Hot Electron Generation and Cathodoluminescence Nanoscopy of Chiral Split Ring Resonators

Swiss roll nanoarrays for chiral plasmonic photocatalysis

Manipulating the chirality at nanoscale has drawn great attention among scientists, considering its pivotal role in various applications of current interest, including nano-optics, biomedicine, and photocatalysis. In this work, we delve into this arena by fabricating chiral Swiss roll nanoarray (SRNA) continuous films employing colloidal lithography. The technique permits the dimension of Swiss roll metamaterials to reduce to nanoscale, thus achieving chiroptical response (circular dichroism (CD)) in the visible region. The interplay between the CD signals and plasmon resonance modes is revealed through theoretical simulations, enabling a deep understanding of chiral plasmonic metamaterials. The polarization-sensitive photocatalytic activity of chiral SRNAs is investigated, noting a marked increase in the reaction rate when the chirality of SRNAs matches with the handedness of circularly polarized light (CPL). Notably, the SRNA continuous films based on substrate possess integration and reusability without complex recycling process, enhancing their practicality in applications like sensing and plasmonic nanochemistry, particularly toward polarization-dependent photocatalysis.

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.

Electron Beam Induced Circularly Polarized Light Emission of Chiral Gold Nanohelices

Chiral plasmonic nanostructures possess a chiroptical response orders of magnitude stronger than that of natural biomolecular systems, making them highly promising for a wide range of biochemical, medical, and physical applications. Despite extensive efforts to artificially create and tune the chiroptical properties of chiral nanostructures through compositional and geometrical modifications, a fundamental understanding of their underlying mechanisms remains limited. In this study, we present a comprehensive investigation of individual gold nanohelices by using advanced analytical electron microscopy techniques. Our results, as determined by angle-resolved cathodoluminescence polarimetry measurements, reveal a strong correlation between the circular polarization state of the emitted far-field radiation and the handedness of the chiral nanostructure in terms of both its dominant circularity and directional intensity distribution. Further analyses, including electron energy-loss measurements and numerical simulations, demonstrate that this correlation is driven by longitudinal plasmonic modes that oscillate along the helical windings, much like straight nanorods of equal strength and length. However, due to the three-dimensional shape of the structures, these longitudinal modes induce dipolar transverse modes with charge oscillations along the short axis of the helices for certain resonance energies. Their radiative decay leads to observed emission in the visible range. Our findings provide insight into the radiative properties and underlying mechanisms of chiral plasmonic nanostructures and enable their future development and application in a wide range of fields, such as nano-optics, metamaterials, molecular physics, biochemistry, and, most promising, chiral sensing via plasmonically enhanced chiral optical spectroscopy techniques.

Cathodoluminescence Nanoscopy: State of the Art and Beyond

Cathodoluminescence (CL) nanoscopy is proven to be a powerful tool to explore nanoscale optical properties, whereby free electron beams achieve a spatial resolution far beyond the diffraction limit of light. With developed methods for the control of electron beams and the collection of light, the dimension of information that CL can access has been expanded to include polarization, momentum, and time, holding promise to provide invaluable insights into the study of materials and optical near-field dynamics. With a focus on the burgeoning field of CL nanoscopy, this perspective outlines the recent advancements and applications of this technique, as illustrated by the salient experimental works. In addition, as an outlook for future research, several appealing directions that may bring about developments and discoveries are highlighted.

Tunable Chiral Plasmonic Activities Enabled via Stimuli Responsive Micro-Origami

Chiral plasmonic nanomaterials with distinctive circularly polarized light-dependent optical responses over a broad range of frequency have great potential for photonic and biomedical applications. However, it still remains challenging to fabricate 3D plasmonic chiral micro-constructs with readily modulated chiroptical properties over the magnitude of ellipticity, mode frequency, and switchable handedness, especially in the vis-NIR range. In this study, polymeric micro-origami-based 3D plasmonic chiral structures are constructed through self-rolling of gold  nanospheres (AuNSs)-decorated polymeric micro-sheets. Spherical AuNSs are assembled as highly ordered linear chains on 2D rectangular micro-sheets by polydimethylsiloxane-wrinkle assisted assembly. Upon rolling the micro-sheets to micro-tubules, the AuNS chains transform into 3D helices. The AuNS-assembled helices induce collective plasmonic modes propagating in a helical manner, leading to a strong chiral response over the vis-NIR range. The circular dichroism (CD) is measured to be as high as hundreds of millidegree, and the position and sign of CD peaks are actively modulated by controlling the orientated angle of AuNS chains, enabled by tuning the collective plasmonic modes. This micro-origami-based strategy incorporates the incompatible 2D assembly technique with 3D chiral structures, opening up an intriguing way toward constructing chiral plasmonic structures and modulating chiroptical effects based on responsive polymeric materials.

