Helical Phases Research Articles

Transfer of optical vortices in a coherently-prepared rare-earth-ion-doped solid

Optical vortices carrying orbital angular momentum (OAM) of light have earned much attention due to their potential applications in the classical and quantum physics. Quantum coherence has been proved to be an important tool for controlling the behavior of light fields. Developing the manipulation method of optical vortices is very useful for high-dimensional information processing encoded with OAM. In this paper, we experimentally investigate the transfer of optical vortices between different light modes in a coherently-prepared solid-state medium. Utilizing the technique of fractional stimulated Raman passage (f-STIRAP), the experimental medium is prepared into a coherent superposition of two ground states. By applying a probe vortex beam to scatter the induced atomic coherence, a new signal is generated with the same vorticity as that of the input probe field. Furthermore, the vortex beams are used as the f-STIRAP preparation pulses, and the helical phases of the vortex beams are mapped into the coherent superposition state. Subsequently, the optical vortices of the preparation pulses are transferred to a new light mode by using the probe beam to interact with the coherent medium. The new signal field accumulates the total topological charges of the applied fields in this coherent process. Such an experimental demonstration will enrich the interactions between optical vortices and matter, and can be helpful in further information processing.

Coexistence of Quantum-Spin-Hall and Quantum-Hall-Topological-Insulating States in Graphene/hBN on SrTiO3 Substrate.

SrTiO3 (STO) substrate, a perovskite oxide material known for its high dielectric constant (ɛ), facilitates the observation of various (high-temperature) quantum phenomena. A quantum Hall topological insulating (QHTI) state, comprising two copies of QH states with antiparallel two ferromagnetic edge-spin overlap protected by the U(1) axial rotation symmetry of spin polarization, has recently been achieved in low magnetic field (B) even as high as ≈100 K in a monolayer graphene/thin hexagonal boron nitride (hBN) spacer placed on an STO substrate, thanks to the high ɛ of STO. Despite the use of the heavy STO substrate, however, proximity-induced quantum spin Hall (QSH) states in 2D TI phases, featuring a topologically protected helical edge spin phase within time-reversal-symmetry, is not confirmed. Here, with the use of a monolayer hBN spacer, it is revealed the coexistence of QSH (at B = 0T) and QHTI (at B ≠ 0) states in the same single graphene sample placed on an STO, with a crossover regime between the two at low B. It is also classified that the different symmetries of the two nontrivial helical edge spin phases in the two states lead to different interaction with electron-puddle quantum dots, caused by a local surface pocket of the STO, in the crossover regime, resulting in a spin dephasing only for the QHTI state. The results obtained using STO substrates open the doors to investigations of novel QH spin states with different symmetries and their correlations with quantum phenomena. This exploration holds value for potential applications in spintronic devices.

Magnetic tunability in tetragonal Mn–Rh–Ir–Sn inverse Heusler compounds

Gaining control over magnetic structure has been an ongoing challenge in materials that form complex, nanoscale, and non-collinear magnetic configurations. Recently, it was predicted that tuning the ratio of the Dzyaloshinskii–Moriya interaction to the uniaxial magnetic anisotropy in tetragonal inverse Heuslers through changes in composition could allow a range of interesting magnetic states to be accessed, from simple ferrimagnetism, to helical and antiskyrmionic phases. Here, we show tunability of the magnetic phase behavior in the Mn–Rh–Sn system through Ir substitution on the Rh substructure. Iridium substitution correlates to an increase in the strength of ferromagnetic exchange couplings, at the expense of antiferromagnetic exchange couplings. However, we do not observe the complex non-collinear magnetic phases proposed previously, likely due to the extremely narrow composition window where these phases are predicted to form in a bulk sample. This work highlights the sensitivity of complex magnetic structures to stoichiometry, which makes them difficult to discover empirically.

