Chiral Objects Research Articles

Sedimentation dynamics of triply twisted Möbius bands: Geometry versus topology

We explore the sedimentation dynamics of triply twisted Möbius bands, each characterized by threefold rotational symmetry but distinguished by its construction and intrinsic geometrical properties. Three types of bands are considered: one with vanishing Gaussian curvature, constructed by isometrically deforming flat rectangular strips through bending without stretching; and two with negative Gaussian curvature, one being constructed by isometrically deforming helicoids. Experiment on these three types of bands, with a focus on varying aspect ratios, reveals a singular phenomenon: while the spin directions of bands not derived from helicoids spin in directions consistent with their inherent chirality, bands derived from helicoids exhibit an aspect-ratio-dependent spin, pointing to the existence of a critical aspect ratio at which geometric factors dominate over chiral influences. Supported by numerical simulations and a detailed analysis of the resistance tensors, we propose the unique response of bands derived from helicoids originates from a complex interaction among geometry, topology, and hydrodynamics. Two explanations are offered for the chiral transition observed in bands derived from helicoid. First, this transition may parallel the dynamics of superhelices, for which competing chiralities influence rotational behavior. Second, the unique geometrical properties of bands derived from helicoids, coupled with deviations between the rotation axis and the local symmetry axis, may underlie the observed aspect-ratio-dependent chiral transition. Our study underscores the significant role of geometrical and topological nuances in determining the behavior of chiral objects suspended in fluids. In addition to offering transformative potential across diverse fields, it promises advancements in mixing, separation processes, and innovative passive swimmers. Published by the American Physical Society 2024

Separating the Material and Geometry Contribution to the Circular Dichroism of Chiral Objects Made from Chiral Media

Separating the Material and Geometry Contribution to the Circular Dichroism of Chiral Objects Made from Chiral Media

Maximal circular polarization isolation and asymmetric polarization conversion by chiral meta-structure

Chiral objects hold immense significance in modern optical technology, particularly due to their ability to manipulate circularly polarized waves. The chiroptical effects observed in naturally known chiral structures are typically very weak, however, the use of engineered meta-structures has proven to be highly effective in overcoming these shortcomings. Despite extensive research efforts, the construction of chiroptical phenomena approaching maximum performance has proven to be challenging, mostly due to the lack of optimal design choices and the existence of material losses. Here we present a metasurface constituting S-shaped building blocks capable of realizing virtually maximum chiroptical phenomena. The structure demonstrates nearly full polarization transmission, conversion to an opposite spin state, and reflection of the opposite spin state at a wavelength of 1549 nm. As a result, the maximum circular dichroism (CD) value reaches up to ≈1 (0.9993) for a given polarization state. Furthermore, reduced symmetry enables the one-way flow of a given polarization state resulting in about unity (0.998) asymmetric transmission (AT) value. Through rigorous numerical simulations, we elucidate the underlying principles driving these extraordinary optical properties. The CD and AT values are record-high demonstrated so far. The single-layer design offers an ultrathin profile, making it compatible with integrated photonics and providing opportunities for applications in compact, lightweight optical devices such as circular polarizers, half-wave plates, and self-polarizing reflectors.

Generalised Jeffery's equations for rapidly spinning particles. Part 2. Helicoidal objects with chirality

In this two-part study, we investigate the motion of rigid, active objects in shear Stokes flow, focusing on bodies that induce rapid rotation as part of their activity. In Part 2, we derive and analyse governing equations for rapidly spinning complex-shaped particles – general helicoidal objects with chirality. Using the multiscale framework that we develop in Part 1 (Dalwadi et al., J. Fluid Mech., vol. 979, 2024, A1), we systematically derive emergent equations of motion for the angular and translational dynamics of these chiral spinning objects. We show that the emergent dynamics due to rapid rotation can be described by effective generalised Jeffery's equations, which differ from the classic versions via the inclusion of additional terms that account for chirality and other asymmetries. Furthermore, we use our analytic results to characterise and quantify the explicit effect of rotation on the effective hydrodynamic shape of the chiral objects, expanding significantly the scope of Jeffery's seminal study.

Chiral Skyrmions Interacting with Chiral Flowers.

