Optofluidic sorting of material chirality by chiral light

Unveiling the Loss Mode Enabled Tunable Plasmonic Chirality at Flat Metal Surface

Plasmonic chirality has garnered significant interest in the past decade due to its enhanced chiral light-matter interactions. Current methods for achieving plasmonic chirality often rely on complex nanostructures or metamaterials, which are hampered by intricate fabrication processes. In this work, we present an approach to generate plasmonic chiral structured surface plasmon polariton (s-SPP) fields on a single, flat metal surface, bypassing elaborate fabrication techniques. The plasmonic chiral s-SPP fields are excited by the superposition of multiple differently oriented transverse magnetic polarized plane waves. We demonstrate, both theoretically and experimentally, the flexible tuning of chiral plasmonic patterns by adjusting the symmetry and phase differences of the incident waves. This method provides a facile mean to optically tailor plasmonic chiral properties on a subwavelength scale, offering potential applications in sensing, enantioselective reactions, imaging, and reconfigurable chiral switches.

Harnessing Enantioselective Optical Forces by Quasibound States in the Continuum

Enantioselective optical forces have garnered significant attention, because they provide a noninvasive means to separate chiral objects. A promising approach to enhance enantioselective optical forces is spatially overlapping and boosting electric and magnetic fields to create giant superchiral fields. Here, we utilize metasurfaces composed of asymmetric silicon dimers that support two distinct quasibound states in the continuum (quasi BICs). By precisely engineering these quasi BICs, we achieve nearly perfect spatial overlap of electric and magnetic fields near their anticrossing point, resulting in a remarkable 10^{4}-fold enhancement of the superchiral field. Consequently, the enantioselective optical force exerting on a single molecule exhibits a substantial increase, with magnitude up to pN/mW μm^{2}. Furthermore, by encircling the anticrossing point, we can switch the handedness of the superchiral field and the enantioselective optical force. Last, we analyze the dynamics of quasi-BIC-assisted chiral separation, highlighting its potential applications in chiral sensing and sorting, circular dichroism spectroscopy, and pharmacology.

Nanoscale Helical Optical Force for Determining Crystal Chirality

The deterministic control of material chirality has been a sought-after goal. As light possesses intrinsic chirality, light-matter interactions offer promising avenues for achieving non-contact, enantioselective optical induction, assembly, or sorting of chiral entities. However, experimental validations are confined to the microscale due to the limited strength of asymmetrical interactions within sub-diffraction limit ranges. In this study, a novel approach is presented to facilitate chirality modulation through chiral crystallization using a helical optical force field originating from localized nanogap surface plasmon resonance. The force field emerges near a gold trimer nanogap and is propelled by linear and angular momentum transfer from the incident light to the resonant nanogap plasmon. By employing Gaussian and Laguerre-Gaussian incident laser beams, notable enantioselectivity is achieved through low-power plasmon-induced chiral crystallization of an organic compound-ethylenediamine sulfate. The findings provide new insights into chirality transmission orchestrated by the exchange of linear and angular momentum between light and nanomaterials.

Enantioselective transport of chiral spheres using focused femtosecond laser pulses

Optical tweezers are commonly used for manipulating chiral particles by tailoring the properties of the electromagnetic field or of the particles themselves. Non-linearity provides additional degree of freedom to control the manipulation by changing the trapping conditions. In this work, we leverage the nonlinear optical properties of a medium by illuminating it with a circularly polarized laser pulse, enabling single particle enantioselection for the chiral spheres immersed in it. By adjusting the power of the laser pulses, we demonstrate stable trapping of chiral spheres with one handedness near the focal region, while spheres with the opposite handedness are repelled. This enables the chiral resolution of racemic mixtures. Additionally, we perturbed the stable equilibrium position of the trap by driving the sample stage, leading to the emergence of a new stable equilibrium position achieved under the action of the Stokes force. Here we show that the chirality of each individually trapped particle can also be characterized by the rotation of the equilibrium position. Since the power of the laser pulses can be experimentally controlled, this scheme is practical to perform enantioselection, chiral characterization, and chiral resolution of a single chiral sphere with arbitrarily small chirality parameters.

Mid-Infrared Optical Force Chromatography of Microspheres Containing Siloxane Bonds

Recent interest in particle sorting using optical forces has grown due to its ability to separate micro- and nanomaterials based on their optical properties. Here, we present a mid-infrared optical force manipulation technique that enables precise sorting of microspheres based on their molecular vibrational properties using a mid-infrared quantum cascade laser. Utilizing the optical pushing force driven by a 9.3 μm mid-infrared evanescent field generated on a prism through total internal reflection, a variety of microspheres, including those composed of Si-O-Si bonds, can be separated in accordance with their absorbance values at 9.3 μm. The experimental results are in good agreement with the optical force calculations using finite-difference time-domain simulation. Thus, each microsphere's displacement and velocity can be predicted from the absorbance value; conversely, the optical properties (e.g., absorbance and complex refractive index in the mid-infrared region) of individual microspheres can be estimated by monitoring their velocity.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Optofluidic Tweezers: Efficient and Versatile Micro/Nano-Manipulation Tools

Optical tweezers (OTs) can transfer light momentum to particles, achieving the precise manipulation of particles through optical forces. Due to the properties of non-contact and precise control, OTs have provided a gateway for exploring the mysteries behind nonlinear optics, soft-condensed-matter physics, molecular biology, and analytical chemistry. In recent years, OTs have been combined with microfluidic chips to overcome their limitations in, for instance, speed and efficiency, creating a technology known as "optofluidic tweezers." This paper describes static OTs briefly first. Next, we overview recent developments in optofluidic tweezers, summarizing advancements in capture, manipulation, sorting, and measurement based on different technologies. The focus is on various kinds of optofluidic tweezers, such as holographic optical tweezers, photonic-crystal optical tweezers, and waveguide optical tweezers. Moreover, there is a continuing trend of combining optofluidic tweezers with other techniques to achieve greater functionality, such as antigen-antibody interactions and Raman tweezers. We conclude by summarizing the main challenges and future directions in this research field.