Cylindrical vector beam multiplexing holography employing spin-decoupled phase modulation metasurface

Abstract Cylindrical vector beams (CVBs) hold considerable promise as high-capacity information carriers for multiplexing holography due to their mode orthogonality. In CVB holography, phase holograms are encoded onto the wave-front of CVBs with different mode orders while preserving their independence during reconstruction. However, a major challenge lies in the limited ability to manipulate the spatial phase and polarization distribution of CVBs independently. To address this challenge, we propose a spin-decoupled phase modulation strategy by leveraging the propagation and geometric phase of composite phase metasurfaces. By exploiting the polarized Poincaré sphere, we show that CVBs can be decomposed into two circularly polarized components with orthogonal polarization states and conjugate phase distributions. This decomposition enables independent control of the phase and polarization distributions of CVBs by modulating the initial phase and phase difference of these two components. Consequently, two holograms with discrete spatial frequency distributions that carry opposite helical phases are encoded to modulate the wave-front of CVBs by the metasurface consisting of Si nanopillars. This allows for us to achieve successful four-channel CVB multiplexing holography. Benefiting from the non-dispersive nature of geometric phase, this metasurface exhibits a broad operating band spanning the entire visible light spectrum (443 nm–633 nm). These suggest that our proposed method offers comprehensive control over the spatial phase and polarization of CVBs, thereby holding significant potential for advancing their application in holography.

Wavefront distortion and compensation for weakly relativistic vortex beams propagating in plasma

The propagation of electromagnetic wave in plasma is one of the long-standing concerns in the field of laser plasma, and it is closely related to the researches of radiation source generation, particle acceleration, and inertial confinement fusion. Recently, the proposal of various schemes for generating intense vortex beams has led to an increasing number of researchers focusing on the interaction between intense vortex beams and plasmas, resulting in significant research progress in various areas, such as particle acceleration, high-order harmonic generation, quasi-static self-generated magnetic fields, and parametric instability. Compared with traditional Gaussian beams, vortex beams, featuring their hollow amplitudes and helical phases, can exhibit novel phenomena during propagating through plasma. In this work, we primarily focus on studying the influence of the propagation process on the wave structure of vortex beams before filamentation occurs. The three-dimensional particle-in-cell simulations show that weakly relativistic vortex beams exhibit wavefront distortion during their propagation in plasma. The distortion degree is closely related to the intensity of the electromagnetic wave and the propagation distance for a given plasma density. This phenomenon is theoretically explained by using a phase correction model that considers the relativistic mass correction of electrons. Additionally, we demonstrate that the wavefront distortion can be compensated for and suppressed by appropriately modulating the initial plasma density, as confirmed by three-dimensional particle simulations. The results of decomposing the wavefront into Laguerre-Gaussian (LG) mode components indicate that the wavefront distortion is primarily caused by high-order <i>p</i> LG modes, and it is independent of other <i>l</i> LG modes. Additionally, we extend the present investigation to the propagation of vortex beams in axially magnetized plasma, where the phase correction model can also effectively explain the occurrence of wavefront distortion. Our work can deepen the understanding of the interaction between plasma and strong vortex beams, and provide some valuable references for designing plasma devices serving as the manipulation of intense vortex beams in future research.

Confinement-induced self-assembly of a diblock copolymer within a non-uniform cylindrical nanopore.

The self-assembly of a diblock copolymer melt confined within a non-uniform cylindrical nanopore is studied using the self-consistent field theory. The non-uniformity manifests in the form of a converging-diverging cylindrical nanopore. The axial variation in pore diameter presents a range of curvatures within the same confinement pore as opposed to a single curvature in a uniform-diameter cylindrical pore. The introduction of multiple curvatures leads to the formation of novel microstructures not accessible in uniform cylindrical confinement. The well-known equilibrium structures like a single helix, double helices, and concentric lamella under cylindrical confinement transition into new morphologies such as hyperboloidal phases, microstructures containing rings with a bead, rings with spheres, and a squeezed helical phase as the pore diameter varies axially. The converging-diverging geometry of the confining pore renders the helical phases seen in the cylindrical pore less favorable. A phase diagram in the parametric space of the block fraction and the ratio of the smallest and largest pore radii has been constructed to depict the order-order transition of various microstructures. The ratio of radii, a measure of the non-uniformity of the pore, along with the pore length brings out some interesting morphologies. The mechanism of these structural transitions is understood as the interplay between the variation in pore curvature attributed to the non-uniformity, the spontaneous curvature of the block copolymer interface, and the enthalpic interaction between the segregated blocks.