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.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Ultrathin Suspended Chiral Metasurfaces for Enantiodiscrimination

Chiral metasurfaces can exhibit a strong circular dichroism, but it is limited by the complicated fabrication procedure and alignment errors. Here, a new type of self-aligned suspended chiral bilayer metasurface with only one-step electron beam lithography exposure is demonstrated. A significant optical chirality of 221° µm^-1 can be realized using suspended metasurfaces with a thickness of 100 nm. Furthermore, this study experimentally demonstrates that such a structure is capable of label-free discrimination of the chiral molecules at zeptomole level, exhibiting a much higher sensitivity (orders of magnitude) compared to the conventional circular dichroism spectroscopy. The fundamental principles for chiral sensing using molecules-metasurfaces interactions are explored. Benefiting from the giant chiroptical response, the proposed metadevice may offer promising applications for ultrathin circular polarizers, chiral molecular detectors, and asymmetry information processing.

Broadband Design of Midinfrared Chiral Metamaterials Based on the Indium Tin Oxide Conical Helix

This paper proposed a periodic midinfrared broadband chiral structure composed of indium tin oxide (ITO) conical helix. The simulation results show that the structure achieves a good broadband CD in the midinfrared. At the wavelength of 9.2 μm, the maximum value of CD is 0.55, the FWHM (full width half maximum) of CD is 7.2 μm, and the wavelength range is from 5.7 μm to 12.9 μm. The simulation results also show that compared with the traditional uniform spiral structure with the same radius, the conical spiral structure can achieve better broadband circular dichroism in the midinfrared band. This provides a new idea for the design of broadband polarization state control devices in the midinfrared band.

Plasmonic Circular Dichroism in Chiral Gold Nanowire Dimers

We report a computational study at the time-dependent density functional theory (TDDFT) level of the chiro-optical spectra of chiral gold nanowires coupled in dimers. Our goal is to explore whether it is possible to overcome destructive interference in single nanowires that damp chiral response in these systems and to achieve intense plasmonic circular dichroism (CD) through a coupling between the nanostructures. We predict a huge enhancement of circular dichroism at the plasmon resonance when two chiral nanowires are intimately coupled in an achiral relative arrangement. Such an effect is even more pronounced when two chiral nanowires are coupled in a chiral relative arrangement. Individual component maps of rotator strength, partial contributions according to the magnetic dipole component, and induced densities allow us to fully rationalize these findings, thus opening the way to the field of plasmonic CD and its rational design.

Inorganic Chiral Hybrid Nanostructures for Tailored Chiroptics and Chirality-Dependent Photocatalysis

Inorganic chiral hybrid nanostructures embedding chirality within distinct material compositions can create novel chiral properties and functionalities absent from achiral ones, but remain largely unexplored. We report for the first time a class of chiral plasmonic metal-semiconductor core-shell nanostructures by employing structurally chiral nanoparticles as chirality inducing templates to grow functional shell materials, which allows us to independently control material parameters including core geometry and shell thickness, as well as handedness of the system. We experimentally and theoretically achieve enhanced and tunable chiroptical activity of the hetero-structures as a result of the core-shell strong coupling effect. As a proof-of-concept demonstration, we show the chiral hybrid nanostructures can drive chirality-dependent photocatalytic hydrogen generation under circularly polarized light. This study enables rational design and functionalization of chiral hybrid nanomaterials towards enhanced chiral light-matter interactions and chiral device applications.

Circularly Polarized Plasmons in Chiral Gold Nanowires via Quantum-Mechanical Design

Time-dependent density functional theory (TDDFT) simulations are conducted on a series of chiral gold nanowires to explore whether an enhancement of circular dichroism at the plasmon resonance is possible and identify its quantum-mechanical origin. We find that in linear two-dimensional chiral nanowires the dichroic response is suppressed by destructive interference of nearly degenerate components with opposite signs, pointing to this phenomenon as a common and likely origin of the difficulty encountered so far in achieving a plasmonic CD response in experiment and suggesting nevertheless that these opposite components could be "decoupled" by using multiwall arrangements. In contrast, we predict a giant dichroic response for nanowires with three-dimensional helical coiling. We rationalize this finding via an electronic structure analysis of longitudinal and transversal plasmonic excitations and their coupling into chiral components, and we propose a simple formula for the chiral response as a function of structural parameters (nanowire length and coiling number).