Uni- and Bidirectional Rotation and Speed Control in Chiral Photonic Micromotors Powered by Light

Liquid crystalline polymers are attractive materials for untethered miniature soft robots. When they contain azo dyes, they acquire light-responsive actuation properties. However, the manipulation of such photoresponsive polymers at the micrometer scale remains largely unexplored. Here, uni- and bidirectional rotation and speed control of polymerized azo-containing chiral liquid crystalline photonic microparticles powered by light is reported. The rotation of these polymer particles is first studied in an optical trap experimentally and theoretically. The micro-sized polymer particles respond to the handedness of a circularly polarized trapping laser due to their chirality and exhibit uni- and bidirectional rotation depending on their alignment within the optical tweezers. The attained optical torque causes the particles to spin with a rotation rate of several hertz. The angular speed can be controlled by small structural changes, induced by ultraviolet (UV) light absorption. After switching off the UV illumination, the particle recovers its rotation speed. The results provide evidence of uni- and bidirectional motion and speed control in light-responsive polymer particles and offer a new way to devise light-controlled rotary microengines at the micrometer scale.

Enantioselective optical trapping of single chiral molecules in the superchiral field vicinity of metal nanostructures

In this study, we theoretically analyzed the optical force acting on single chiral molecules in the plasmon field induced by metallic nanostructures. Using the extended discrete dipole approximation, we quantitatively examined the optical response of single chiral molecules in the localized plasmon by numerically analyzing the internal polarization structure of the molecules obtained from quantum chemical calculations, without phenomenological treatment. We evaluated the chiral gradient force due to the optical chirality gradient of the superchiral field near the metallic nanostructures for chiral molecules. Our calculation method can be used to evaluate the molecular-orientation dependence and rotational torque by considering the chiral spatial structure inside the molecules. We theoretically showed that the superchiral field induced by chiral plasmonic nanostructures can be used to selectively optically capture the enantiomers of a single chiral molecule.

Ultrafast chirality: the road to efficient chiral measurements

Today we are witnessing the electric-dipole revolution in chiral measurements. Here we reflect on its lessons and outcomes, such as the perspective on chiral measurements using the complementary principles of "chiral reagent" and "chiral observer", the hierarchy of scalar, vectorial and tensorial enantio-sensitive observables, the new properties of the chiro-optical response in the ultrafast and non-linear domains, and the geometrical magnetism associated with the chiral response in photoionization. The electric-dipole revolution is a landmark event. It has opened routes to extremely efficient enantio-discrimination with a family of new methods. These methods are governed by the same principles but work in vastly different regimes - from microwaves to optical light; they address all molecular degrees of freedom - electronic, vibrational and rotational, and use flexible detection schemes, i.e. detecting photons or electrons, making them applicable to different chiral phases, from gases to liquids to amorphous solids. The electric-dipole revolution has also enabled enantio-sensitive manipulation of chiral molecules with light. This manipulation includes exciting and controlling ultrafast helical currents in vibronic states of chiral molecules, enantio-sensitive control of populations in electronic, vibronic and rotational molecular states, and opens the way to efficient enantio-separation and enantio-sensitive trapping of chiral molecules. The word "perspective" has two meanings: an "outlook" and a "point of view". In this perspective article, we have tried to cover both meanings.

Optical gradient force on chiral particles

When a chiral nanoparticle is optically trapped using a circularly polarized laser beam, a circular polarization (CP)–dependent gradient force can be induced on the particle. We investigated the CP-dependent gradient force exerted on three-dimensional chiral nanoparticles. The experimental results showed that the gradient force depended on the handedness of the CP of the trapping light and the particle chirality. The analysis revealed that the spectral features of the CP handedness–dependent gradient force are influenced not only by the real part of the refractive index but also by the electromagnetic field perturbed by the chiral particle resonant with the incident light. This is in sharp contrast to the well-known behavior of the gradient force, which is governed by the real part of the refractive index. The extended aspect of the chiral optical force obtained here can provide novel methodologies on chirality sensing, manipulation, separation, enantioselective biological reactions, and other fields.

Polariscopy with optical near-fields

Polarisation analysis of light-matter interactions established for propagating optical far-fields is now extended into an evanescent field as demonstrated in this study using an attenuated total reflection (ATR) setup and a synchrotron source at THz frequencies. Scalar intensity E², rather than a vector E-field, is used for absorbance analysis of the s- and p-components of the linearly polarised incident light. Absorption and phase changes induced by the sample and detected at the transmission port of the ATR accessory revealed previously non-accessible anisotropy in the absorption-dispersion properties of the sample probed by the evanescent optical near-field. Mapping of the sample's anisotropy perpendicular to its surface by the non-propagating light field is validated and the cos² θ absorbance dependence was observed for the angle θ, where θ = 0° is aligned with the sample's surface. A four-polarisation method is presented for the absorbance mapping and a complimentary retardance spectrum is retrieved from the same measurement of the angular dependence of transmittance in structurally complex poly-hydroxybutyrate (PHB) and poly-L-lactic acid (PLLA) samples with amorphous and banded-spherulite (radially isotropic) crystalline regions. A possibility of all 3D mapping of anisotropy (polarisation tomography) is outlined.