The chiral nature of active matter plays an important role in the dynamics of active matter interacting with chiral structures. Skyrmions are chiral objects, and their interactions with chiral nanostructures can lead to intriguing phenomena. Here, we explore the random-walk dynamics of a thermally activated chiral skyrmion interacting with a chiral flower-like obstacle in a ferromagnetic layer, which could create topology-dependent outcomes. It is a spontaneous mesoscopic order-from-disorder phenomenon driven by the thermal fluctuations and topological nature of skyrmions that exists only in ferromagnetic and ferrimagnetic systems. The interactions between the skyrmions and chiral flowers at finite temperatures can be utilized to control the skyrmion position and distribution without applying any external driving force or temperature gradient. The phenomenon that thermally activated skyrmions are dynamically coupled to chiral flowers may provide a new way to design topological sorting devices.

A theory of pitch for the hydrodynamic properties of molecules, helices, and achiral swimmers at low Reynolds number.

We present a theory for pitch, a matrix property that is linked to the coupling of rotational and translational motion of rigid bodies at low Reynolds numbers. The pitch matrix is a geometric property of objects in contact with a surrounding fluid, and it can be decomposed into three principal axes of pitch and their associated moments of pitch. The moments of pitch predict the translational motion in a direction parallel to each pitch axis when the object is rotated around that axis and can be used to explain translational drift, particularly for rotating helices. We also provide a symmetrized boundary element model for blocks of the resistance tensor, allowing calculation of the pitch matrix for arbitrary rigid bodies. We analyze a range of chiral objects, including chiral molecules and helices. Chiral objects with a Cn symmetry axis with n > 2 show additional symmetries in their pitch matrices. We also show that some achiral objects have non-vanishing pitch matrices, and we use this result to explain recent observations of achiral microswimmers. We also discuss the small but non-zero pitch of Lord Kelvin's isotropic helicoid.

Chiral Metasurfaces: A Review of the Fundamentals and Research Advances

Chirality, the absence of mirror symmetry, is predominant in nature. The chiral nature of the electromagnetic field behaves differently with chiral matter for left circularly polarized and right circularly polarized light. The chiroptical behavior in the sensing of naturally occurring chiral objects is weak, and improving the chiroptical response enhances the chiral sensing platform. This review covers the fundamental concepts of chiral metasurfaces and various types of single- and multi-layered chiral metasurfaces. In addition, we discuss tunable and deep-learning-based chiral metasurfaces. Tunability is achieved by manipulating the meta-atom’s property in response to external stimuli for applications such as optical modulation, chiral photonics, advanced sensing, and adaptive optics. Deep-learning modeling techniques, such as CNNs and GANs, offer efficient learning of the complex relationships in data, enabling the optimization and accurate prediction of chiral metasurface properties. The challenges in the design and fabrication of chiral metasurface include achieving broadband performance and scalability and addressing material limitations. Chiral metasurface performance is evaluated by optical rotation, circular dichroism enhancement, and tunability, which are quantified through the spectroscopic measurement of circular dichroism and optical rotation. Chiral metasurface progress enables applications, including metaholography, metalenses, and chiral sensing. Chiral sensing improves the detection of pharmaceuticals and biomolecules, increasing the sensitivity and accuracy of analytical diagnostics.

Collectively Enhanced Giant Circular Dichroism of Germanium Nanohelix Square Lattice Arrays

Circular dichroism is a unique chiroptical signature of the chirality of a system and is a prevalent way to characterize and distinguish systems on a fundamental level and for their technological applicability. Thus, engineering and maximizing the chiroptical response of a single chiral object or a metasurface composed of chiral entities is a formidable task. Current efforts strongly focus on individual metallic nanostructures and their periodic ensembles to harvest on (resonant) plasmonic properties and interactions. This route, however, waives the advantages of high‐refractive‐index nanoscale materials embracing low dissipative losses at optical wavelengths and electromagnetic fields penetrating and propagating in such materials. Herein, a strong circular dichroism is demonstrated in square lattices of nanohelices made of the high‐refractive‐index semiconductor germanium, with dissymmetry factors outperforming metal‐based ensembles. The observation of a much higher dissymmetry emerges for illumination with spatially coherent light, in comparison to spatially incoherent light. High dissymmetry is attributed to cooperative coupling between single helices, resulting from the combination of dielectric resonances of both the individual helical building blocks and the highly ordered lattice.