Luminescent and photoconductive liquid crystalline lamellar and helical network phases of achiral polycatenars

Lamellar and helical network phases of achiral polycatenars. Functional achiral triple chain π-conjugated rod-like molecules were investigated for their photoconductivity, photoluminescence, phosphorescence, and electroluminescence properties.

Electric field- and light-responsive oxadiazole bent-core polycatenar liquid crystals

Columnar and helical network phases of achiral polycatenars. Functional oxadiazole-based bent-core molecules were investigated for their response to light and electric fields.

Arbitrary manipulations of broadband terahertz focused Poincaré beams enabled by anisotropic silicon metasurfaces

Focused Poincaré beams (FPBs) carrying inhomogeneous polarization properties describe the paraxial solution of the vector Helmholz equation in cylindrical coordinate systems, with potential for classical and quantum applications. Here, we theoretically demonstrate a generalized strategy for generating FPBs based on the spin decoupling mechanism. Silicon metasurfaces with high degrees of freedom allow the proposed design to generate vector beams carrying arbitrary topological charges in a broadband terahertz (THz) frequency range. By adopting a purely phase-based modulation approach, the single-layer polarization-multiplexed metasurface can act as a bridge between the conventional Poincaré sphere (PS) and the higher-order Poincaré sphere (HOPS), preserving the corresponding mapping relations. Benefiting from the independence of the on-demand tailored helical phases, we further evaluate the coaxial evolutionary behavior of FPBs, generating structured THz fields with incremental topological charges. It is foreseeable that this work will provide an efficient meta-platform for the generation of broadband THz structured fields and facilitate their application in information encryption and quantum scientific information.

Task-adaptive physical reservoir computing

Reservoir computing is a neuromorphic architecture that may offer viable solutions to the growing energy costs of machine learning. In software-based machine learning, computing performance can be readily reconfigured to suit different computational tasks by tuning hyperparameters. This critical functionality is missing in ‘physical’ reservoir computing schemes that exploit nonlinear and history-dependent responses of physical systems for data processing. Here we overcome this issue with a ‘task-adaptive’ approach to physical reservoir computing. By leveraging a thermodynamical phase space to reconfigure key reservoir properties, we optimize computational performance across a diverse task set. We use the spin-wave spectra of the chiral magnet Cu2OSeO3 that hosts skyrmion, conical and helical magnetic phases, providing on-demand access to different computational reservoir responses. The task-adaptive approach is applicable to a wide variety of physical systems, which we show in other chiral magnets via above (and near) room-temperature demonstrations in Co8.5Zn8.5Mn3 (and FeGe).

Cycloplatinated(II) metallomesogens and their binary-mediated chirality transfer and amplificated deep-red circularly polarized luminescence with ultrahigh dissymmetry factor over 0.13