Identification and separation of chiral particles by focused circularly polarized vortex beams

The identification and separation of chiral substances are of importance in the biological, chemical, and pharmaceutical industries. Here, we demonstrate that a focused circularly polarized vortex beam can, in the focal plane, selectively trap and rotate chiral dipolar particles via radial and azimuthal optical forces. The handedness and topological charge of the incident beam have strong influence on identifying and separating behavior: left- and right-handed circular polarizations lead to opposite effects on the particle of trapping and rotating, while the sign of topological charge will change the particle's rotation direction. Such effects are a direct result of the handedness and topological charge manifesting themselves in the directions of the spin angular momentum (SAM) and Poynting vector. The research provides insight into the chiral light-matter interaction and may find potential application in the identification and separation of chiral nanoparticles.

Optical separation and discrimination of chiral particles by vector beams with orbital angular momentum

Chirality describes a reduced symmetry and abounds in nature. The handedness-dependent response usually occurs only when a chiral object interacts with another chiral entity. Light carrying orbital angular momentum (OAM) is inherently chiral due to the helical wave front. Here, we put forward a scheme that enables optical separation and simultaneous discrimination of single chiral particles using focused vector beams with OAM. Such focused vector vortex beams carrying radial-splitting optical chirality can selectively trap one enantiomer inside or outside the intensity maxima depending on the sign of the OAM. The particles with different chirality parameters can be trapped on different orbits and experience enhanced orbital motion. Moreover, the magnitude of OAM as well as the size of particle plays an important role in the chiral separation and discrimination. In addition to particle manipulation, the discussion of OAM in chiral light-matter interactions has potential application in, for example, optical enantioseparation or chiral detection.

Optical Trapping Separation of Chiral Nanoparticles by Subwavelength Slot Waveguides

Enantiomer separation opens great opportunities to develop the technologies of pharmaceutics, chemicals, and biomedicine, but faces daunting challenges. Here, we discover a considerable chiral-dependent trapping force to separate nanometer-scale enantiomers in a new silicon-based waveguide platform. The electromagnetic chirality gradient of strongly confined evanescent fields can be largely enhanced by the counterpropagating slot waveguides so that the resulting chiral gradient forces can shift the trapping equilibrium positions of dielectric gradient forces. Especially, there exists a transitional width for the slot waveguides to exchange the trapping equilibrium positions between two opposite enantiomers. Our thoroughly numerical investigations demonstrate that the chiral-separable slot waveguides here can offer high efficiency and feasibility of separating chiral nanoparticles, and may pave a route toward new on-chip chiral optical tweezers or optofluidic transport systems for large-scale chiral separation.

Formulation of resonant optical force based on the microscopic structure of chiral molecules

Optical manipulation, exemplified by Ashkin's optical tweezers, is a promising technique in the fields of bioscience and chemistry, as it enables the non-destructive and non-contact selective transport or manipulation of small particles. To realize the separation of chiral molecules, several researchers have reported on the use of light and discussed feasibility of selection. Although the separation of micrometer-sized chiral molecules has been experimentally demonstrated, the separation of nanometer-sized chiral molecules, which are considerably smaller than the wavelength of light, remains challenging. Therefore, we formulated an optical force under electronic resonance to enhance the optical force and enable selective manipulation. In particular, we incorporated the microscopic structures of molecular dipoles into the nonlocal optical response theory. The analytical expression of optical force could clarify the mechanism of selection exertion of the resonant optical force on chiral molecules. Furthermore, we quantitatively evaluated the light intensity and light exposure time required to separate a single molecule in a solvent. The results can facilitate the design of future schemes for the selective optical manipulation of chiral molecules.

Enantio-detection via cavity-assisted three-photon processes

We propose a method for enantio-detection of chiral molecules based on a cavity-molecule system, where the left- and right-handed molecules are coupled with a cavity and two classical light fields to form cyclic three-level models. Via the cavity-assisted three-photon processes based on the cyclic three-level model, photons are generated continuously in the cavity even in the absence of external driving to the cavity. However, the photonic fields generated from the three-photon processes of left- and right-handed molecules differ with the phase difference π according to the inherent properties of electric-dipole transition moments of enantiomers. This provides a potential way to detect the enantiomeric excess of chiral mixture by monitoring the output field of the cavity.

On-chip transporting arresting and characterizing individual nano-objects in biological ionic liquids

Understanding and controlling the individual behavior of nanoscopic matter in liquids, the environment in which many such entities are functioning, is both inherently challenging and important to many natural and man-made applications. Here, we transport individual nano-objects, from an assembly in a biological ionic solution, through a nanochannel network and confine them in electrokinetic nanovalves, created by the collaborative effect of an applied ac electric field and a rationally engineered nanotopography, locally amplifying this field. The motion of so-confined fluorescent nano-objects is tracked, and its kinetics provides important information, enabling the determination of their particle diffusion coefficient, hydrodynamic radius, and electrical conductivity, which are elucidated for artificial polystyrene nanospheres and subsequently for sub-100-nm conjugated polymer nanoparticles and adenoviruses. The on-chip, individual nano-object resolution method presented here is a powerful approach to aid research and development in broad application areas such as medicine, chemistry, and biology.