Thermal Effects on Optical Chirality, Mechanics, and Associated Symmetry Properties

A review is provided here about the thermal effects on optical chirality. To this goal, chiral objects dispersed in an embedding fluid are examined for their magnetoelectric coupling. Thermal effects on several chiral meta-atoms and their ensembles are examined. To this goal, DNA-like helical structures are examined in detail. The mechanical aspect of thermo-elasticity is reviewed along with transverse deformations while drawing analogies from condensed-matter physics. In this respect, the chirality-induced spin selection is reviewed along with the temperature-mediated electron–phonon interactions. A wide range of materials, such as polymers and biological cells, are also examined for temperature effects. A transition temperature delineating a sign flip in the chirality parameter is identified as well. Chirality-associated functionalities such as ratchet motions, switching, and modulations are investigated for their respective thermal effects. Issues of fabricating chiral meta-atoms are also discussed.

Achiral and Chiral Optical Force Within Topological Optical Lattices Generated with Plasmonic Metasurfaces and Tunable Incident Beam

An optical lattice is a platform that is used in studies in multiple branches of physics, e.g., ultracold atoms and microfluidic sorting. Unlike traditional complex optical systems, surface plasmon polaritons enable the generation of optical lattices on a chip. Here, we propose an on-chip scheme to transform five topological optical lattices using tunable polarization and a phase vortex. Using the dipolar approximation and numerical simulations, we demonstrate that the spin density force is the main force for sorting chiral particles in the out-of-plane direction when their radius is more than 100 nm. A microflow system is proposed to trap particles with electric optical lattices and sort chiral particles with spin optical lattices. Our findings offer possible routes toward realization of on-chip optical lattices and enable their potential application in both biology and chemistry, e.g., in simultaneous trapping or excitation of multiple objects and identification and sorting of chiral objects.

Enantiomeric discrimination by chiral electromagnetic resonance enhancement.

Circularly polarized light interacts preferentially with the biomolecules to generate spectral fingerprints reflecting their primary and secondary structure in the ultraviolet region of the electromagnetic spectrum. The spectral features can be transferred to the visible and near-infrared regions by coupling the biomolecules with plasmonic assemblies made of noble metals. Nanoscale gold tetrahelices were used to detect the presence of chiral objects that are 40 times smaller in size by using plane-polarized light of 550 nm wavelength. The emergence of chiral hotspots in the gaps between 80 nm long tetrahelices differentiate between weakly scattering S- vs R-molecules with optical constants similar to that of organic solvents. Simulations map the spatial distribution of the scattered field to reveal enantiomeric discrimination with selectivity up to 0.54.

Bidirectional wave-propelled capillary spinners

When a solid body floats at the interface of a vibrating liquid bath, the motion of the object generates outwardly propagating surface waves. We here demonstrate that chiral objects on a vibrating fluid interface are set into steady rotation, with the angular speed and direction of rotation controlled by the interplay between object geometry and driving parameters. Scaling laws and a simplified model of the wavefield reveal the underlying physical mechanism of rotation, while collapsing measurements of the angular velocity across parameters. Leveraging the control over the chiral object’s direction of rotation, we demonstrate that a body with an asymmetric mass distribution and chirality can be remotely steered along two-dimensional trajectories via modulation of the driving frequency. This accessible and tunable macroscopic system serves as a potential platform for explorations of chiral active and driven matter, and demonstrates a mechanism by which wave-mediated forces can be manipulated for directed propulsion.