Revealing the Chiroptical Response of Plasmonic Nanostructures at the Nanofemto Scale

The spatiotemporal origin of plasmonic chiroptical responses in nanostructures remains unexplored and unclear. Here, two orthogonally oriented Au nanorods as a prototype were investigated, with a giant chiroptical response caused by antisymmetric and symmetric mode excitations for obliquely incident left-handed circular polarization (LCP) and right-handed circular polarization (RCP) light. Time-resolved photoemission electron microscopy (PEEM) was employed to measure the near-field spatial distributions, spectra, and spatiotemporal dynamics of plasmonic modes associated with the chiroptical responses at the nanofemto scale, verifying the characteristic near-field distributions at the resonant wavelengths of the two modes and a very large spectral dichroism for LCP and RCP. More importantly, eigenmode excitations and their contributions to the ultrafast plasmonic chiroptical response in the space-time domain were directly revealed, promoting a full understanding of the ultrafast chiral origin in complex nanostructures. These findings open a way to design chiroptical nanophotonic devices for spatiotemporal control of chiral light-matter interactions.

Broadband chiral silicon photonic circuits based on fork-type inversely tapered nanowire waveguides

Optical spin or circular polarization provides a new degree of freedom to control light-matter interaction in the fundamentals and applications of light. To broaden the bandwidth of chiral (spin-controlled) coupling in photonic integrated circuits, we propose fork-type inversely tapered nanowire waveguides to compensate for the out-of-step phase evolution of adiabatic coupling between ${{\rm TE}_0}$ and ${{\rm TM}_0}$ (${{\rm TE}_1}$) modes excited from the $x$- and $y$-polarization components of spin polarized light in free space, respectively. We design and simulate two kinds of devices based on air and ${{\rm Si}_3}{{\rm N}_4}$ up-claddings to show the feasibility of broadening the bandwidth of chiral silicon photonic circuits by using fork-type inverse tapers. Numerical results show that the bandwidth can approach 70 nm under high directionality of above 0.90. This broadband chiral coupling via the new phase synchronizing technique with fork-type inverse tapers may pave the way to develop on-chip spin photonics or polarization-based photonic integrated devices.

Chiro-optical response of a wafer scale metamaterial with ellipsoidal metal nanoparticles

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

Control of light-valley interactions in 2D transition metal dichalcogenides with nanophotonic structures

Electronic valley in two-dimensional transition-metal dichalcogenides (2D TMDCs) offers a new degree of freedom for information storage and processing. The valley pseudospin can be optically encoded by photons with specific helicity, enabling the construction of electronic information devices with both high performance and low power consumption. Robust detection, manipulation and transport of the valley pseudospins at room temperature are still challenging because of the short lifetime of valley-polarized carriers and excitons. Integrating 2D TMDCs with nanophotonic objects such as plasmonic nanostructures provides a competitive solution to address the challenge. The research in this field is of practical interest and can also present rich physics of light-matter interactions. In this minireview, recent progress on using nanophotonic strategies to enhance the valley polarization degree, especially at room temperature, is highlighted. Open questions, major challenges, and interesting future developments in manipulating the valley information in 2D semiconductors with the help of nanophotonic structures will also be discussed.

Selectively steering photon spin angular momentum via electron-induced optical spin Hall effect

The development of the optical spin Hall effect (OSHE) realizes the splitting of different spin components, contributing to the manipulation of photon spin angular momentum that acts as the information carrier for quantum technology. However, OSHE with optical excitation lacks active control of photon angular momentum at deep subwavelength scale because of the optical diffraction limit. Here, we experimentally demonstrate a selective manipulation of photon spin angular momentum at a deep subwavelength scale via electron-induced OSHE in Au nanoantennas. The inversion of the OSHE radiation pattern is observed by angle-resolved cathodoluminescence polarimetry with the electron impact position shifting within 80 nm in a single antenna unit. By this selective steering of photon spin, we propose an information encoding with robustness, privacy, and high level of integration at a deep subwavelength scale for the future quantum applications.

Discrete metal nanoparticles with plasmonic chirality

From a geometrical perspective, a chiral object does not have mirror planes or inversion symmetry. It exhibits the same physical properties as its mirror image (enantiomer), except for the chiroptical activity, which is often the opposite. Recent advancements have identified particularly interesting implications of chirality on the optical properties of metal nanoparticles, which are intimately related to localized surface plasmon resonance phenomena. Although such resonances are usually independent of the circular polarization of light, specific strategies have been applied to induce chirality, both in assemblies and at the single-particle level. In this tutorial review, we discuss the origin of plasmonic chirality, as well as theoretical models that have been proposed to explain it. We then summarise recent developments in the synthesis of discrete nanoparticles with plasmonic chirality by means of wet-chemistry methods. We conclude with a discussion of promising applications for discrete chiral nanoparticles. We expect this tutorial review to be of interest to researchers from a wide variety of disciplines where chiral plasmonics can be exploited at the nanoparticle level, such as chemical sensing, photocatalysis, photodynamic or photothermal therapies, etc.