Separating and trapping of chiral nanoparticles with dielectric photonic crystal slabs

Chiral separation is a crucial step in many chemical synthesis processes, particularly for pharmaceuticals. Here we present a novel method for the realization of both separating and trapping of enantiomers using the dielectric photonic crystal (PhC) slabs, which possess quasi-fourfold degenerate Bloch modes (overlapping double degenerate transverse-electric-like and transverse-magnetic-like modes). Based on the designed structure, a large gradient of optical chirality appears near the PhC slab, leading to the extreme enhancement of chiral optical forces about 3 orders of magnitude larger than those obtained with circularly polarized lights. In this case, our method provides a reference for realizing all-optical enantiopure syntheses.

Review of optical sensing and manipulation of chiral molecules and nanostructures with the focus on plasmonic enhancements [Invited]

Chiral molecules are ubiquitous in nature; many important synthetic chemicals and drugs are chiral. Detecting chiral molecules and separating the enantiomers is difficult because their physiochemical properties can be very similar. Here we review the optical approaches that are emerging for detecting and manipulating chiral molecules and chiral nanostructures. Our review focuses on the methods that have used plasmonics to enhance the chiroptical response. We also review the fabrication and assembly of (dynamic) chiral plasmonic nanosystems in this context.

Enantioselection and chiral sorting of single microspheres using optical pulling forces

We put forward a novel, twofold scheme that enables, at the same time, all-optical enantioselection and sorting of single multipolar chiral microspheres based on optical pulling forces exerted by two non-collinear, non-structured, circularly polarized light sources. Our chiral resolution method can be externally controlled by varying the angle between their incident wavevectors, allowing for fine-tuning of the range of chiral indices for enantioselection. Enantioselectivity is achieved by choosing angles such that only particles with the same handedness of the light sources are pulled. This proposal allows one to achieve all-optical sorting of chiral microspheres with arbitrarily small chiral parameters, thus outperforming current optical methods.

Simulations and experimental demonstration of three different regimes of optofluidic manipulation

It has been demonstrated that optically controlled microcurrents can be used to capture and move around a variety of microscopic objects ranging from cells and nanowires to whole live worms. Here, we present our findings on several new regimes of optofluidic manipulation that can be engineered using careful design of microcurrents. We theoretically optimize these regimes using COMSOL Multiphysics and present three sets of simulations and corresponding optofluidic experiments. In the first regime, we use local fluid heating to create a microcurrent with a symmetric toroid shape capturing particles in the center. In the second regime, the microcurrent shifts and tilts because external fluid flow is introduced into the microfluidic channel. In the third regime, the whole microfluidic channel is tilted, and the resulting microcurrent projects particles in a fan-like fashion. All three configurations provide interesting opportunities to manipulate small particles in fluid droplets and microfluidic channels.

Orbital angular momentum beams generation from 61 channels coherent beam combining femtosecond digital laser

We report on the use of a 61 beamlets coherent beam combination femtosecond fiber amplifiers as a digital laser source to generate high-power orbital angular momentum beams. Such an approach opens the path for higher-order non-symmetrical user-defined far field distributions.

Novel optofluidic concepts enabled by topological microfluidics-INVITED

The coupling between flow and director orientation of liquid crystals (LCs) has been long utilized to devise wide-ranging applications spanning modern displays, medical and environmental solutions, and bio-inspired designs and applications. LC-based optofluidic platforms offer a non-invasive handle to modulate light and material fields, both locally and dynamically. The flow-driven reorientation of the LC molecules can tailor distinct optical and mechanical responses in microfluidic confinements, and harness the coupling therein. Yet the synergy between traditional optofluidics with isotropic fluids and LC microfluidics remains at its infancy. Here, we discuss emerging optofluidic concepts based on Topological Microfluidics, leveraging microfluidic control of topological defects and defect landscapes. With a specific focus on the role of surface anchoring and microfluidic geometry, we present recent and ongoing works that harness flow-controlled director and defect configurations to modulate optical fields. The flow-induced optical attributes, and the corresponding feedback, is enhanced in the vicinity of the topological defects which geenerate distinct isotropic opto-material properties within an anisotropic matrix. By harnessing the rich interplay of confining geometry, anchoring and micro-scale nematodynamics, topological microfluidics offers a promising platform to ideate the next generation of optofluidic and optomechnical concepts.

Enantioselective optical trapping of chiral nanoparticles using a transverse optical needle field with a transverse spin

Since the fundamental building blocks of life are built of chiral amino acids and chiral sugar, enantiomer separation is of great interest in plenty of chemical syntheses. Light-chiral material interaction leads to a unique chiral optical force, which possesses opposite directions for specimens with different handedness. However, usually the enantioselective sorting is challenging in optical tweezers due to the dominating achiral force. In this work, we propose an optical technique to sort chiral specimens by use of a transverse optical needle field with a transverse spin (TONFTS), which is constructed through reversing the radiation patterns from an array of paired orthogonal electric dipoles located in the focal plane of a 4Pi microscopy and experimentally generated with a home-built vectorial optical field generator. It is demonstrated that the transverse component of the photonic spin gives rise to the chiral optical force perpendicular to the direction of the light's propagation, while the transverse achiral gradient force would be dramatically diminished by the uniform intensity profile of the optical needle field. Consequently, chiral nanoparticles with different handedness would be laterally sorted by the TONFTS and trapped at different locations along the optical needle field, providing a feasible route toward all-optical enantiopure chemical syntheses and enantiomer separations in pharmaceuticals.