Incorporating supramolecular chirality and high emission into metallomesogens has great significance in the field of flexible electronic devices because of their emergent capacities for enlarged chiroptical activities and either thermotropic or solution-processability. In this work, an enantiomeric pair of chiral Pt(II) metallomesogens (S)/(R)-Pt1 were prepared through laboratory-synthesized chiral picolinic acid ligands and phenylpyridine derivatives to study their mesophase behaviors, chiroptical properties, and chirality transfer and amplification. The solutions of (S)/(R)-Pt1 emitted green phosphorescence at 508 nm, but the amorphous form displayed orange-red phosphorescence at 616 nm. Upon gentle grinding or heating, the emission color of Pt1 was dramatically changed with a bathochromic shift of 47 and 51 nm, respectively, because of emerged intermolecular π···π and Pt⋯Pt interactions in all aggregation states. Furthermore, the chiroptical properties and LC self-assembly model were proposed by Density Functional Theory (DFT) calculations and chiroptical spectrum measurements. The enantiotopic (S)/(R)-Pt1 complexes exhibited electronic circular dichroism (CD) and circularly polarized luminescence (CPL) activities in solution with a smaller asymmetry factor (±5 × 10−4) by chiral disturbance. This pair of exclusively enantiomeric complexes exhibited smectic mesophase behavior in the range of 31.4–76.5 °C but induced unsuccessfulness of helical packing in LC states due to weak induction ability. Contrastively, binary-component mediated supramolecular helical formation and amplificated deep-red CPL switch with ultrahigh dissymmetry factor over ±0.13 in helical twist grain boundary phases (TGBA*) mesophase has been realized.

Directionally duplexed all-dielectric metalens for multifunctional structured light generation.

Directionally duplexed metalenses manipulated by the geometric phase of a silicon nano-bar are theoretically designed to generate multifunctional structured light. It is numerically demonstrated that incident light with different linear and circular polarization states, along forward and backward propagation directions, can be differentially converted into multiple focusing structured beams of arbitrary topological charges, either of vector light with azimuthally variant polarizations or of vortex light with helical phases. Due to the all-silicon and nonresonant metastructural design, the resultant high working efficiencies of our proposed metalens are promising for applications such as optical communication, nanoparticle manipulation, and other direction-duplexed multifunctional optical meta-devices.

Harnessing Skyrmion Hall Effect by Thickness Gradients in Wedge-Shaped Samples of Cubic Helimagnets.

The skyrmion Hall effect, which is regarded as a significant hurdle for skyrmion implementation in thin-film racetrack devices, is theoretically shown to be suppressed in wedge-shaped nanostructures of cubic helimagnets. Under an applied electric current, ordinary isolated skyrmions with the topological charge 1 were found to move along the straight trajectories parallel to the wedge boundaries. Depending on the current density, such skyrmion tracks are located at different thicknesses uphill along the wedge. Numerical simulations show that such an equilibrium is achieved due to the balance between the Magnus force, which instigates skyrmion shift towards the wedge elevation, and the force, which restores the skyrmion position near the sharp wedge boundary due to the minimum of the edge-skyrmion interaction potential. Current-driven dynamics is found to be highly non-linear and to rest on the internal properties of isolated skyrmions in wedge geometries; both the skyrmion size and the helicity are modified in a non-trivial way with an increasing sample thickness. In addition, we supplement the well-known theoretical phase diagram of states in thin layers of chiral magnets with new characteristic lines; in particular, we demonstrate the second-order phase transition between the helical and conical phases with mutually perpendicular wave vectors. Our results are useful from both the fundamental point of view, since they systematize the internal properties of isolated skyrmions, and from the point of view of applications, since they point to the parameter region, where the skyrmion dynamics could be utilized.

Full-Stokes parameters detection enabled by a non-interleaved fiber-compatible metasurface.

Polarization of the optical field determines the way of light-matter interaction, which lays the foundation for various applications such as chiral spectroscopy, biomedical imaging, and machine vision. Currently, with the rise of the metasurface, miniaturized polarization detectors have attracted extensive interest. However, due to the limitation of the working area, it is still a challenge to integrate polarization detectors on the fiber end face. Here, we propose a design of compact non-interleaved metasurface that can be integrated on the tip of a large-mode-area photonic crystal fiber (LMA-PCF) to realize full-Stokes parameters detection. Through concurrent control over the dynamic phase and Pancharatnam-Berry (PB) phase, different helical phases are assigned to the two orthogonal circular polarization bases, of which the amplitude contrast and relative phase difference can be represented by two non-overlapped foci and an interference ring pattern, respectively. Therefore, the determination of arbitrary polarization states through the proposed ultracompact fiber-compatible metasurface can be achieved. Moreover, we calculated full-Stokes parameters according to simulation results and obtained that the average detection deviation is relatively low at 2.84% for 20 elucidated samples. The novel metasurface exhibits excellent polarization detection performance and overcomes the limitation of the small integrated area, which provides insights into the further practical explorations of ultracompact polarization detection devices.

