Chiral Scatterometry on Chemically Synthesized Single Plasmonic Nanoparticles

Chiral plasmonic-dielectric coupling enables strong near-infrared chiroptical responses from helicoidal core-shell nanoparticles

Helicoid plasmonic nanoparticles with intrinsic chirality are an emerging class of artificial chiral materials with tailorable properties. The ability to extend their chiroplasmonic responses to the near-infrared (NIR) range is critically important for biomedical and nanophotonic applications, yet the rational design of such materials remains challenging. Herein, a strategy employing chiral plasmon-dielectric coupling is proposed to manipulate the chiroptical responses into the NIR region with high optical anisotropy. Through this strategy, the helicoid Au@Cu2O nanoparticles with structural chirality are designed and synthesized with tunable and enriched NIR chiroptical responses. Specially, a high optical anisotropy (g-factor) with a value of 0.35 is achieved in the NIR region, and multi-band chiroptical behaviors are observed. Spectral and electromagnetic simulations elucidate that strong coupling between chiroplasmonic core and chiral dielectric shell with high refractive index contributes to the rich and strong chiroptical responses, which are related to the interplay between various emerged and enhanced electric and magnetic multipolar resonance modes proved by multipole expansion analysis. Moreover, the helicoid Au@Cu2O nanoparticles display greater polarization rotation capability than the helicoid Au nanoparticles. This work offers mechanistic insights into chiral plasmon-dielectric coupling and suggests a general approach of creating NIR chiroplasmonic materials.

Expanding the Horizons of Machine Learning in Nanomaterials to Chiral Nanostructures

Machine learning holds significant research potential in the field of nanotechnology, enabling nanomaterial structure and property predictions, facilitating materials design and discovery, and reducing the need for time-consuming and labor-intensive experiments and simulations. In contrast to their achiral counterparts, the application of machine learning for chiral nanomaterials is still in its infancy, with a limited number of publications to date. This is despite the great potential of machine learning to advance the development of new sustainable chiral materials with high values of optical activity, circularly polarized luminescence, and enantioselectivity, as well as for the analysis of structural chirality by electron microscopy. In this review, an analysis of machine learning methods used for studying achiral nanomaterials is provided, subsequently offering guidance on adapting and extending this work to chiral nanomaterials. An overview of chiral nanomaterials within the framework of synthesis-structure-property-application relationships is presented and insights on how to leverage machine learning for the study of these highly complex relationships are provided. Some key recent publications are reviewed and discussed on the application of machine learning for chiral nanomaterials. Finally, the review captures the key achievements, ongoing challenges, and the prospective outlook for this very important research field.

Circular Differential Photocurrent Mapping of Hot Electron Response from Individual Plasmonic Nanohelicoids

Chiral plasmonic nanocrystals have recently attracted increasing attention in circular polarization-dependent photocatalysis driven by hot carriers. While being concealed in traditional ensemble measurements, the individual chiral photocatalytic activity of nanocrystals can exclusively be revealed by directly correlating the circular differential photocurrent response to helical morphologies using single-particle techniques. Herein, we develop a method named circular differential photocurrent mapping (CDPM) and demonstrate that CDPM can be used to characterize the circular differential hot electron (CDHE) response from individual Au nanohelicoids (AuNHs) on a TiO2 photoanode in a photoelectrochemical cell. The single-particle circular differential scattering and CDHE measurements were interpreted with calculations performed on a model in direct correlation to the helical morphologies of the nanocrystal. While CDHE response was found inactive at a dipolar resonance of 750 nm, helicity-convoluted sites of HE generation were identified on the AuNH at a specific higher-order mode of 550 nm, resulting in a significant response of CDHE in association with the handedness of the AuNH. Details of circular differential contributions were further resolved by examining the efficiencies of individual AuNHs in terms of g-factors. Our study provides a powerful microscopic method at the single-particle level for the photocatalytic characterization of chiral nanocrystals, gaining fundamental insights into the photocatalysis of chirality, especially toward plasmon-induced asymmetrical photochemistry or photoelectrochemistry.

Cu2+-Dominated Chirality Transfer from Chiral Molecules to Concave Chiral Au Nanoparticles

Foreign ions as additives are of great significance for realizing excellent control over the morphology of noble metal nanostructures in the state-of-the-art seed-mediated growth method; however, they remain largely unexplored in chiral synthesis. Here, we report on a Cu2+-dominated chiral growth strategy that can direct the growth of concave chiral Au nanoparticles with C3-dominant chiral centers. The introduction of trace amounts of Cu2+ ions in the seed-mediated chiral growth process is found to dominate the chirality transfer from chiral molecules to chiral nanoparticles, leading to the formation of chiral nanoparticles with a concave VC geometry. Both experimental and theoretical results further demonstrate the correlation between the nanoparticle structure and optical chirality for the concave chiral nanoparticle. The Cu2+ ion is found to dominate the chiral growth by selectively activating the deposition of Au atoms along the [110] and [111] directions, facilitating the formation of the concave VC. We further demonstrate that the Cu2+-dominated chiral growth strategy can be employed to generate a variety of concave chiral nanoparticles with enriched geometric chirality and desired chiroptical properties. Concave chiral nanoparticles also exhibit appealing catalytic activity and selectivity toward electrocatalytic oxidation of enantiomers in comparison to helicoidal nanoparticles. The ability to tune the geometric chirality in a controlled manner by simply manipulating the Cu2+ ions as additives opens up a promising strategy for creating chiral nanomaterials with increasing architectural diversity for chirality-dependent optical and catalytic applications.