Tailoring the gradient and scattering forces for longitudinal sorting of generic-size chiral particles

Based on the concepts of conservative and non-conservative optical forces (COF and NCOF), we analyze the physical mechanism of longitudinal chirality sorting along the direction of light propagation in some simple optical fields. It is demonstrated, both numerically and analytically for particle of arbitrary size, that the sorting relies solely on the NCOF, which switches its direction when particle chirality is reversed. For particles larger than half of the optical wavelength λ, the NCOF far surpasses its counterpart COF, enabling the longitudinal chirality sorting. When the particle is much smaller than λ, however, the COF outweighs the NCOF, destroying the sorting mechanism. A scenario is thus proposed that totally eliminates the COF while leaving the sorting NCOF unchanged, extending the applicability of longitudinal chirality sorting to small particles.

Generation of controllable chiral optical fields by vector beams

Chirality is common in nature, describing not only the geometrical property of a three-dimensional object, but also an intrinsic feature of an optical field. Chiral optical fields are attracting increasing attention due to their potential applications in chiral light-matter interaction. Here we demonstrate a strategy to realize a controllable chiral optical field by tightly focusing two tailored vector beams in a 4π optical microscopic system. By modulating the wavefronts of the incident vector beams with appropriately designed phase masks, a chiral optical field with multiple spots carrying switchable handedness or controllable chirality can be generated. The location, the number and the handedness of such chiral spots can be arbitrarily adjusted depending on the actual application requirements. In addition to trapping and manipulating multiple particles, this controllable chiral optical field may find applications in enantioselective separation, chiral detection and chiral sensing at the nanoscale.

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

Lateral optical forces induced by linearly polarized laser beams have been predicted to deflect dipolar particles with opposite chiralities toward opposite transversal directions. These "chirality-dependent" forces can offer new possibilities for passive all-optical enantioselective sorting of chiral particles, which is essential to the nanoscience and drug industries. However, previous chiral sorting experiments focused on large particles with diameters in the geometrical-optics regime. Here, we demonstrate, for the first time, the robust sorting of Mie (size ~ wavelength) chiral particles with different handedness at an air-water interface using optical lateral forces induced by a single linearly polarized laser beam. The nontrivial physical interactions underlying these chirality-dependent forces distinctly differ from those predicted for dipolar or geometrical-optics particles. The lateral forces emerge from a complex interplay between the light polarization, lateral momentum enhancement, and out-of-plane light refraction at the particle-water interface. The sign of the lateral force could be reversed by changing the particle size, incident angle, and polarization of the obliquely incident light.

Nanophotonic Platforms for Chiral Sensing and Separation

Chirality in Nature can be found across all length scales, from the subatomic to the galactic. At the molecular scale, the spatial dissymmetry in the atomic arrangements of pairs of mirror-image molecules, known as enantiomers, gives rise to fascinating and often critical differences in chemical and physical properties. With increasing hierarchical complexity, protein function, cell communication, and organism health rely on enantioselective interactions between molecules with selective handedness. For example, neurodegenerative and neuropsychiatric disorders including Alzheimer's and Parkinson's diseases have been linked to distortion of chiral-molecular structure. Moreover, d-amino acids have become increasingly recognized as potential biomarkers, necessitating comprehensive analytical methods for diagnosis that are capable of distinguishing l- from d-forms and quantifying trace concentrations of d-amino acids. Correspondingly, many pharmaceuticals and agrochemicals consist of chiral molecules that target particular enantioselective pathways. Yet, despite the importance of molecular chirality, it remains challenging to sense and to separate chiral compounds. Chiral-optical spectroscopies are designed to analyze the purity of chiral samples, but they are often insensitive to the trace enantiomeric excess that might be present in a patient sample, such as blood, urine, or sputum, or pharmaceutical product. Similarly, existing separation schemes to enable enantiopure solutions of chiral products are inefficient or costly. Consequently, most pharmaceuticals or agrochemicals are sold as racemic mixtures, with reduced efficacy and potential deleterious impacts.Recent advances in nanophotonics lay the foundation toward highly sensitive and efficient chiral detection and separation methods. In this Account, we highlight our group's effort to leverage nanoscale chiral light-matter interactions to detect, characterize, and separate enantiomers, potentially down to the single molecule level. Notably, certain resonant nanostructures can significantly enhance circular dichroism for improved chiral sensing and spectroscopy as well as high-yield enantioselective photochemistry. We first describe how achiral metallic and dielectric nanostructures can be utilized to increase the local optical chirality density by engineering the coupling between electric and magnetic optical resonances. While plasmonic nanoparticles locally enhance the optical chirality density, high-index dielectric nanoparticles can enable large-volume and uniform-sign enhancements in the optical chirality density. By overlapping these electric and magnetic resonances, local chiral fields can be enhanced by several orders of magnitude. We show how these design rules can enable high-yield enantioselective photochemistry and project a 2000-fold improvement in the yield of a photoionization reaction. Next, we discuss how optical forces can enable selective manipulation and separation of enantiomers. We describe the design of low-power enantioselective optical tweezers with the ability to trap sub-10 nm dielectric particles. We also characterize their chiral-optical forces with high spatial and force resolution using combined optical and atomic force microscopy. These optical tweezers exhibit an enantioselective optical force contrast exceeding 10 pN, enabling selective attraction or repulsion of enantiomers based on the illumination polarization. Finally, we discuss future challenges and opportunities spanning fundamental research to technology translation. Disease detection in the clinic as well as pharmaceutical and agrochemical industrial applications requiring large-scale, high-throughput production will gain particular benefit from the simplicity and relative low cost that nanophotonic platforms promise.