Fundamental Cause of Bio-Chirality: Space-Time Symmetry—Concept Review

The search for fundamental determinants of bio-molecular chirality is a hot topic in biology, clarifying the meaning of evolution and the enigma of life’s origin. The question of origin may be resolved assuming that non-biological and biological entities obey nature’s universal laws grounded on space-time symmetry (STS) and space-time relativity (SPR). The fabric of STS is our review’s primary subject. This symmetry, encompassing the behavior of elementary particles and galaxy structure, imposes its fundamental laws on all hierarchical levels of the biological world. From the perspective of STS, objects across spatial scales may be classified as chiral or achiral concerning a specific space-related symmetry transformation: mirror reflection. The chiral object is not identical (i.e., not superimposable) to its mirror image. In geometry, distinguish two kinds of chiral objects. The first one does not have any reflective symmetry elements (a point or plane of symmetry) but may have rotational symmetry axes (dissymmetry). The second one does not have any symmetry elements (asymmetry). As the form symmetry deficiency, Chirality is the critical structural feature of natural systems, including sub-atomic particles and living matter. According to the Standard Model (SM) theory and String Theory (StrT), elementary particles associated with the four fundamental forces of nature determine the existence of micro- and galaxy scales of nature. Therefore, the inheritance of molecular symmetry from the symmetry of elementary particles indicates a bi-directional (internal [(micro-scale) and external (galaxy sale)] causal pathway of prevalent bio-chirality. We assume that the laws of the physical world impact the biological matter’s appearance through both extremities of spatial dimensions. The extended network of multi-disciplinary experimental evidence supports this hypothesis. However, many experimental results are derived and interpreted based on the narrow-view prerogative and highly specific terminology. The current review promotes a holistic approach to experimental results in two fast-developing, seemingly unrelated, divergent branches of STS and biological chirality. The generalized view on the origin of prevalent bio-molecular chirality is necessary for understanding the link between a diverse range of biological events. The chain of chirality transfer links ribosomal protein synthesis, cell morphology, and neuronal signaling with the laterality of cognitive functions.

DNA double helix, a tiny electromotor.

Flowing fluid past chiral objects has been used for centuries to power rotary motion in man-made machines. By contrast, rotary motion in nanoscale biological or chemical systems is produced by biasing Brownian motion through cyclic chemical reactions. Here we show that a chiral biological molecule, a DNA or RNA duplex rotates unidirectionally at billions of revolutions per minute when an electric field is applied along the duplex, with the rotation direction being determined by the chirality of the duplex. The rotation is found to be powered by the drag force of the electro-osmotic flow, realizing the operating principle of a macroscopic turbine at the nanoscale. The resulting torques are sufficient to power rotation of nanoscale beads and rods, offering an engineering principle for constructing nanoscale systems powered by electric field.

3D-printed complex-shaped chiral organosilica porous monoliths that display enantioselectivity

We describe a new process for fabricating chiral organosilica 3D complex structures by combining digital light processing 3D printing with a sol-gel polycondensation process. The fabricated low-density objects have a high surface area with hierarchical porosity based on micropores resulting from the materials’ design, and on macropores in the structure resulting from the 3D printing design. Thus, several 3D objects having complex shapes were printed by the polycondensation of 3-acryloxypropyltrimethoxysilane (APTMS) and chiral silane monomers that were obtained by reacting (1R,2R)-cyclohexane-1,2-diamine or (1S,2S)-cyclohexane-1,2-diamine with (3-Isocyanatopropyl)triethoxysilane. The dual-function monomer APTMS enabled both localized photopolymerization and polycondensation. Printed gyroids, cubes, and disk-shaped chiral monoliths successfully revealed the enantioselective adsorption of tryptophan enantiomers. It was found that the macroscopic shape of the monolith affects the adsorption performance and its enantioselectivity. High enantioselectivity was obtained when the objects were formed from a chiral silane synthesized from (1R,2R)-cyclohexane-1,2-diamine: L-tryptophan was adsorbed ~10 fold higher than D-tryptophan. When the chiral object was fabricated using a chiral silane monomer prepared from (1S,2S)-cyclohexane-1,2-diamine, the enantioselectivity of the adsorption was reversed towards the D-tryptophan isomer. The new approach utilizes the 3D printing methodologies developed here for all-printed enantioselective separation columns; the printed macroporosity facilitates efficient flow, and the meso/microporous walls facilitate enantioselectivity. • A new UV-curable chiral hybrid material was synthesized. • 3D printing of complex structures of chiral organosilica material by stereolithography. • The printed objects exhibit highly selective enantiomeric adsorption. • The printed objects have high macro and micro surface areas. • The process enables the fabrication of new improved customized columns for enantiomeric separation.