Tuning the Geometry and Optical Chirality of Pentatwinned Au Nanoparticles with 5-Fold Rotational Symmetry

Chirality transfer from chiral molecules to chiral nanomaterials represents an important topic for exploring the origin of chirality in many natural and artificial systems. Moreover, developing a promising class of chiral nanomaterials holds great significance for various applications, including sensing, photonics, catalysis, and biomedicine. Here we demonstrate the geometric control and tunable optical chirality of chiral pentatwinned Au nanoparticles with 5-fold rotational symmetry using the seed-mediated chiral growth method. A distinctive growth pathway and optical chirality are observed using pentatwinned decahedra as seeds, in comparison with the single-crystal Au seeds. By employing different peptides as chiral inducers, pentatwinned Au nanoparticles with two distinct geometric chirality (pentagonal nanostars and pentagonal prisms) are obtained. The intriguing formation and evolution of geometric chirality with the twinned structure are analyzed from a crystallographic perspective upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. Moreover, the interesting effects of the molecular structure of peptides on tuning the geometric chirality of pentatwinned Au nanoparticles are also explored. Finally, we theoretically and experimentally investigate the far-field and near-field optical properties of chiral pentatwinned Au nanoparticles through numerical simulations and single-particle chiroptical measurements. The ability to tune the geometric chirality in a controlled manner represents an important step toward the development of chiral nanomaterials with increasing architectural complexity for chiroptical applications.

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

Universal Chiral-Plasmon-Induced Upward and Downward Transfer of Circular Dichroism to Achiral Molecules

Electromagnetic chirality transfer represents an effective means of the nanoscale manipulation of optical chirality. While most of the previous reports have exclusively focused on the circular dichroism (CD) transfer from UV-responsive chiral molecules toward visible-resonant achiral colloidal nanoparticles, here we demonstrate a reverse process in which plasmonic chirality can be transferred to achiral molecules, either upward from visible to UV or downward from visible to near infrared (NIR). By hybridizing achiral UV- or NIR-responsive dye molecules with chiral metal nanoparticles in solution, we observe a chiral-plasmon-induced CD (CPICD) signal at the intrinsically achiral molecular absorption bands. Full-wave electromagnetic modeling reveals that both near-field Coulomb interaction and far-field radiative coupling contribute to the observed CPICD, indicating that the mechanism considered here is universal for different material systems and types of optical resonances. Our study provides a set of design guidelines for broadband nanophotonic chiral sensing from the UV to NIR spectral regime.

Multicomponent chiral plasmonic hybrid nanomaterials: recent advances in synthesis and applications

Chiral hybrid nanomaterials with multiple components provide a highly promising approach for the integration of desired chirality with other functionalities into one single nanoscale entity. However, precise control over multicomponent chiral plasmonic hybrid nanomaterials to enable their application in diverse and complex scenarios remains a significant challenge. In this review, our focus lies on the recent advances in the preparation and application of multicomponent chiral plasmonic hybrid nanomaterials, with an emphasis on synthetic strategies and emerging applications. We first systematically elucidate preparation methods for multicomponent chiral plasmonic hybrid nanomaterials encompassing the following approaches: physical deposition approach, galvanic replacement reaction, chiral molecule-mediated, chiral heterostructure, circularly polarized light-mediated, magnetically induced, and chiral assembly. Furthermore, we highlight emerging applications of multicomponent chiral plasmonic hybrid nanomaterials in chirality sensing, enantioselective catalysis, and biomedicine. Finally, we provide an outlook on the challenges and opportunities in the field of multicomponent chiral plasmonic hybrid nanomaterials. In-depth investigations of these multicomponent chiral hybrid nanomaterials will pave the way for the rational design of chiral hybrid nanostructures with desirable functionalities for emerging technological applications.

Homochiral Nanopropeller via Chiral Active Surface Growth

Under the control of chiral ligand glutathione and in the presence of hexadecyltrimethylammonium bromide, Au deposition on Au seeds is known to give chiral nanostructures. We have previously shown that the protruding chiral patterns, as opposed to flat facets, are likely caused by active surface growth, where nonuniform ligand coverage could be responsible for the focused growth at a few active sites. By pushing the limit of such a growth mode, here, we use decahedral seeds to prepare homochiral nanopropellers with intricate patterns of deep valleys and protruding ridges. Control experiments show that the focused growth depends on the rates of Au deposition by changing either the seed concentration or the reductant concentration, consistent with the proposed mechanism. The dynamic growth competition between the ligand-deficient active sites and the ligand-rich surfaces gradually focuses the growth onto a few active sites, causing the expansion of grooves, squeezing of steep ridges, and a surprising 36° rotation of the pentagonal outline. The imbalanced deposition on the prochiral slopes is responsible for the tilted grooves, the twisted walls, and thus the well-separated and distorted blades, which become the origin of the chiroptical responses.

Chiral Plasmonic Hybrid Nanostructures: A Gateway to Advanced Chiroptical Materials

Chirality introduces a new dimension of functionality to materials, unlocking new possibilities across various fields. When integrated with plasmonic hybrid nanostructures, this attribute synergizes with plasmonic and other functionalities, resulting in unprecedented chiroptical materials that push the boundaries of the system's capabilities. Recent advancements have illuminated the remarkable chiral light-matter interactions within chiral plasmonic hybrid nanomaterials, allowing for the harnessing of their tunable optical activity and hybrid components. These advancements have led to applications in areas such as chiral sensing, catalysis, and spin optics. Despite these promising developments, there remains a need for a comprehensive synthesis of the current state-of-the-art knowledge, as well as a thorough understanding of the construction techniques and practical applications in this field. This review begins with an exploration of the origins of plasmonic chirality and an overview of the latest advancements in the synthesis of chiral plasmonic hybrid nanostructures. Furthermore, representative emerging categories of hybrid nanomaterials are classified and summarized, elucidating their versatile applications. Finally, the review engages with the fundamental challenges associated with chiral plasmonic hybrid nanostructures and offer insights into the future prospects of this advanced field.