Enantioselective manipulation of single chiral nanoparticles using optical tweezers

We put forward an enantioselective method for chiral nanoparticles using optical tweezers. We demonstrate that the optical trapping force in a typical, realistic optical tweezing setup with circularly-polarized trapping beams is sensitive to the chirality of core-shell nanoparticles, allowing for efficient enantioselection. It turns out that the handedness of the trapped particles can be selected by choosing the appropriate circular polarization of the trapping beam. The chirality of each individual trapped nanoparticle can be characterized by measuring the rotation of the equilibrium position under the effect of a transverse Stokes drag force. We show that the chirality of the shell gives rise to an additional twist, leading to a strong enhancement of the optical torque driving the rotation. Both methods are shown to be robust against variations of size and material parameters, demonstrating that they are particularly useful in (but not restricted to) several situations of practical interest in chiral plasmonics, where enantioselection and characterization of single chiral nanoparticles, each and every one with its unique handedness and optical properties, are in order. In particular, our method could be employed to unveil the chiral response arising from disorder in individual plasmonic raspberries, synthesized by close-packing a large number of metallic nanospheres around a dielectric core.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Characterization of optofluidic devices for the sorting of sub-micrometer particles

In this work, we investigate methods of fabricating a device for the optical actuation of nanoparticles. To create the microfluidic channel, we pursued three fabrication methods: SU-8 to molded polydimethylsiloxane soft lithography, laser etching of glass, and deep reactive ion etching of fused silica. We measured the surface roughness of the etched sidewalls, and the laser power transmission through each device. We then measured the radiation pressure on 0.5-µm particles in the best-performing fabricated device (etched fused silica) and in a square glass capillary.

Generation of optical chirality patterns with plane waves, evanescent waves and surface plasmon waves

We systematically investigate the generation of optical chirality patterns by applying the superposition of two waves in three scenarios, namely free-space plane waves, evanescent waves of totally reflected light at dielectric interface and propagating surface plasmon waves on a metallic surface. In each scenario, the general analytical solution of the optical chirality pattern is derived for different polarization states and propagating directions of the two waves. The analytical solutions are verified by numerical simulations. Spatially structured optical chirality patterns can be generated in all scenarios if the incident polarization states and propagation directions are correctly chosen. Optical chirality enhancement can be obtained from the constructive interference of free-space circularly polarized light or enhanced evanescent waves of totally reflected light. Surface plasmon waves do not provide enhanced optical chirality unless the near-field intensity enhancement is sufficiently high. The structured optical chirality patterns may find applications in chirality sorting, chiral imaging and circular dichroism spectroscopy.

Influence of chirality on fluorescence and resonance energy transfer

Electronically excited molecules frequently exhibit two distinctive decay mechanisms that rapidly generate optical emission: one is direct fluorescence and the other is energy transfer to a neighboring component. In the latter, the process leading to the ensuing "indirect" fluorescence is known as FRET, or fluorescence resonance energy transfer. For chiral molecules, both fluorescence and FRET exhibit discriminatory behavior with respect to optical and material handedness. While chiral effects such as circular dichroism are well known, as too is chiral discrimination for FRET in isolation, this article presents a study on a stepwise mechanism that involves both. Chirally sensitive processes follow excitation through the absorption of circularly polarized light and are manifest in either direct or indirect fluorescence. Following recent studies setting down the symmetry principles, this analysis provides a rigorous, quantum outlook that complements and expands on these works. Circumventing expressions that contain complicated tensorial components, our results are amenable for determining representative numerical values for the relative importance of the various coupling processes. We discover that circular dichroism exerts a major influence on both fluorescence and FRET, and resolving the engagement of chirality in each component reveals the distinct roles of absorption and emission by, and between, donor and acceptor pairs. It emerges that chiral discrimination in the FRET stage is not, as might have been expected, the main arbiter in the stepwise mechanism. In the concluding discussion on various concepts, attention is focused on the validity of helicity transfer in FRET.

Separation of chiral enantiomers by optical force and torque induced by tightly focused vector polarized hollow beams

Enantioseparation is important for biology, chemistry and even pharmaceutical industries. We propose an approach for discriminating and separating chiral enantiomers by tightly focused vector polarized hollow beams, which possess a transverse spin angular momentum that can rotate the chiral particles along the transverse direction. We demonstrate the different optomechanical behaviours of the particles upon illumination with different vector polarized (azimuthally and radially) hollow beams by numerically calculating the optical force and spin torque. It is believed that this interesting approach may have potential applications in enantioseparation due to its simplicity and accessibility.

Orientational structures in cholesteric droplets with homeotropic surface anchoring

The dependency of orientational structures in cholesteric droplets with homeotropic surface anchoring on the helicity parameter has been studied by experiment and simulations. We have observed a sequence of structures, in which the director configurations and topological defects were identified by comparison of polarized microscopy pictures with simulated textures. A toron-like and low-symmetry intermediate layer-like structures have been revealed and studied in detail. The ranges of stability of the observed structures have been summarized in a general diagram and explained by the helicity parameter dependence of the free energy terms.

Mechanical chiral resolution

Mechanical interactions of chiral objects with their environment are well-established at the macroscale, like a propeller on a plane or a rudder on a boat. At the colloidal scale and smaller, however, such interactions are often not considered or deemed irrelevant due to Brownian motion. As we will show in this tutorial review, mechanical interactions do have significant effects on chiral objects at all scales, and can be induced using shearing surfaces, collisions with walls or repetitive microstructures, fluid flows, or by applying electrical or optical forces. Achieving chiral resolution by mechanical means is very promising in the field of soft matter and to industry, but has not received much attention so far.

