacetylenes (H-PAs) that are synthesized at low (-12 °C) and high (28 °C) temperature show right- and left-handedness, respectively, in terms of the fibrils, fibril bundles, and spiral morphology. In addition, the helical sense of H-PA is opposite that of the N*-LC because of the peculiar polymerization mechanism for acetylene in the N*-LC. The current N*-LC is the first chiral reaction field that has not only the thermally invertible helical sense but also the chemical functions and stability needed to serve as the medium for polymer reactions.

Cross Helicity Reversals in Magnetic Switchbacks

We consider 2D joint distributions of normalised residual energy $\sigma_r(s,t)$ and cross helicity $\sigma_c(s,t)$ during one day of Parker Solar Probe's (PSP's) first encounter as a function of wavelet scale $s$. The broad features of the distributions are similar to previous observations made by HELIOS in slow solar wind, namely well correlated and fairly Alfv\'enic, except for a population with negative cross helicity which is seen at shorter wavelet scales. We show that this population is due to the presence of magnetic switchbacks, brief periods where the magnetic field polarity reverses. Such switchbacks have been observed before, both in HELIOS data and in Ulysses data in the polar solar wind. Their abundance and short timescales as seen by PSP in its first encounter is a new observation, and their precise origin is still unknown. By analysing these MHD invariants as a function of wavelet scale we show that MHD waves do indeed follow the local mean magnetic field through switchbacks, with net Elsasser flux propagating inward during the field reversal, and that they therefore must be local kinks in the magnetic field and not due to small regions of opposite polarity on the surface of the Sun. Such observations are important to keep in mind as computing cross helicity without taking into account the effect of switchbacks may result in spurious underestimation of $\sigma_c$ as PSP gets closer to the Sun in later orbits.

Extended dynamical symmetries of Landau levels in higher dimensions

Continuum models for time-reversal (TR) invariant topological insulators (Tis) in d ≥ 3 dimensions are provided by harmonic oscillators coupled to certain SO(d) gauge fields. These models are equivalent to the presence of spin-orbit (SO) interaction in the oscillator Hamiltonians at a critical coupling strength (equivalent to the harmonic oscillator frequency) and leads to flat Landau Level (LL) spectra and therefore to infinite degeneracy of either the positive or the negative helicity states depending on the sign of the SO coupling. Generalizing the results of [1] to d ≥ 4, we construct vector operators commuting with these Hamiltonians and show that SO(d, 2) emerges as the non-compact extended dynamical symmetry. Focusing on the model in four dimensions, we demonstrate that the infinite degeneracy of the flat spectra can be fully explained in terms of the discrete unitary representations of SO(4,2), i.e. the doubletons. The degeneracy in the opposite helicity branch is finite, but can still be explained exploiting the complex conjugate doubleton representations. Subsequently, the analysis is generalized to d-dimensions, distinguishing the cases of odd and even d. We also determine the spectrum generating algebra in these models and briefly comment on the algebraic organization of the LL states w.r.t. an underlying “deformed” AdS geometry as well as on the organization of the surface states under open boundary conditions in view of our results.

Controlled transformation of skyrmions and antiskyrmions in a non-centrosymmetric magnet.

Control of topological spin textures in magnetic systems may enable future spintronic applications. Magnetic field pulses can switch the vortex polarity1 or the winding number of magnetic bubbles2. Thermal energy can reverse the helicity of skyrmions3 and induce the transformation between meron and skyrmion by modifying the in-plane anisotropy4,5. Among the various topological spin textures, skyrmions6,7 and antiskyrmions8-10 are nanometric spin-whirling structures carrying integer topological charges (N) of -1 and +1 (refs. 7,11,12), respectively, and can be observed in real space8,13. They exhibit different dynamical properties under current flow14-18, for example, opposite signs for the topological Hall effect. Here we observe, in real space, transformations among antiskyrmions, non-topological (NT) bubbles and skyrmions (with N of +1, 0 and -1, respectively) and their lattices in a non-centrosymmetric Heusler magnet, Mn1.4Pt0.9Pd0.1Sn, with D2d symmetry. Lorentz transmission electron microscopy images under out-of-plane magnetic fields show a square lattice of square-shaped antiskyrmions near the Curie temperature and a triangular lattice of elliptically deformed skyrmions with opposite helicities at lower temperatures. The clockwise and counter-clockwise helicities of the skyrmions originate from Dzyaloshinskii-Moriya interactions with opposite signs along the [100] and [010] directions, respectively. A variation of the in-plane magnetic field induces a topological transformation from antiskyrmions to NT-bubbles and to skyrmions, which is accompanied by a change of the lattice geometry. We also demonstrate control of the helicity of skyrmions by variations of the in-plane magnetic field. These results showcase the control of the topological nature of spin configurations in complex magnetic systems.

Momentum-Space Geometric Structure of Helical Evanescent Waves and Its Implications on Near-Field Directionality

In this paper, a momentum-space geometrical structure in helical evanescent electromagnetic waves is revealed. It is shown that for every helical evanescent wave on a helicity-dependent half tangent line in momentum space, the orientation of each of its field, spin, and Poynting vectors is the same. This geometric structure is revealed in a remarkable relation between the far-field and near-field components of the angular spectrum. Any general evanescent wave vector is linked to two points on the ${k}_{\ensuremath{\rho}}={k}_{0}$ circle of propagating wave vectors via two helicity-dependent tangent lines. Knowing the field of a general dipolar source on the ${k}_{\ensuremath{\rho}}={k}_{0}$ circle is sufficient to determine its entire evanescent angular spectrum. By applying this concept, we gain insights into near-field directionality by showing that every zero in the angular spectrum is a helicity singularity where two half tangent lines of opposite helicity intersect. A powerful method for the synthetic design of near-field directional sources is also devised, using structured helical illumination to gain full control of the near-field directionality. The results provide fundamental insight into helical evanescent waves and have implications in areas where chiral light-matter interaction plays a central role.