Chiral Light Emission from a Sphere Revealed by Nanoscale Relative-Phase Mapping

Circularly polarized light (CPL) is currently receiving much attention as a key ingredient for next-generation information technologies, such as quantum communication and encryption. CPL photon generation used in those applications is commonly realized by coupling achiral optical quantum emitters to chiral nanoantennas. Here, we explore a different strategy consisting in exciting a nanosphere-the ultimate symmetric structure-to produce CPL emission along an arbitrary direction. Specifically, we demonstrate chiral emission from a silicon nanosphere induced by an electron beam based on two different strategies: either shifting the relative phase of degenerate orthogonal dipole modes or interfering electric and magnetic modes. We prove these concepts both theoretically and experimentally by visualizing the phase and polarization using a fully polarimetric four-dimensional cathodoluminescence method. Besides their fundamental interest, our results support the use of free-electron-induced light emission from spherically symmetric systems as a versatile platform for the generation of chiral light with on-demand control over the phase and degree of polarization.

Chiral surface plasmon-enhanced chiral spectroscopy: principles and applications

In this review, the development context and scientific research results of chiral surface plasmons (SPs) in recent years are classified and described in detail. First, the principle of chiral SPs is introduced through classical and quantum theory. Following this, the classification and properties of different chiral structures, as well as the superchiral near-field, are introduced in detail. Second, we describe the excitation and propagation properties of chiral SPs, which lays a good foundation for the application of chiral SPs and their chiral spectra in various fields. After that, we have summarized the recent research results of chiral SPs and their applications in the areas of biology, two-dimensional materials, topological materials, analytical chemistry, chiral sensing, chiral optical force, and chiral light detection. Chiral SPs are a new type of optical phenomenon that have useful application potential in many fields and are worth exploring.

Molecular Chirality Detection with Periodic Arrays of Three-Dimensional Twisted Metamaterials

Rapid detection of the handiness of chiral molecules is an important topic for pharmaceutical industries because chiral drugs with opposing handiness sometimes exhibit unwanted side effects. In this research, a rapid optical method is proposed to determine the handiness of the chiral drug "Thalidomide". The platform is a large array of three-dimensional (3D) twisted metamaterials fabricated with a novel method by combining nanospherical-lens lithography (NLL) and hole-mask lithography (HML). The fabrication is high-throughput and the twisted metamaterials cover a large area. Strong circular dichroism (CD) response is observed in the near-infrared (NIR) region, which enables the chiral detection to be performed by a low-cost and portable spectroscope system. The proposed nanofabrication method significantly improves the capabilities of NLL and HML, which can be quickly adapted to fabricate various periodic 3D metamaterials. In addition, the results of this research pave the road for the rapid penetration of nanophotonics into the pharmaceutical industries.

Plasmonic Chirality and Circular Dichroism in Bioassembled and Nonbiological Systems: Theoretical Background and Recent Progress

Nature is chiral, thus chirality is a key concept required to understand a multitude of systems in physics, chemistry, and biology. The field of optics offers valuable tools to probe the chirality of nanosystems, including the measurement of circular dichroism, the differential interaction strength between matter and circularly polarized light with opposite helicity. Simultaneously, the use of plasmonic systems with giant light-interaction cross-sections opens new paths to investigate and manipulate systems on the nanoscale. Consequently, the interest in chiral plasmonic and hybrid systems has continually grown in recent years, due to their potential applications in biosensing, polarization-encoded optical communication, polarization-selective chemical reactions, and materials with polarization-dependent light-matter interaction. Experimentally, chiral properties of nanostructures can be either created artificially using modern fabrication techniques involving inorganic materials, or borrowed from nature using bioassembly or biomolecular templating. Herein, the recent progress in the field of plasmonic chirality is summarized, with a focus on both the theoretical background and the experimental advances in the study of chirality in various systems, including molecular-plasmonic assemblies, chiral plasmonic nanostructures, chiral assemblies of interacting plasmonic nanoparticles, and chiral metal metasurfaces and metamaterials. The growth prospects of this field are also discussed.