Optical lateral forces and torques induced by chiral surface-plasmon-polaritons and their potential applications in recognition and separation of chiral enantiomers

Surface plasmon polaritons carry an intrinsic transverse spin angular momentum which is locked to their propagation direction due to the quantum spin Hall effect of light. We study the chirality-sorting lateral optical forces arising from this phenomenon in a Kretschmann configuration. We show that the characteristics of surface plasmon polaritons are affected by small changes of the environment's chirality. This property can be utilized to detect the medium's chirality in the macro-world. Furthermore, we explain how the the lateral forces and optical torques interact with the chirality of nano-particles located or adsorbed in the vicinity of the surface in the micro-world. Finally, we demonstrate that introducing one handedness of chirality to the dielectric medium in the Kretschmann configuration will benefit the discrimination of chiral enantiomers compared with the nonchiral case. Our work presents physical insights into chirality-selective lateral forces using surface plasmon polaritons and provides a new approach to discriminating and separating chiral enantiomers in the chemical and pharmaceutical industries.

Chiral Optical Stern-Gerlach Newtonian Experiment

We report on a chiral optical Stern-Gerlach experiment where chiral liquid crystal microspheres are selectively displaced by means of optical forces arising from optical helicity gradients. The present Newtonian experimental demonstration of an effect predicted at molecular scale [New J. Phys. 16, 013020 (2014)NJOPFM1367-263010.1088/1367-2630/16/1/013020] is a first instrumental step in an area restricted so far to theoretical discussions. Extending the Stern-Gerlach experiment legacy to chiral light-matter interactions should foster further studies, for instance towards the elaboration of chirality-enabled quantum technologies or spin-based optoelectronics.

Chiral optical tweezers for optically active particles in the T-matrix formalism

Modeling optical tweezers in the T-matrix formalism has been of key importance for accurate and efficient calculations of optical forces and their comparison with experiments. Here we extend this formalism to the modeling of chiral optomechanics and optical tweezers where chiral light is used for optical manipulation and trapping of optically active particles. We first use the Bohren decomposition to deal with the light scattering of chiral light on optically active particles. Thus, we show analytically that all the observables (cross sections, asymmetry parameters) are split into a helicity dependent and independent part and study a practical example of a complex resin particle with inner copper-coated stainless steel helices. Then, we apply this chiral T-matrix framework to optical tweezers where a tightly focused chiral field is used to trap an optically active spherical particle, calculate the chiral behaviour of optical trapping stiffnesses and their size scaling, and extend calculations to chiral nanowires and clusters of astrophysical interest. Such general light scattering framework opens perspectives for modeling optical forces on biological materials where optically active amino acids and carbohydrates are present.

Optical transport of fluorescent diamond particles inside a tapered capillary

Optical forces provide an efficient way to sort particles and biological materials according to their optical properties. However, both enhanced optical forces and a large interaction volume are needed in order to optically sort a large number of nanoparticles. We investigate the use of a tapered glass capillary as an optofluidic platform for optical manipulation and optical sorting applications. Tapered capillaries with micrometre and sub-micrometre sizes are fabricated. After filling the tapered capillary with a colloidal solution of red fluorescent diamond particles, a green laser light is coupled into the capillary. The tapered capillary acts both as a microfluidic channel and as an optical waveguide, making it possible for the light to interact with the particles inside the sample solution. Using an incident laser power of few tens of milliwatts, we achieve optical transportation of the brightest particles inside the tapered part of the capillary. Particle velocities as high as few tens of micrometres per second are measured.

Compound droplets derived from a cholesteric suspension of cellulose nanocrystals

This study reports microfluidic generation of Janus droplets that consist of a liquid crystal component (a cholesteric aqueous suspension of cellulose nanocrystals, Ch-CNC) and a mineral oil (MO) component. The composition of the droplets was controlled by varying the relative flow rates of MO and Ch-CNC suspension. The shape of the Ch-CNC component of the droplets was changed from a truncated sphere to a hemisphere to a crescent moon. For these three Ch-CNC phase shapes, the Ch packing of the CNC pseudolayers was preserved, however the Ch pitch reduced, which was ascribed to the change in CNC orientation at the Ch-CNC/MO interface from perpendicular to parallel. The shape of the compound droplets was tuned from a dumbbell to a sphere by reducing interfacial tension between the Ch-CNC suspension and MO phases. Photopolymerization of the monomer mixture introduced in the Ch-CNC phase of the droplets and the removal of the sacrificial MO phase enabled the generation of Ch microgels. The results of this work can be used for exploring new applications of Janus colloids and the fabrication of programmable active soft matter.

Gradient and scattering forces of anti-reflection-coated spheres in an aplanatic beam

Anti-reflection coatings (ARCs) enable one to trap high dielectric spheres that may not be trappable otherwise. Through rigorously calculating the gradient and scattering forces, we directly showed that the improved trapping performance is due to the reduction in scattering force, which originates from the suppression of backscattering by ARC. We further applied ray optics and wave scattering theories to thoroughly understand the underlying mechanism, from which, we inferred that ARC only works for spherical particles trapped near the focus of an aplanatic beam, and it works much better for large spheres. For this reason, in contradiction to our intuition, large ARC-coated spheres are sometimes more trappable than their smaller counter parts. Surprisingly, we discovered a scattering force free zone for a large ARC-coated sphere located near the focus of an aplanatic beam. Our work provides a quantitative study of ARC-coated spheres and bridges the gap between the existing experiments and current conceptual understandings.