Chiral Gold Nanorods with Five-Fold Rotational Symmetry and Orientation-Dependent Chiroptical Properties of Their Monomers and Dimers

Chiral plasmonic nanoparticles have attracted much attention because of their strong chiroptical responses and broad scientific applications. However, the types of chiral plasmonic nanoparticles have remained limited. Herein we report on a new type of chiral nanoparticle, chiral Au nanorod (NR) with five-fold rotational symmetry, which is synthesized using chiral molecules. Three different types of Au seeds (Au elongated nanodecahedrons, nanodecahedrons, and nanobipyramids) are used to study the growth behaviors. Different synthesis parameters, including the chiral molecules, surfactant, reductant, seeds, and Au precursor, are systematically varied to optimize the chiroptical responses of the chiral Au NRs. The chiral scattering measurements on the individual chiral Au NRs and their dimers are performed. Intriguingly, the chiroptical signals of the individual chiral Au NRs and their end-to-end dimers are similar, while those of the side-by-side dimers are largely reduced. Theoretical calculations and numerical simulations reveal that the different chiroptical responses of the chiral NR dimers are originated from the coupling effect between the plasmon resonance modes. Our study enriches chiral plasmonic nanoparticles and provides valuable insight for the design of plasmonic nanostructures with desired chiroptical properties.

Geometric Control and Optical Properties of Intrinsically Chiral Plasmonic Nanomaterials

Intrinsically chiral plasmonic nanomaterials exhibit intriguing geometry-dependent chiroptical properties, which is due to the combination of plasmonic features with geometric chirality. Thus, chiral plasmonic nanomaterials have become promising candidates for applications in biosensing, asymmetric catalysis, biomedicine, photonics, etc. Recent advances in geometric control and optical tuning of intrinsically chiral plasmonic nanomaterials have further opened up a unique opportunity for their widespread applications in many emerging technological areas. Here, the recent developments in the geometric control of chiral plasmonic nanomaterials are reviewed with special attention given to the quantitative understanding of the chiroptical structure-property relationship. Several important optical spectroscopic tools for characterizing the optical chirality of plasmonic nanomaterials at both ensemble and single-particle levels are also discussed. Three emerging applications of chiral plasmonic nanomaterials, including enantioselective sensing, enantioselective catalysis, and biomedicine, are further highlighted. It is envisioned that these advanced studies in chiral plasmonic nanomaterials will pave the way toward the rational design of chiral nanomaterials with desired optical properties for diverse emerging technological applications.

Plasmonic Nanostructure Engineering with Shadow Growth

Physical shadow growth is a vacuum deposition technique that permits a wide variety of 3D-shaped nanoparticles and structures to be fabricated from a large library of materials. Recent advances in the control of the shadow effect at the nanoscale expand the scope of nanomaterials from spherical nanoparticles to complex 3D shaped hybrid nanoparticles and structures. In particular, plasmonically active nanomaterials can be engineered in their shape and material composition so that they exhibit unique physical and chemical properties. Here, the recent progress in the development of shadow growth techniques to realize hybrid plasmonic nanomaterials is discussed. The review describes how fabrication permits the material response to be engineered and highlights novel functions. Potential fields of application with a focus on photonic devices, biomedical, and chiral spectroscopic applications are discussed.

Halide-assisted differential growth of chiral nanoparticles with threefold rotational symmetry

Enriching the library of chiral plasmonic nanoparticles that can be chemically mass-produced will greatly facilitate the applications of chiral plasmonics in areas ranging from constructing optical metamaterials to sensing chiral molecules and activating immune cells. Here we report on a halide-assisted differential growth strategy that can direct the anisotropic growth of chiral Au nanoparticles with tunable sizes and diverse morphologies. Anisotropic Au nanodisks are employed as seeds to yield triskelion-shaped chiral nanoparticles with threefold rotational symmetry and high dissymmetry factors. The averaged scattering g-factors of the L- and D-nanotriskelions are as large as 0.57 and - 0.49 at 650 nm, respectively. The Au nanotriskelions have been applied in chiral optical switching devices and chiral nanoemitters. We also demonstrate that the manipulation of the directional growth rate enables the generation of a variety of chiral morphologies in the presence of homochiral ligands.