Giant polarization anisotropic optical response from anodic aluminum oxide templates embedded with plasmonic metamaterials

Plasmonic metamaterials enable extraordinary manipulation of key constitutive properties of light at a subwavelength scale and thus have attracted significant interest. Here, we report a simple and convenient nanofabrication method for a novel meta-device by glancing deposition of gold into anodic aluminum oxide templates on glass substrates. A methodology with the assistance of ellipsometric measurements to examine the anisotropy and optical activity properties is presented. A tunable polarization conversion in both transmission and reflection is demonstrated. Specifically, giant broadband circular dichroism for reflection at visible wavelengths is experimentally realized by oblique incidence, due to the extrinsic chirality resulting from the mutual orientation of the metamaterials and the incident beam. This work paves the way for practical applications for large-area, low-cost polarization modulators, polarization imaging, displays, and bio-sensing.

Room-temperature valleytronic transistor

Valleytronics, based on the valley degree of freedom rather than charge, is a promising candidate for next-generation information devices beyond complementary metal-oxide-semiconductor (CMOS) technology^1-4. Although many intriguing valleytronic properties have been explored based on excitonic injection or the non-local response of transverse current schemes at low temperature^4-7, demonstrations of valleytronic building blocks similar to transistors in electronics, especially at room temperature, remain elusive. Here, we report a solid-state device that enables a full sequence of generating, propagating, detecting and manipulating valley information at room temperature. Chiral nanocrescent plasmonic antennae⁸ are used to selectively generate valley-polarized carriers in MoS₂ through hot-electron injection under linearly polarized infrared excitation. These long-lived valley-polarized free carriers can be detected in a valley Hall configuration^9-11 even without charge current, and can propagate over 18 μm by means of drift. In addition, electrostatic gating allows us to modulate the magnitude of the valley Hall voltage. The electrical valley Hall output could drive the valley manipulation of a cascaded stage, rendering the device able to serve as a transistor free of charge current with pure valleytronic input/output. Our results demonstrate the possibility of encoding and processing information by valley degree of freedom, and provide a universal strategy to study the Berry curvature dipole in quantum materials.

Au Nanospirals Transferred onto PDMS Film Exhibiting Circular Dichroism at Visible Wavelengths

We propose a thin, single-layered circular dichroic filter with Au nanospiral structures on a polydimethylsiloxane (PDMS) thin film that has strong circular dichroism at visible wavelengths. Au nanospiral structures with a diameter of 70 nm were fabricated by cryogenic glancing angle deposition on a substrate with a nanodot array template patterned with the block copolymer PS-PDMS. The Au nanospiral structures were transferred onto a transparent and flexible PDMS thin film to fabricate a thin, single-layered circular dichroic filter. The filter had a very large circular dichroism peak of -830 mdeg at 630 nm. The results show that the Au nanospiral structures transferred onto PDMS thin film exhibit large circular dichroism at visible wavelengths.

Oliveira O, Mejía-Salazar JR. Chiral Plasmonics and Their Potential for Point-of-Care Biosensing Applications

There has been growing interest in using strong field enhancement and light localization in plasmonic nanostructures to control the polarization properties of light. Various experimental techniques are now used to fabricate twisted metallic nanoparticles and metasurfaces, where strongly enhanced chiral near-fields are used to intensify circular dichroism (CD) signals. In this review, state-of-the-art strategies to develop such chiral plasmonic nanoparticles and metasurfaces are summarized, with emphasis on the most recent trends for the design and development of functionalizable surfaces. The major objective is to perform enantiomer selection which is relevant in pharmaceutical applications and for biosensing. Enhanced sensing capabilities are key for the design and manufacture of lab-on-a-chip devices, commonly named point-of-care biosensing devices, which are promising for next-generation healthcare systems.

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.

L-shaped ITO structures fabricated by oblique angle deposition technique for mid-infrared circular dichroism

This paper proposes a mid-infrared chiral structure, which consists of L-shaped indium tin oxide (ITO) films formed on self-assembled monolayer polystyrene microspheres in two orthogonal directions by oblique angle deposition technique. Experimental results demonstrate that the structure exhibit circular dichroism (CD) responses in the range of 2.5 - 4 µm. As the thickness difference of the ITO films in the two orthogonal directions increases, the CD response enhances. The reason is that the ITO films produce cross dipoles and their bigger differences in thickness bring to bigger phase differences in optical chirality. The experimental results also demonstrate that the CD signals are evidently stronger than those of the structure consisting of silver in the mid-infrared band. This work provides a new idea for the fabrication of mid-infrared chiral structures, which have potential applications in the polarization state control of mid-infrared lasers.