Topological skyrmion phases of matter

We introduce topological phases of matter defined by skyrmions in the ground state spin—or pseudospin—expectation value textures in the Brillouin zone, the chiral and helical topological skyrmion phases of matter. These phases are protected by a symmetry present in centrosymmetric superconductors. We consider a tight-binding model for spin-triplet superconductivity in transition metal oxides and find it realizes each of these topological skyrmion phases. The chiral phase is furthermore realized for a parameter set characterizing Sr2RuO4 with spin-triplet superconductivity. We also find two types of topological phase transitions by which the skyrmion number can change. The second type occurs without the closing of energy gaps in a system described by a quadratic Hamiltonian without breaking the protecting symmetries when atomic spin–orbit coupling is non-negligible and there is a suitable additional degree of freedom. This contradicts the ‘flat band’ limit assumption important in use of entanglement spectrum and Wilson loops, and in construction of the ten-fold way classification scheme of topological phases of matter. We furthermore predict two kinds of bulk-boundary correspondence signatures—one for measurements which execute a partial trace over degrees of freedom other than spin, which yields quantized transport signatures—and a second resulting from skyrmions trapping defects with their own non-trivial topology that is discussed in a second work, which yields generalizations of unpaired Majorana zero-modes.

Conformational and Supramolecular Aspects in Chirality of Flexible Camphor-Containing Schiff Base as an Inducer of Helical Liquid Crystals.

The experimental and theoretical study of influence of the conformational state and association on the chirality of the stereochemically nonrigid biologically active bis-camphorolidenpropylenediamine (CPDA) and its ability to induce the helical mesophase of alkoxycyanobiphenyls liquid-crystalline binary mixture was carried out. On the basis of quantum-chemical simulation of the CPDA structure, four relatively stable conformers were detected. A comparison of the calculated and experimental electronic circular dichroism (ECD) and 1H, 13C, 15N NMR spectra, as well as specific optical rotation and dipole moments, allowed to establish the most probable trans-gauche conformational state (tg) of dicamphorodiimine and CPDA dimer with a predominantly mutually parallel arrangement of molecular dipoles. The induction of helical phases in LC mixtures based on cyanobiphenyls and bis-camphorolidenpropylenediamine was studied by polarization microscopy. The clearance temperatures and the helix pitch of the mesophases were measured. The helical twisting power (HTP) was calculated. The decrease in HTP with increasing dopant concentration was shown to be connected with the CPDA association process in the LC phase. The effect of camphor-containing chiral dopants of various structures on nematic LCs was compared. The values of the permittivity and birefringence components of the CPDA solutions in CB-2 were measured experimentally. A strong effect of this dopant on the anisotropic physical properties of the induced chiral nematic was established. A significant decrease in the dielectric anisotropy was associated with the 3D compensation of the LC dipoles during the formation of the helix.

Small-angle neutron scattering of long-wavelength magnetic modulations in reduced sample dimensions.

Magnetic small-angle neutron scattering (SANS) is ideally suited to providing direct reciprocal-space information on long-wavelength magnetic modulations, such as helicoids, solitons, merons or skyrmions. SANS of such structures in thin films or micro-structured bulk materials is strongly limited by the tiny scattering volume vis a vis the prohibitively high background scattering by the substrate and support structures. Considering near-surface scattering just above the critical angle of reflection, where unwanted signal contributions due to substrate or support structures become very small, it is established that the scattering patterns of the helical, conical, skyrmion lattice and fluctuation-disordered phases in a polished bulk sample of MnSi are equivalent for conventional transmission and near-surface SANS geometries. This motivates the prediction of a complete repository of scattering patterns expected for thin films in the near-surface SANS geometry for each orientation of the magnetic order with respect to the scattering plane.