Chiral Spectroscopy of Nanostructures

ConspectusChirality is ubiquitous in the universe and in living creatures over detectable length scales from the subatomic to the galactic, as exemplified in the two extremes by subatomic particles (neutrinos) and spiral galaxies. Between them are living creatures that display multiple levels of chirality emerging from hierarchically assembled asymmetric building blocks. Not too far from the bottom of this pyramid are the foundational building blocks with chiral atomic centers on sp³ carbon atoms exemplified by l-amino acids and d-sugars that are self-assembled into higher-order structures with increasing dimensions forming highly complex, amazingly functional, and energy-efficient living systems. The organization and materials employed in their construction inspired scientists to replicate complex living systems via the self-assembly of chiral components. Multiple studies pointed to unexpected and unique electromagnetic properties of chiral structures with nanoscale and microscale dimensions, including giant circular dichroism and collective circularly polarized scattering that their constituent units did not possess.To address the wide variety of chiral geometries observed in continuous materials, singular particles, and their complex systems, multiple analytic techniques are needed. Simultaneously, their spectroscopic properties create a pathway to multiple applications. For example, mirror-asymmetric vibrations at chiral centers formed by sp³ carbon atoms lead to optical activity for the infrared (IR) wavelength regions. At the same time, understanding the optical activity in, for example, the IR region enables biomedical applications because multiple modalities of biomedical imaging and vibrational optical activity (VOA) of biomolecules are known for IR range. In turn, VOA can be realized in both absorption and emission modalities due to large magnetic transition moments, as vibrational circular dichroism (VCD) or Raman optical activity (ROA) spectroscopy. In addition to the VOA, in the range of longer wavelengths, lattice vibrational mode or phononic behavior occurs in chiral crystals and nanoassemblies, which can be readily detected by terahertz circular dichroism (TCD) spectroscopy. Meanwhile, chiral self-assembly can induce circularly polarized light emission (CPLE) regardless of the existence of chirality in coassembled fluorophores. The CPLE from self-assembled chiral materials is particularly interesting because the CPLE can originate from both circularly polarized luminescence and circularly polarized scattering (CPS). Furthermore, because self-assembled nanostructures often exhibit stronger optical activity than their building blocks owing to dimension and resonance effects, the optical activity of single assembled nanostructures can be investigated by using microscopic technology combined with chiral optics. Here, we describe the state of the art for spectroscopic methods for the comprehensive analysis of chiral nanomaterials at various photon wavelengths, addressed with special attention given to new tools emerging both for materials with self-organized hierarchical chirality and single-particle spectroscopy.

Practical Considerations for Simulating the Plasmonic Properties of Metal Nanoparticles

Simulating the plasmonic properties of colloidally derived metal nanoparticles with accuracy to their experimentally observed measurements is challenging due to the many structural and compositional parameters that influence their scattering and absorption properties. Correlation between single nanoparticle scattering measurements and simulated spectra emphasize these strong structural and compositional relationships, providing insight into the design of plasmonic nanoparticles. This Perspective builds from this history to highlight how the structural features of models used in simulation methods such as those based on the Finite-Difference Time-Domain (FDTD) method and Discrete Dipole Approximation (DDA) are of critical consideration for correlation with experiment and ultimately prediction of new nanoparticle properties. High-level characterizations such as electron tomography are discussed as ways to advance the accuracy of models used in such simulations, allowing the plasmonic properties of structurally complex nanoparticles to be better understood. However, we also note that the field is far from bringing experiment and simulation into agreement for plasmonic nanoparticles with complex compositions, reflecting analytical challenges that inhibit accurate model generation. Potential directions for addressing these challenges are also presented.

Chiral Plasmonic Pinwheels Exhibit Orientation-Independent Linear Differential Scattering under Asymmetric Illumination

Plasmonic nanoantennas have considerably stronger polarization-dependent optical properties than their molecular counterparts, inspiring photonic platforms for enhancing molecular dichroism and providing fundamental insight into light-matter interactions. One such insight is that even achiral nanoparticles can yield strong optical activity when they are asymmetrically illuminated from a single oblique angle instead of evenly illuminated. This effect, called extrinsic chirality, results from the overall chirality of the experimental geometry and strongly depends on the orientation of the incident light. Although extrinsic chirality has been well-characterized, an analogous effect involving linear polarization sensitivity has not yet been discussed. In this study, we investigate the differential scattering of rotationally symmetric chiral plasmonic pinwheels when asymmetrically irradiated with linearly polarized light. Despite their high rotational symmetry, we observe substantial linear differential scattering that is maintained over all pinwheel orientations. We demonstrate that this orientation-independent linear differential scattering arises from the broken mirror and rotational symmetries of our overall experimental geometry. Our results underscore the necessity of considering both the rotational symmetry of the nanoantenna and the experimental setup, including illumination direction and angle, when performing plasmon-enhanced chiroptical characterizations. Our results demonstrate spectroscopic signatures of an effect analogous to extrinsic chirality for linear polarizations.

Synthesis of Bitten Gold Nanoparticles with Single-Particle Chiroptical Responses

The introduction of structural complexity to nanoparticles brings them interesting properties. Regularity breaking has been challenging in the chemical synthesis of nanoparticles. Most reported chemical methods for synthesizing irregular nanoparticles are complicated and laborious, largely hindering the exploration of structural irregularity in nanoscience. In this study, the authors have combined seed-mediated growth and Pt(IV)-induced etching to synthesize two types of unprecedented Au nanoparticles, bitten nanospheres and nanodecahedrons, with size control. Each nanoparticle has an irregular cavity on it. They exhibit distinct single-particle chiroptical responses. Perfect Au nanospheres and nanorods without any cavity do not show optical chirality, which demonstrates that the geometrical structure of the bitten opening plays a decisive role in the generation of chiroptical responses.

Biomimetic mineralization based on self-assembling peptides

Biomimetic science has attracted great interest in the fields of chemistry, biology, materials science, and energy. Biomimetic mineralization is the process of synthesizing inorganic minerals under the control of organic molecules or biomolecules under mild conditions. Peptides are the motifs that constitute proteins, and can self-assemble into various hierarchical structures and show a high affinity for inorganic substances. Therefore, peptides can be used as building blocks for the synthesis of functional biomimetic materials. With the participation of peptides, the morphology, size, and composition of mineralized materials can be controlled precisely. Peptides not only provide well-defined templates for the nucleation and growth of inorganic nanomaterials but also have the potential to confer inorganic nanomaterials with high catalytic efficiency, selectivity, and biotherapeutic functions. In this review, we systematically summarize research progress in the formation mechanism, nanostructural manipulation, and applications of peptide-templated mineralized materials. These can further inspire researchers to design structurally complex and functionalized biomimetic materials with great promising applications.