Emerging Electron Microscopy Techniques for Probing Functional Interfaces in Energy Materials

Interfaces play a fundamental role in many areas of chemistry. However, their localized nature requires characterization techniques with high spatial resolution in order to fully understand their structure and properties. State-of-the-art atomic resolution or in situ scanning transmission electron microscopy and electron energy-loss spectroscopy are indispensable tools for characterizing the local structure and chemistry of materials with single-atom resolution, but they are not able to measure many properties that dictate function, such as vibrational modes or charge transfer, and are limited to room-temperature samples containing no liquids. Here, we outline emerging electron microscopy techniques that are allowing these limitations to be overcome and highlight several recent studies that were enabled by these techniques. We then provide a vision for how these techniques can be paired with each other and with in situ methods to deliver new insights into the static and dynamic behavior of functional interfaces.

Spatially and spectrally resolved orbital angular momentum interactions in plasmonic vortex generators

Understanding the near-field electromagnetic interactions that produce optical orbital angular momentum (OAM) is crucial for integrating twisted light into nanotechnology. Here, we examine the cathodoluminescence (CL) of plasmonic vortices carrying OAM generated in spiral nanostructures. The nanospiral geometry defines a photonic local density of states that is sampled by the electron probe in a scanning transmission electron microscope (STEM), thus accessing the optical response of the plasmonic vortex with high spatial and spectral resolution. We map the full spectral dispersion of the plasmonic vortex in spiral structures designed to yield increasing topological charge. Additionally, we fabricate nested nanospirals and demonstrate that OAM from one nanospiral can be coupled to the nested nanospiral, resulting in enhanced luminescence in concentric spirals of like handedness with respect to concentric spirals of opposite handedness. The results illustrate the potential for generating and coupling plasmonic vortices in chiral nanostructures for sensitive detection and manipulation of optical OAM.

Chiral Plasmonic Nanocrystals for Generation of Hot Electrons: Toward Polarization-Sensitive Photochemistry

The use of biomaterials, with techniques such as DNA-directed assembly or biodirected synthesis, can surpass top-down fabrication techniques in creating plasmonic superstructures in terms of spatial resolution, range of functionality, and fabrication speed. In particular, by enabling a very precise placement of nanoparticles in a bioassembled complex or through the controlled biodirected shaping of single nanoparticles, plasmonic nanocrystals can show remarkably strong circular dichroism (CD) signals. We show that chiral bioplasmonic assemblies and single nanocrystals can enable polarization-sensitive photochemistry based on the generation of energetic (hot) electrons. It is now established that hot plasmonic electrons can induce surface photochemistry or even reshape plasmonic nanocrystals. We show that merging chiral plasmonic nanocrystal systems and the hot-election generation effect offers unique possibilities in photochemistry, such as polarization-sensitive photochemistry promoting nonchiral molecular reactions, chiral photoinduced growth of a colloid at the atomic level, and chiral photochemical destruction of chiral nanocrystals. In contrast, for chiral molecular systems, the equivalent of the described effects is challenging to observe because molecular species typically exhibit very small CD signals. Moreover, we compare our findings with traditional chiral photochemistry at the molecular level, identifying new, different regimes for chiral photochemistry with possibilities that are unique for plasmonic colloidal systems. In this study, we bring together the concept of hot-electron generation and the field of chiral colloidal plasmonics. Using chiral plasmonic nanorod complexes as a model system, we demonstrate remarkably strong CD in both optical extinction and generation rates of hot electrons. Studying the regime of steady-state excitation, we discuss the influence of geometrical and material parameters on the chiral effects involved in the generation of hot electrons. Optical chirality and the chiral hot-electron response in the nanorod dimers result from complex interparticle interactions, which can appear in the weak coupling regime or in the form of Rabi splitting. Regarding practical applications, our study suggests interesting opportunities in polarization-sensitive photochemistry, in chiral recognition or separation, and in promoting chiral crystal growth at the nanoscale.

Imaging of Plasmonic Chiral Radiative Local Density of States with Cathodoluminescence Nanoscopy

Chiral light-matter interactions as an emerging aspect of quantum optics enable exceptional physical phenomena and advanced applications in nanophotonics through the nanoscale exploitation of photon-emitter interactions. The chiral radiative properties of quantum emitters strongly depend on the photonic environment, which can be drastically altered by plasmonic nanostructures with a high local density of states (LDOS). Hence, precise knowledge of the chiral photonic environment is essential for manipulating the chirality of light-matter interactions, which requires high resolution chiral characterization techniques. In this work, chiral radiative LDOS distributions of single plasmonic nanostructures that directly govern the chiral radiative spontaneous decay of quantum emitters are imaged at the nanoscale by using cathodoluminescence nanoscopy, enabling precise and highly efficient control of chiral photon emission for chiroptical technologies. Radiative LDOS hot-spots with the chirality larger than 93% are obtained by properly designing chiral plasmonic modes of Au nanoantennas. After fabricating monolayered WSe₂ nanodisks (NDs) at chiral radiative LDOS hot-spots and forming ND/Au hybrid nanostructures, the chiral radiative properties of WSe₂ NDs are significantly modified, leading to chiral photoluminescence. Our experimental concept and method provide an effective way to characterize and manipulate chiral light-matter interactions at the nanoscale, facilitating future applications in chiral quantum nanophotonics such as single-photon sources and light emission devices.