Acoplanarity, Aromaticity, Chirality, and Helical Twisting Power of Chlorin e6 13(N)-Methylamide-15,17-dimethyl Ester Complexes: Effect of a Metal

The experimental and theoretical study of the influence of metal complexing on geometry, aromaticity, chirality, and the ability to twist the nematic phase by complexes based on modified natural chlorin e6 was carried out. The geometry optimization of the chlorin e6 13(N)-methylamide-15,17-dimethyl ester (MADMECl) and its Zn, Cu, and Ni complexes by DFT (CAM-B3LYP/6–31 G(d,p) functional) method was performed. Based on these calculations, the acoplanarity degree of the macrocyclic ligand and the distortion energy of its dianion were estimated, which allowed the arrangement of the MADMECl complexes in the series Ni > Cu > Zn. Aromaticity was evaluated using the NICS criterion (nuclear independent chemical shift). An increase in the degree of aromaticity of the macrocycle upon complex formation was established. At the same time, the aromaticity of the inner conjugation contour corresponds to the same series as the acoplanarity, while the outer π-delocalization is characterized by the reverse sequence. An experimental evaluation of the electron circular dichroism of the Soret and the Q-bands, as well as the g-factor of dissymmetry, was carried out. The growth of these parameters with an increase in the degree of acoplanarity and aromaticity of the internal conjugation contour was determined. The induction of helical phases in mixtures of nematic liquid crystals (LCs) based on cyanobiphenyls and MADMECl macrocyclic metal complexes was studied by polarization microscopy, and the clearance temperatures and helix pitch of the mesophases were measured. A strong effect of the metal on the phase transition temperature and helical twisting power was established.

All-graphene geometric terahertz metasurfaces for generating multi-dimensional focused vortex beams

Simultaneously assembling the functions of lenses and phase vortices into geometric metasurfaces on an ultra-compact platform can potentially be used for light trapping and edge imaging, helping to enhance their applications in polarized optical systems. Here, we propose and theoretically investigate an all-graphene geometric metasurface operating in the terahertz (THz) band for generating multidimensional vortices. Focused scalar vortex beams with polarization-independent properties can be generated by introducing the superposition of two helical phases with the same topological charge within orthogonal circularly polarized (CP) channels. Focus shift can be further tolerated by tailoring the conventional helical phase distribution. Embedding polarization modulation into the proposed design enables the generation of multiple focused vortices with inhomogeneous polarization properties in the longitudinal direction. The typical conjugate phase is capable of generating aggregated vortices with vectorial characteristics by switching the incident polarization mode, and can be extended to arbitrary orders. Using the way in which the graphene Fermi energy was changed in the simulation provides strong evidence for dynamically tuning the focusing efficiency of the generated beam. Benefiting from the proposed method that manipulates the orthogonal CP components separately, the design scheme has greater degree of freedom and can find potential applications in meta-optics, high-tolerance edge imaging, and high-capacity optical communications.

Generation and manipulation of multi-twisted beams via azimuthal shift factors

We develop the effective experimental approach to generate multi-twisted beams (MTBs) with twisted intensity lobes by superimposing helical phases consisting of multiple independent sub-phases with different azimuthal shift factors. The MTBs' energy flows and propagation properties are also investigated, indicating that such beams exhibit twisted properties. The azimuthal shift factor determines the twisted intensity distributions, and the number of twisted lobes depends on the sub-phase number. The bright lobes of a MTB possess the shapes of thin spiral lines, and the intensity pattern depends on the topological charge. Diverse MTBs can be generated by flexibly manipulating the azimuthal shift factors and the sub-phase number. Also, various mirror-symmetrical twisted beams are constructed using the matrix flip scheme, further enriching the light structures of MTBs. Numerical simulation and experimental results are consistent. Furthermore, the capture and guide of microspheres via the MTBs are experimentally executed and demonstrate the feasibility and practicability of our generated MTBs. The various MTBs will likely give rise to potential applications in fabricating chiral nanostructures and manipulating microparticles.