Photothermal Circular Dichroism Measurements of Single Chiral Gold Nanoparticles Correlated with Electron Tomography

Chemically synthesized metal nanoparticles with morphological chiral features are known to exhibit strong circular dichroism. However, we still lack understanding of the correlation between morphological and chiroptical features of plasmonic nanoparticles. To shed light on that question, single nanoparticle experiments are required. We performed photothermal circular dichroism measurements of single chiral and achiral gold nanoparticles and correlated the chiroptical response to the 3D morphology of the same nanoparticles retrieved by electron tomography. In contrast to an ensemble measurement, we show that individual particles within the ensemble display a broad distribution of strength and handedness of circular dichroism signals. Whereas obvious structural chiral features, such as helical wrinkles, translate into chiroptical ones, nanoparticles with less obvious chiral morphological features can also display strong circular dichroism signals. Interestingly, we find that even seemingly achiral nanoparticles can display large g-factors. The origin of this circular dichroism signal is discussed in terms of plasmonics and other potentially relevant factors.

Optically Probing the Chirality of Single Plasmonic Nanostructures and of Single Molecules: Potential and Obstacles

Circular dichroism (CD) is a standard method for the analysis of biomolecular conformation. It is very reliable when applied to molecules, but requires relatively large amounts of solution. Plasmonics offer the perspective of enhancement of CD signals, which would extend CD spectrometry to smaller amounts of molecules and to weaker chiral signals. However, plasmonic enhancement comes at the cost of additional complications: averaging over all orientations is no longer possible or reliable, linear dichroism leaks into CD signals because of experimental imperfections, scattering and its interference with the incident beam must be taken into account, and the interaction between chiral molecules and possibly chiral plasmonic structures considerably complicates the interpretation of measured signals. This Perspective aims to explore the motivations and problems of plasmonic chirality and to re-evaluate present and future solutions.

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.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy

The nanoscale morphology of metal nanostructures directly defines their optical, catalytic and electronic properties and even small morphological changes can cause significant property variations. On the one hand, this dependence allows for precisely tuning and exploring properties by shape engineering; next to advanced synthesis protocols, post-synthesis modification through tailored laser modification has become an emerging tool to do so. On the other hand, with this interconnection also comes the quest for detailed structure-property correlation and understanding of laser-induced reshaping processes on the individual nanostructure level beyond ensemble averages. With the development of single-particle (ultrafast) optical spectroscopy techniques and advanced electron microscopy such understanding can in principle be gained at the femtosecond temporal and atomic spatial scale, respectively. However, accessing both on the same individual nanostructure is far from straightforward as it requires the combination of optical spectroscopy and electron microscopy. In this Minireview, we highlight key studies from recent years that performed such correlative measurements on the same individual metal nanostructure either in a consecutive ex situ manner or in situ inside the electron microscope. We demonstrate that such a detailed correlation is critical for revealing the full picture of the structure-property relationship and the physics behind light-induced nanostructure modifications. We put emphasis on the advantages and disadvantages of each methodology as well as on the unique information that one can gain only by correlative studies performed on the same individual nanostructure and end with an outlook on possible further development of this field in the near future.

A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Quantification of the Helical Morphology of Chiral Gold Nanorods

Chirality in inorganic nanoparticles and nanostructures has gained increasing scientific interest, because of the possibility to tune their ability to interact differently with left- and right-handed circularly polarized light. In some cases, the optical activity is hypothesized to originate from a chiral morphology of the nanomaterial. However, quantifying the degree of chirality in objects with sizes of tens of nanometers is far from straightforward. Electron tomography offers the possibility to faithfully retrieve the three-dimensional morphology of nanomaterials, but only a qualitative interpretation of the morphology of chiral nanoparticles has been possible so far. We introduce herein a methodology that enables us to quantify the helicity of complex chiral nanomaterials, based on the geometrical properties of a helix. We demonstrate that an analysis at the single particle level can provide significant insights into the origin of chiroptical properties.

Second Harmonic Optical Circular Dichroism of Plasmonic Chiral Helicoid-III Nanoparticles

While plasmonic particles can provide optical resonances in a wide spectral range from the lower visible up to the near-infrared, often, symmetry effects are utilized to obtain particular optical responses. By breaking certain spatial symmetries, chiral structures arise and provide robust chiroptical responses to these plasmonic resonances. Here, we observe strong chiroptical responses in the linear and nonlinear optical regime for chiral L-handed helicoid-III nanoparticles and quantify them by means of an asymmetric factor, the so-called g-factor. We calculate the linear optical g-factors for two distinct chiroptical resonances to -0.12 and -0.43 and the nonlinear optical g-factors to -1.45 and -1.63. The results demonstrate that the chirality of the helicoid-III nanoparticles is strongly enhanced in the nonlinear regime.

Three-dimensional artificial chirality towards low-cost and ultra-sensitive enantioselective sensing

Artificial chiral structures have potential applications in the field of enantioselective signal sensing. Advanced nanofabrication methods enable a large diversity in geometric structures and broad selectivity of materials, which can be exploited to manufacture artificial three-dimensional chiral structures. Various chiroptical phenomena exploiting spin and orbital angular momentum at the nanoscale have been continuously exploited as a way to effectively detect enantiomers. This review introduces precisely controlled bottom-up and large-area top-down metamaterial fabrication methods to solve the limitations of high manufacturing cost and low production speed. Particle synthesis, self-assembly, glanced angled vapor deposition, and three-dimensional plasmonic nanostructure printing are introduced. Furthermore, emerging sensitive chiral sensing methods such as cavity-enhanced chirality, photothermal circular dichroism, and helical dichroism of single particles are discussed. The continuous progress of nanofabrication technology presents the strong potential for developing artificial chiral structures for applications in biomedical, pharmaceutical, nanophotonic systems.

