Crafting chirality in three dimensions via a novel fabrication technique for bound states in the continuum metasurfaces.

An additional deposition step was added to a multi-step electron beam lithographic fabrication process to unlock the height dimension as an accessible parameter for resonators comprising unit cells of quasi-bound states in the continuum metasurfaces, which is essential for the geometric design of intrinsically chiral structures.

Polarization-based colour tuning of mixed colloidal quantum-dot thin films using direct patterning.

Colloidal quantum-dots (cQDs) are finding increasingly widespread application in photonics and optoelectronics, providing high brightness and record-wide colour gamuts. However, the external quantum efficiencies in thin-film device architectures are still limited due to losses into waveguide modes and different strategies are being explored to promote the outcoupling of emission. Here we use a template-stripping-based direct-patterning strategy to fabricate linear gratings at the surface of cQD thin films. The linear gratings enhance optical outcoupling through Bragg scattering, yielding bright emission with a strong degree of linear polarization. By patterning linear gratings with different periodicities and orientations onto a film of mixed-colour cQDs, we demonstrate polarization-based active colour tuning of the thin-film emission.

Nanoscale Bouligand Multilayers: Giant Circular Dichroism of Helical Assemblies of Plasmonic 1D Nano-Objects.

Chirality is found at all length scales in nature, and chiral metasurfaces have recently attracted attention due to their exceptional optical properties and their potential applications. Most of these metasurfaces are fabricated by top-down methods or bottom-up approaches that cannot be tuned in terms of structure and composition. By combining grazing incidence spraying of plasmonic nanowires and nanorods and Layer-by-Layer assembly, we show that nonchiral 1D nano-objects can be assembled into scalable chiral Bouligand nanostructures whose mesoscale anisotropy is controlled with simple macroscopic tools. Such multilayer helical assemblies of linearly oriented nanowires and nanorods display very high circular dichroism up to 13 000 mdeg and giant dissymmetry factors up to g ≈ 0.30 over the entire visible and near-infrared range. The chiroptical properties of the chiral multilayer stack are successfully modeled using a transfer matrix formalism based on the experimentally determined properties of each individual layer. The proposed approach can be extended to much more elaborate architectures and gives access to template-free and enantiomerically pure nanocomposites whose structure can be finely tuned through simple design principles.

Fluorescence-Detected Circular Dichroism of a Chiral Molecular Monolayer with Dielectric Metasurfaces.

Strong enhancement of molecular circular dichroism (CD) has the potential to enable efficient asymmetric photolysis, a method of chiral separation that has conventionally been impeded by insufficient yield and low enantiomeric excess. Here, we study experimentally how predicted enhancements in optical chirality density near resonant silicon nanodisks boost CD. We use fluorescence-detected circular dichroism (FDCD) spectroscopy to measure indirectly the differential absorption of circularly polarized light by a monolayer of optically active molecules functionalized to silicon nanodisk arrays. Importantly, the molecules and nanodisk antennas have spectrally coincident resonances, and our fluorescence technique allows us to deconvolute absorption in the nanodisks from the molecules. We find that enhanced FDCD signals depend on nanophotonic resonances, in good agreement with simulated differential absorption and optical chirality density, while no signal is detected from molecules adsorbed on featureless silicon surfaces. These results verify the potential of nanophotonic platforms to be used for asymmetric photolysis with lower energy requirements.

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.

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.

Carbon Nanotube Chirality Determines Properties of Encapsulated Linear Carbon Chain.

Long linear carbon chains (LLCCs) encapsulated inside double-walled carbon nanotubes (DWCNTs) are regarded as a promising realization of carbyne, the truly one-dimensional allotrope of carbon. While the electronic and vibronic properties of the encapsulated LLCC are expected to be influenced by its nanotube host, this dependence has not been investigated experimentally so far. Here we bridge this gap by studying individual LLCCs encapsulated in DWCNTs with tip-enhanced Raman scattering (TERS). We reveal that the nanotube host, characterized by its chirality, determines the vibronic and electronic properties of the encapsulated LLCC. By choice of chirality, the fundamental Raman mode (C-mode) of the chain is tunable by ∼95 cm-1 and its band gap by ∼0.6 eV, suggesting this one-dimensional hybrid system to be a promising building block for nanoscale optoelectronics. No length dependence of the chain's C-mode frequency is evident, making LLCCs a close to perfect representation of carbyne.

Chiral Light Design and Detection Inspired by Optical Antenna Theory.

Chiral metallic nanostructures can generate evanescent fields which are more highly twisted than circularly polarized light. However, it remains unclear how best to exploit this phenomenon, hindering the optimal utilization of chiral electromagnetic fields. Here, inspired by optical antenna theory, we address this challenge by introducing chiral antenna parameters: the chirality flux efficiency and the chiral antenna aperture. These quantities, which are based on chirality conservation, quantify the generation and dissipation of chiral light. We then present a label-free experimental technique, chirality flux spectroscopy, which measures the chirality flux efficiency, providing valuable information on chiral near fields in the far field. This principle is verified theoretically and experimentally with two-dimensionally chiral coupled nanorod antennas, for which we show that chiral near and far fields are linearly dependent on the magnetoelectric polarizability. This elementary system confirms our concept to quantify chiral electromagnetic fields and paves the way toward broadly tunable chiral optical applications including ultrasensitive detection of molecular chirality or optical information storage and transfer.

Polarization Multiplexing of Fluorescent Emission Using Multiresonant Plasmonic Antennas.

Combining the ability to localize electromagnetic fields at the nanoscale with a directional response, plasmonic antennas offer an effective strategy to shape the far-field pattern of coupled emitters. Here, we introduce a family of directional multiresonant antennas that allows for polarization-resolved spectral identification of fluorescent emission. The geometry consists of a central aperture surrounded by concentric polygonal corrugations. By varying the periodicity of each axis of the polygon individually, this structure can support multiple resonances that provide independent control over emission directionality for multiple wavelengths. Moreover, since each resonant wavelength is directly mapped to a specific polarization orientation, spectral information can be encoded in the polarization state of the out-scattered beam. To demonstrate the potential of such structures in enabling simplified detection schemes and additional functionalities in sensing and imaging applications, we use the central subwavelength aperture as a built-in nanocuvette and manipulate the fluorescent response of colloidal-quantum-dot emitters coupled to the multiresonant antenna.

Ultraviolet Plasmonic Chirality from Colloidal Aluminum Nanoparticles Exhibiting Charge-Selective Protein Detection.

Chiral aluminum nanoparticles, dispersed in water, are prepared, which provide strong ultraviolet plasmonic circular dichroism, high-energy superchiral near-fields, and charge-selective protein detection.

Subdiffusive exciton transport in quantum dot solids.

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.