Nanoparticle-based Chemiluminescence for Chiral Discrimination of Thiol-Containing Amino Acids

The ability to recognize the molecular chirality of enantiomers is extremely important owing to their critical role in drug development and biochemistry. Convenient discrimination of enantiomers has remained a challenge due to lack of unsophisticated methods. In this work, we have reported a simple strategy for chiral recognition of thiol-containing amino acids including penicillamine (PA), and cysteine (Cys). We have successfully designed a nanoparticle-based chemiluminescence (CL) system based on the reaction between cadmium telluride quantum dots (CdTe QDs) and the enantiomers. The different interactions of CdTe QDs with PA enantiomers or Cys enantiomers led to different CL intensities, resulting in the chiral recognition of these enantiomers. The developed method showed the ability for determination of enantiomeric excess of PA and Cys. It has also obtained an enantioselective concentration range from 1.15 to 9.2 mM for PA. To demonstrate the potential application of this method, the designed platform was applied for the quantification of PA in urine and tablet samples. For the first time, we presented a novel practical application of nanoparticle-based CL system for chiral discrimination.

Mixtures of Blue Phase Liquid Crystal with Simple Liquids: Elastic Emulsions and Cubic Fluid Cylinders

We numerically investigate the behavior of a phase-separating mixture of a blue phase I liquid crystal with an isotropic fluid. The resulting morphology is primarily controlled by an inverse capillary number, χ, setting the balance between interfacial and elastic forces. When χ and the concentration of the isotropic component are both low, the blue phase disclination lattice templates a cubic array of fluid cylinders. For larger χ, the isotropic phase arranges primarily into liquid emulsion droplets which coarsen very slowly, rewiring the blue phase disclination lines into an amorphous elastic network. Our blue phase-simple fluid composites can be externally manipulated: an electric field can trigger a morphological transition between cubic fluid cylinder phases with different topologies.

Three-Dimensional Enantiomeric Recognition of Optically Trapped Single Chiral Nanoparticles

We optically trap freestanding single metallic chiral nanoparticles using a standing-wave optical tweezer. We also incorporate within the trap a polarimetric setup that allows us to perform in situ chiral recognition of single enantiomers. This is done by measuring the S_{3} component of the Stokes vector of a light beam scattered off the trapped nanoparticle in the forward direction. This unique combination of optical trapping and chiral recognition, all implemented within a single setup, opens new perspectives towards the control, recognition, and manipulation of chiral objects at nanometer scales.

Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials

Deep-learning framework has significantly impelled the development of modern machine learning technology by continuously pushing the limit of traditional recognition and processing of images, speech, and videos. In the meantime, it starts to penetrate other disciplines, such as biology, genetics, materials science, and physics. Here, we report a deep-learning-based model, comprising two bidirectional neural networks assembled by a partial stacking strategy, to automatically design and optimize three-dimensional chiral metamaterials with strong chiroptical responses at predesignated wavelengths. The model can help to discover the intricate, nonintuitive relationship between a metamaterial structure and its optical responses from a number of training examples, which circumvents the time-consuming, case-by-case numerical simulations in conventional metamaterial designs. This approach not only realizes the forward prediction of optical performance much more accurately and efficiently but also enables one to inversely retrieve designs from given requirements. Our results demonstrate that such a data-driven model can be applied as a very powerful tool in studying complicated light-matter interactions and accelerating the on-demand design of nanophotonic devices, systems, and architectures for real world applications.

Wavelength-tunable and shape-reconfigurable photonic capsule resonators containing cholesteric liquid crystals

Cholesteric liquid crystals (CLCs) have a photonic bandgap due to the periodic change of refractive index along their helical axes. The CLCs containing optical gain have served as band-edge lasing resonators. In particular, CLCs in a granular format provide omnidirectional lasing, which are promising as a point light source. However, there is no platform that simultaneously achieves high stability in air and wavelength tunability. We encapsulate CLCs with double shells to design a capsule-type laser resonator. The fluidic CLCs are fully enclosed by an aqueous inner shell that promotes the planar alignment of LC molecules along the interface. The outer shell made of silicone elastomer protects the CLC core and the inner shell from the surroundings. Therefore, the helical axes of the CLCs are radially oriented within the capsules, which provide a stable omnidirectional lasing in the air. At the same time, the fluidic CLCs enable the fine-tuning of lasing wavelength with temperature. The capsules retain their double-shell structure during the dynamic deformation. Therefore, the CLCs in the core maintain the planar alignment along the deformed interface, and a lasing direction can be varied from omnidirectional to bi- or multidirectional, depending on the shape of deformed capsules.

Stimuli-Driven Control of the Helical Axis of Self-Organized Soft Helical Superstructures

Supramolecular and macromolecular functional helical superstructures are ubiquitous in nature and display an impressive catalog of intriguing and elegant properties and performances. In materials science, self-organized soft helical superstructures, i.e., cholesteric liquid crystals (CLCs), serve as model systems toward the understanding of morphology- and orientation-dependent properties of supramolecular dynamic helical architectures and their potential for technological applications. Moreover, most of the fascinating device applications of CLCs are primarily determined by different orientations of the helical axis. Here, the control of the helical axis orientation of CLCs and its dynamic switching in two and three dimensions using different external stimuli are summarized. Electric-field-, magnetic-field-, and light-irradiation-driven orientation control and reorientation of the helical axis of CLCs are described and highlighted. Different techniques and strategies developed to achieve a uniform lying helix structure are explored. Helical axis control in recently developed heliconical cholesteric systems is examined. The control of the helical axis orientation in spherical geometries such as microdroplets and microshells fabricated from these enticing photonic fluids is also explored. Future challenges and opportunities in this exciting area involving anisotropic chiral liquids are then discussed.