Second harmonic Rayleigh scattering optical activity of single Ag nanohelices in a liquid

Determining the chirality of molecules and nanoparticles often relies on circular dichroism and optical rotation: two chiral optical (chiroptical) effects in the linear optical regime. Although these linear effects are weak compared to nonlinear chiroptical effects, they have the advantage of being measured in isotropic liquids - free from the complications of anisotropy. Recently, a nonlinear effect: hyper-Rayleigh scattering optical activity (HRS OA) has been shown to reliably distinguish between the two chiral forms of Ag nanohelices, suspended in isotropic liquids. However, this first demonstration of HRS OA also opened new questions. For instance, at a fundamental level, it is not clear what the role of interactions between nanoparticles is. Moreover, the influence of the ultrafast pulse chirp is unknown. Here, we demonstrate HRS OA from well below two Ag nanohelices in the illumination volume, precluding any interactions. Additionally, we performed the first measurements of HRS depolarization ratios in this system and find a value of ≈1. We also show that HRS is highly robust against the chirp of the ultrafast pulses. An important reason for the strong (down to single nanohelix) sensitivity of our experiments is the large chiroptical interaction at the fundamental frequency; this point is illustrated with two sets of numerical simulations of the electromagnetic near-fields. Our results highlight HRS OA as a highly sensitive experimental method for characterization of chiral solutions/suspensions, in tiny illumination volumes.

Enantiomer-dependent immunological response to chiral nanoparticles

Chirality is a unifying structural metric of biological and abiological forms of matter. Over the past decade, considerable clarity has been achieved in understanding the chemistry and physics of chiral inorganic nanoparticles^1-4; however, little is known about their effects on complex biochemical networks^5,6. Intermolecular interactions of biological molecules and inorganic nanoparticles show some commonalities^7-9, but these structures differ in scale, in geometry and in the dynamics of chiral shapes, which can both impede and strengthen their mirror-asymmetric complexes. Here we show that achiral and left- and right-handed gold biomimetic nanoparticles show different in vitro and in vivo immune responses. We use irradiation with circularly polarized light (CPL) to synthesize nanoparticles with controllable nanometre-scale chirality and optical anisotropy factors (g-factors) of up to 0.4. We find that binding of nanoparticles to two proteins from the family of adhesion G-protein-coupled receptors (AGPCRs)-namely cluster-of-differentiation 97 (CD97) and epidermal-growth-factor-like-module receptor 1 (EMR1)-results in the opening of mechanosensitive potassium-efflux channels, the production of immune signalling complexes known as inflammasomes, and the maturation of mouse bone-marrow-derived dendritic cells. Both in vivo and in vitro immune responses depend monotonically on the g-factors of the nanoparticles, indicating that nanoscale chirality can be used to regulate the maturation of immune cells. Finally, left-handed nanoparticles show substantially higher (1,258-fold) efficiency compared with their right-handed counterparts as adjuvants for vaccination against the H9N2 influenza virus, opening a path to the use of nanoscale chirality in immunology.

Nanophotonic Approaches for Chirality Sensing

Chiral nanophotonic materials are promising candidates for biosensing applications because they focus light into nanometer dimensions, increasing their sensitivity to the molecular signatures of their surroundings. Recent advances in nanomaterial-enhanced chirality sensing provide detection limits as low as attomolar concentrations (10^-18 M) for biomolecules and are relevant to the pharmaceutical industry, forensic drug testing, and medical applications that require high sensitivity. Here, we review the development of chiral nanomaterials and their application for detecting biomolecules, supramolecular structures, and other environmental stimuli. We discuss superchiral near-field generation in both dielectric and plasmonic metamaterials that are composed of chiral or achiral nanostructure arrays. These materials are also applicable for enhancing chiroptical signals from biomolecules. We review the plasmon-coupled circular dichroism mechanism observed for plasmonic nanoparticles and discuss how hotspot-enhanced plasmon-coupled circular dichroism applies to biosensing. We then review single-particle spectroscopic methods for achieving the ultimate goal of single-molecule chirality sensing. Finally, we discuss future outlooks of nanophotonic chiral systems.

Measuring the optical properties of nanoscale biogenic spherulites

Recent studies of optical reflectors as part of the vision apparatus in the eyes of decapod crustaceans revealed assemblies of nanoscale spherulites - spherical core-shell nanoparticles with radial birefringence. Simulations performed on the system highlighted the advantages of optical anisotropy in enhancing the functionality of these structures. So far, calculations of the nanoparticle optical properties have relied on refractive indices obtained using ab-initio calculations. Here we describe a direct measurement of the tangential refractive index of the spherulites, which corresponds to the in-plane refractive index of crystalline isoxanthopterin nanoplatelets. We utilize measurements of scattering spectra of individual spherulites and determine the refractive index by analyzing the spectral signatures of scattering resonances. Our measurements yield a median tangential refractive index of 1.88, which is in reasonable agreement with theoretical predictions. Furthermore, our results indicate that the optical properties of small spherulite assemblies are largely determined by the tangential index.

X-ray-Based Techniques to Study the Nano-Bio Interface

X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo.