Chirality-dependent roughness of magnetic domain walls

Dzyaloshinskii–Moriya interaction (DMI) plays a key role in the formation and stability of chiral magnetic objects, such as skyrmions and chiral domain-walls (DWs), which are used as building blocks of emerging high-density and high-speed devices. Here, we demonstrate that the DMI determines also the roughness of chiral DWs. Such DMI-dependent chiral roughness was observed from magnetic multilayer films of the Pt/Co/Pt heterostructure. Despite the stochastic nature of the DW roughness, the films clearly showed the roughness variation with respect to the DW chirality in balance with the DMI. The present analysis of the DW roughness offers a way to quantitatively determine the DMI-induced effective field as an essential parameter for the stability and dynamic characteristics of chiral DWs.

Onset of Chirality in Plasmonic Meta-Molecules and Dielectric Coupling.

Chirality is a fundamental feature in all domains of nature, ranging from particle physics over electromagnetism to chemistry and biology. Chiral objects lack a mirror plane and inversion symmetry and therefore cannot be spatially aligned with their mirrored counterpart, their enantiomer. Both natural molecules and artificial chiral nanostructures can be characterized by their light-matter interaction, which is reflected in circular dichroism (CD). Using DNA origami, we assemble model meta-molecules from multiple plasmonic nanoparticles, representing meta-atoms accurately positioned in space. This allows us to reconstruct piece by piece the impact of varying macromolecular geometries on their surrounding optical near fields. Next to the emergence of CD signatures in the instance that we architect a third dimension, we design and implement sign-flipping signals through addition or removal of single particles in the artificial molecules. Our data and theoretical modeling reveal the hitherto unrecognized phenomenon of chiral plasmonic-dielectric coupling, explaining the intricate electromagnetic interactions within hybrid DNA-based plasmonic nanostructures.

Optical tractor beam for a cluster of plasmonic and dielectric and chiral Mie objects

A light beam pushes any object towards the direction of its propagation as light or quantum of light, photon, carries momentum. However, a light beam can also pull a particle backward towards the source — regarded as an optical tractor beam. Despite a series of phenomenal works being done in the field of optical manipulation using optical tractor beams, optical setup that can enable the optical pulling of all objects (i.e., dielectric, plasmonic and chiral) is still extremely rare. This work stands as the very first proposal of a simple and generic all-optical setup, where attaining optical tractor beam effect over all objects within a cluster is facilitated. Such optical pulling effect is observed only over a dielectric substrate (but not over plasmonic substrate) when two simple plane waves superimpose appropriately maintaining certain conditions. The emergence of such counterintuitive force has been explained based on distinct physical phenomena. For instance, in case of the dielectric and chiral nanoparticles, origin of the pulling force has been explained based on the field induced multipole radiation; whereas, in case of the plasmonic scatterer, the pulling has been explained based on the reversal of induced current density within the object.

Spin-momentum locking in optical rectification

The authors report the direct electrical detection of spin-momentum locking, a mechanism underlying the surface-plasmon-polariton waves, via direct measurement of the optical rectification current induced in an asymmetric metallic grating by a circularly polarized light. The work could potentially be useful for the surface-plasmon-mediated electrical detection of chiral objects to distinguish enantiomers that are otherwise indistinguishable.

Cylindrical wave excitation of a chiral cylindrical object covered with a metasurface

Cylindrical wave scattering characteristics of an infinite chiral cylindrical object of circular cross section and covered with a metasurface has been studied numerically. The field expressions inside a chiral cylindrical object have been derived using the wave field decomposition approach. Analytic expressions of expansion coefficients inside chiral cylinder and scattering coefficients outside chiral cylinder have also been derived in the simplest forms. The influences of chirality parameter, surface reactance of metasurface and locations of cylindrical wave source upon scattering gain have been investigated. It is studied that by varying the chirality and surface reactance, one can significantly control the scattering gain. The proposed theory is also extended to the two dimensional strip grid metasurface covered frequency dispersive chiral cylinder. It is found that the specifically designed two dimensional strip grid metasurface can be used to significantly reduce the back scattering gain from a chiral cylinder at a certain frequency of interest.