3D Janus plasmonic helical nanoapertures for polarization-encrypted data storage

Helical structures have attracted considerable attention due to their inherent optical chirality. Here, we report a unique type of 3D Janus plasmonic helical nanoaperture with direction-controlled polarization sensitivity, which is simply fabricated via the one-step grayscale focused ion beam milling method. Circular dichroism in transmission of as large as 0.72 is experimentally realized in the forward direction due to the spin-dependent mode coupling process inside the helical nanoaperture. However, in the backward direction, the nanoaperture acquires giant linear dichroism in transmission of up to 0.87. By encoding the Janus metasurface with the two nanoaperture enantiomers having specified rotation angles, direction-controlled polarization-encrypted data storage is demonstrated for the first time, where a binary quick-response code image is displayed in the forward direction under the circularly polarized incidence of a specified handedness, while a distinct grayscale image is revealed in the backward direction under linearly polarized illumination with a specified azimuthal angle. We envision that the proposed Janus helical nanoapertures will provide an appealing platform for a variety of applications, which will range from multifunctional polarization control, enantiomer sensing, data encryption and decryption to optical information processing.

Enhanced circular dichroism of tilted zigzag-shaped nanohole arrays

Circular dichroism (CD) of nanostructures is in great demand for applications in biological molecules, photocurrent devices, and photocatalysis. Planar nanostructures can be prepared in a concise manner, and their CD effects have gained much research interest. In this study, tilted zigzag-shaped nanohole (TZSN) arrays are proposed, and the CD effect is studied by the finite element method. A strong resonance occurs in the gap by tuning the charge distributions between adjacent nanoholes. Meanwhile, the CD effect of TZSN arrays is strongly dependent on the structural parameters of TZSN. Results provide a novel method for tuning the CD effects of nanohole arrays on a film.

Free-Standing Plasmonic Chiral Metamaterials with 3D Resonance Cavities

Hollow nanocone array (HNCA) films (cm × cm), composed of two Ag and Au nanoshells, are fabricated via a low-cost and efficient colloidal lithography technique. The relative position of the Ag and Au nanoshells can be controlled to generate various chiral asymmetries. A pronounced chiroptical response is observed in the ultraviolet-visible region with the anisotropy factor up to 10^-1, which is rooted in the asymmetric current oscillations and electric field distributions. Beyond previous reports on plasmonic chiral metamaterials, the HNCA can be free-standing and further transferred to other functional and flexible substrates, such as polydimethylsiloxane (PDMS), highly curved surfaces, prepatterned films, and hydrogels, while keeping the original features. The good transferability would make HNCA more flexible in specific applications. Furthermore, the chiral HNCAs offer a series of chiral resonance cavities, which are conducive for the research of chiral sensing, confinement, chiral signal transmission, and amplification. Overall, this work provides a scalable metamaterial to tune the plasmonic chiral response, and HNCA would be a promising candidate of the components in chiral optical devices and sensors.

Cathodoluminescence as a probe of the optical properties of resonant apertures in a metallic film

Here we present the results of an investigation of resonances of azimuthal trimer arrangements of rectangular slots in a gold film on a glass substrate using cathodoluminescence (CL) as a probe. The variation in the CL signal collected from specific locations on the sample as a function of wavelength and the spatial dependence of emission into different wavelength bands provides considerable insight into the resonant modes, particularly sub-radiant modes, of these apertures. By comparing our experimental results with electromagnetic simulations we are able to identify a Fabry-Pérot mode of these cavities as well as resonances associated with the excitation of surface plasmon polaritons on the air-gold boundary. We obtain evidence for the excitation of dark (also known as sub-radiant) modes of apertures and aperture ensembles.