Focused Ion Beam Processing for 3D Chiral Photonics Nanostructures

The focused ion beam (FIB) is a powerful piece of technology which has enabled scientific and technological advances in the realization and study of micro- and nano-systems in many research areas, such as nanotechnology, material science, and the microelectronic industry. Recently, its applications have been extended to the photonics field, owing to the possibility of developing systems with complex shapes, including 3D chiral shapes. Indeed, micro-/nano-structured elements with precise geometrical features at the nanoscale can be realized by FIB processing, with sizes that can be tailored in order to tune optical responses over a broad spectral region. In this review, we give an overview of recent efforts in this field which have involved FIB processing as a nanofabrication tool for photonics applications. In particular, we focus on FIB-induced deposition and FIB milling, employed to build 3D nanostructures and metasurfaces exhibiting intrinsic chirality. We describe the fabrication strategies present in the literature and the chiro-optical behavior of the developed structures. The achieved results pave the way for the creation of novel and advanced nanophotonic devices for many fields of application, ranging from polarization control to integration in photonic circuits to subwavelength imaging.

Chiral Plasmonic Metamaterials with Tunable Chirality

Chiral hollow nanovolcano array (HNVA) film and chiral hollow nanoshells (HNSs) are simultaneously fabricated via a new strategy of colloidal lithography technique. The chirality of both chiral plasmonic nanostructures, which arises from the asymmetric charge oscillation and electric field distributions, can be well controlled by regulating the opening-angle of the nanounits during the metal depositions. The large-area HNVA films exhibit strong chiroptical responses in the ultraviolet-visible region with g-factor of 0.15 and possess remarkable transferability for better adaptability of different application situations. The chiral HNSs, which are simultaneously obtained during the deposition, is equipped with adjustable chirality and integrability. The obtained HNVA films were transferred to specific substrates, e.g., polydimethylsiloxane (PDMS), hydrogels, and high-curvature surfaces, maintaining the original chiroptical properties and excellent mechanical strength. Deformable chiral flexible metamaterial is obtained by incorporating the chiral HNSs in the hydrogel, enabling the ultrasensitive detection of water content in the hydrogel. Overall, this work will contribute to the study of chiral metamaterials by providing two kinds of newly developed chiral plasmonic metamaterials with tunable chirality and inspiring progressing ways for the flexible devices of artificial chirality.

Electromagnetic chirality: from fundamentals to nontraditional chiroptical phenomena

Chirality arises universally across many different fields. Recent advancements in artificial nanomaterials have demonstrated chiroptical responses that far exceed those found in natural materials. Chiroptical phenomena are complicated processes that involve transitions between states with opposite parities, and solid interpretations of these observations are yet to be clearly provided. In this review, we present a comprehensive overview of the theoretical aspects of chirality in light, nanostructures, and nanosystems and their chiroptical interactions. Descriptions of observed chiroptical phenomena based on these fundamentals are intensively discussed. We start with the strong intrinsic and extrinsic chirality in plasmonic nanoparticle systems, followed by enantioselective sensing and optical manipulation, and then conclude with orbital angular momentum-dependent responses. This review will be helpful for understanding the mechanisms behind chiroptical phenomena based on underlying chiral properties and useful for interpreting chiroptical systems for further studies.

A Single Chiral Nanoparticle Induced Valley Polarization Enhancement

Valley polarization is among the most critical attributes of atomically thin materials. However, increasing contrast from monolayer transition metal dichalcogenides (TMDs) has so far been challenging. In this work, a large degree of circular polarization up to 45% from a monolayer WS₂ is achieved at room temperature by using a single chiral plasmonic nanoparticle. The increased contrast is attributed to the selective enhancement of both the excitation and the emission rate having one particular handedness of the circular polarization, together with accelerated radiative recombination of valley excitons due to the Purcell effect. The experimental results are corroborated by the optical simulation using the finite-difference time-domain (FDTD) method. Additionally, the single chiral nanoparticle enables the observation of valley-polarized luminescence with a linear excitation. The results provide a promising pathway to enhance valley contrast from monolayer TMDs and utilize them for nanophotonic devices.

Polarized evanescent waves reveal trochoidal dichroism

The ability of certain materials to discriminate between two opposite light polarizations is the basic principle behind several technologies such as liquid crystal displays and three-dimensional glasses. While there are numerous forms of light polarization, only linear and circular polarizations, which have wave motion in a flat sheet or helix, respectively, are typically used. Here, we utilize trochoidal polarizations with cartwheeling wave motion. We demonstrate that single gold nanorod dimers can discriminate between trochoidal fields rotating in opposite directions, which we term trochoidal dichroism. Trochoidal dichroism forms an additional classification of polarized light–matter interaction and could inspire the development of optical studies uniquely sensitive to molecules with cartwheeling charge motion, potentially relevant for probing key light-harvesting antennas.

Matter’s sensitivity to light polarization is characterized by linear and circular polarization effects, corresponding to the system’s anisotropy and handedness, respectively. Recent investigations into the near-field properties of evanescent waves have revealed polarization states with out-of-phase transverse and longitudinal oscillations, resulting in trochoidal, or cartwheeling, field motion. Here, we demonstrate matter’s inherent sensitivity to the direction of the trochoidal field and name this property trochoidal dichroism. We observe trochoidal dichroism in the differential excitation of bonding and antibonding plasmon modes for a system composed of two coupled dipole scatterers. Trochoidal dichroism constitutes the observation of a geometric basis for polarization sensitivity that fundamentally differs from linear and circular dichroism. It could also be used to characterize molecular systems, such as certain light-harvesting antennas, with cartwheeling charge motion upon excitation.

Experimental and Theoretical Observation of Photothermal Chirality in Gold Nanoparticle Helicoids

Single-particle spectroscopy is central to the characterization of plasmonic nanostructures and understanding of light-matter interactions in chiral nanosystems. Although chiral plasmonic nanostructures are generally characterized by their circular differential extinction and scattering, single-particle absorption studies can extend our understanding of light-matter interactions. Here, we introduce an experimental observation of photothermal chirality which originated from circular differential absorption of chiral plasmonic nanostructures. Using luminescence ratio thermometry, we identify the optical and photothermal handedness and an absolute temperature difference of 6 K under the right and left circularly polarized light. We observe a circular differential extinction parameter (g_ext) of -0.13 in colloidally prepared gold helicoids and compare our findings with numerical simulations using finite element methods. The simulated data showed that circular differential absorption and the maximum temperature of a small cluster of helical nanoparticles are polarization-dependent. We observed an intensity-dependent photothermal g-factor from chiral helicoids that decreases slightly at higher temperatures. We also measure a range of optical g-factors from several gold helicoids, which are attributed to the heterogeneity of helicoids in nanoparticles during synthesis. The principles of differential photothermal response of chiral nanomaterials and heat generation described here can be potentially used for thermal photocatalysis, energy conversion, and electronic applications.

Uniform Chiral Gap Synthesis for High Dissymmetry Factor in Single Plasmonic Gold Nanoparticle

Synthesis of chiral plasmonic materials has been highlighted for the last decades with their optical properties and versatile potential applications. Recently reported aqueous-based amino acid- and peptide-directed synthesis of chiral plasmonic gold nanoparticles with 432 point-group symmetry shows exceptionally high chiroptic response within 100 nm scales. Despite its already excellent chiroptic response, a single-nanoparticle dark field scattering study revealed that full chiroptic potential of chiral gold nanoparticle is limited with its overall synthetic uniformity. Based on this knowledge, we present a multi-chirality-evolution step synthesis method for the enhancement of chiroptic response through an increase in particle uniformity. Detailed time variant study and interrelationship study of reaction parameters allowed the systematic construction of design principles for chiral nanoparticles with exceptional chiroptic response. With the application of precisely controlled growth kinetic to two distinct growth regimes, modified chiral gold nanoparticles showed significantly improved uniformity, achieving an improved dissymmetry factor of g = 0.31. We expect that our strategy will aid in precise morphology and property control for chiral nanomaterials, which can be used in various plasmonic metamaterial applications.

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.

Enhanced Chiral Sensing with Dielectric Nanoresonators

Chiro-sensitive molecular detection is highly relevant as many biochemical compounds, the building blocks of life, are chiral. Optical chirality is conventionally detected through circular dichroism (CD) in the UV range, where molecules naturally absorb. Recently, plasmonics has been proposed as a way to boost the otherwise very weak CD signal and translate it to the visible/NIR range, where technology is friendlier. Here, we explore how dielectric nanoresonators can contribute to efficiently differentiate molecular enantiomers. We study the influence of the detuning between electric (ED) and magnetic dipole (MD) resonances in silicon nanocylinders on the quality of the CD signal. While our experimental data, supported by numerical simulations, demonstrate that dielectric nanoresonators can perform even better than their plasmonic counterpart, exhibiting larger CD enhancements, we do not observe any significant influence of the optical chirality.

Circular Dichroism Measurement of Single Metal Nanoparticles Using Photothermal Imaging

Circular dichroism (CD) spectroscopy is a powerful optical technique for the study of chiral materials and molecules. It gives access to an enantioselective signal based on the differential absorption of right and left circularly polarized light, usually obtained through polarization analysis of the light transmitted through a sample of interest. CD is routinely used to determine the secondary structure of proteins and their conformational state. However, CD signals are weak, limiting the use of this powerful technique to ensembles of many molecules. Here, we experimentally realize the concept of photothermal circular dichroism, a technique that combines the enantioselective signal from circular dichroism with the high sensitivity of photothermal microscopy, achieving a superior signal-to-noise ratio to detect chiral nano-objects. As a proof of principle, we studied the chiral response of single plasmonic nanostructures with CD in the visible range, demonstrating a signal-to-noise ratio better than 40 with only 30 ms integration time for these nanostructures. The high signal-to-noise ratio allows us to quantify the CD signal for individual nanoparticles. We show that we can distinguish relative absorption differences for right circularly and left circularly polarized light as small as g_min = 4 × 10^-3 for a 30 ms integration time with our current experimental settings. The enhanced sensitivity of our technique extends CD studies to individual nano-objects and opens CD spectroscopy to numbers of molecules much lower than those in conventional experiments.

Optical Helicity and Optical Chirality in Free Space and in the Presence of Matter

The inherently weak nature of chiral light–matter interactions can be enhanced by orders of magnitude utilizing artificially-engineered nanophotonic structures. These structures enable high spatial concentration of electromagnetic fields with controlled helicity and chirality. However, the effective design and optimization of nanostructures requires defining physical observables which quantify the degree of electromagnetic helicity and chirality. In this perspective, we discuss optical helicity, optical chirality, and their related conservation laws, describing situations in which each provides the most meaningful physical information in free space and in the context of chiral light–matter interactions. First, an instructive comparison is drawn to the concepts of momentum, force, and energy in classical mechanics. In free space, optical helicity closely parallels momentum, whereas optical chirality parallels force. In the presence of macroscopic matter, the optical helicity finds its optimal physical application in the case of lossless, dual-symmetric media, while, in contrast, the optical chirality provides physically observable information in the presence of lossy, dispersive media. Finally, based on numerical simulations of a gold and silicon nanosphere, we discuss how metallic and dielectric nanostructures can generate chiral electromagnetic fields upon interaction with chiral light, offering guidelines for the rational design of nanostructure-enhanced electromagnetic chirality.