Deep-Subwavelength Resolving and Manipulating of Hidden Chirality in Achiral Nanostructures

The chiral state of light plays a vital role in light-matter interactions and the consequent revolution of nanophotonic devices and advanced modern chiroptics. As the light-matter interaction goes into the nano- and quantum world, numerous chiroptical technologies and quantum devices require precise knowledge of chiral electromagnetic modes and chiral radiative local density of states (LDOS) distributions in detail, which directly determine the chiral light-matter interaction for applications such as chiral light detection and emission. With classical optical techniques failing to directly measure the chiral radiative LDOS, deep-subwavelength imaging and control of circular polarization (CP) light associated phenomena are introduced into the agenda. Here, we simultaneously reveal the hidden chiral electromagnetic mode and acquire its chiral radiative LDOS distribution of a single symmetric nanostructure at the deep-subwavelength scale by using CP-resolved cathodoluminescence (CL) microscopy. The chirality of the symmetric nanostructure under normally incident light excitation, resulting from the interference between the symmetric and antisymmetric modes of the V-shaped nanoantenna, is hidden in the near field with a giant chiral distribution (∼99%) at the arm-ends, which enables the circularly polarized CL emission from the radiative LDOS hot-spot and the following active helicity control at the deep-subwavelength scale. The proposed V-shaped nanostructure as a functional unit is further applied to the helicity-dependent binary encoding and the two-dimensional display applications. The proposed physical principle and experimental configuration can promote the future chiral characterization and manipulation at the deep-subwavelength scale and provide direct guidelines for the optimization of chiral light-matter interactions for future quantum studies.

Advanced Plasmonic Materials for Dynamic Color Display

Plasmonic structures exhibit promising applications in high-resolution and durable color generation. Research on advanced hybrid plasmonic materials that allow dynamically reconfigurable color control has developed rapidly in recent years. Some of these results may give rise to practically applicable reflective displays in living colors with high performance and low power consumption. They will attract broad interest from display markets, compared with static plasmonic color printing, for example, in applications such as digital signage, full-color electronic paper, and electronic device screens. In this progress report, the most promising recent examples of utilizing advanced plasmonic materials for the realization of dynamic color display are highlighted and put into perspective. The performances, advantages, and disadvantages of different technologies are discussed, with emphasis placed on both the potential and possible limitations of various hybrid materials for dynamic plasmonic color display.

Polarization- and wavelength-resolved near-field imaging of complex plasmonic modes in Archimedean nanospirals

Asymmetric nanophotonic structures enable a wide range of opportunities in optical nanotechnology because they support efficient optical nonlinearities mediated by multiple plasmon resonances over a broad spectral range. The Archimedean nanospiral is a canonical example of a chiral plasmonic structure because it supports even-order nonlinearities that are not generally accessible in locally symmetric geometries. However, the complex spiral response makes nanoscale experimental characterization of the plasmonic near-field structure highly desirable. Here we employ high-efficiency, high-spatial-resolution cathodoluminescence imaging in a scanning transmission electron microscope to describe the spatial, spectral, and polarization response of plasmon modes in the nanospiral geometry.

Chiral Metamaterials of Plasmonic Slanted Nanoapertures with Symmetry Breaking

We propose a universal design scheme for a new type of chiral metamaterials based on plasmonic slanted nanoapertures simply milled in a single metal layer. Strong optical chirality is introduced by tilting nanoapertures with almost arbitrary shape along a certain direction to break all the mirror symmetries. As a typical example, chiral metamaterial based on slanted split-ring apertures is demonstrated with giant circular dichroism in transmission (CDT) over 78% at 760 nm. We reveal that the high CDT originates from the circularly dichroic mode coupling process in the slanted nanoapertures induced by spin-dependent field overlap conditions. Furthermore, tunable CDT is presented through the in-plane rotation of nanoapertures to form chiral images with controllable image contrast. Besides, chiral metamaterials with slanted nanoapertures of two other shapes including L-shaped aperture and rectangular aperture are also presented with large circular dichroism. We envision that our demonstrated chiral metamaterials enable promising platforms for a variety of applications in nonlinear optics, chiral imaging and sensing, spectroscopy, and polarization manipulation.

Reveal and Control of Chiral Cathodoluminescence at Subnanoscale

Circularly polarized light is crucial for the modern physics research. Highly integrated nanophotonic device further requires the control of circularly polarized light at subnanoscale. Here, we report the tuning of chiral cathodoluminescence (CL) on single Au nanostructure under electron stimulation. The detected CL helicity is found ultrasensitive with the electron impinging position on the structure, and a helicity switch is achieved within a 1.86 nm electron-beam movement, which is applied to construct ternary notation sequence. The proposed configuration provides a delicate platform for the CL helicity control, which opens a way for the future chiral applications at subnanoscale like information coding and quantum communication.

On chip chirality-distinguishing beamsplitter

The chirality of photons plays a fundamental role in light-matter interactions. However, a limiting factor in photonic integrated circuits is the lack of a miniaturized component, which can distinguish the chirality in a low cost and integrated manner. Herein we numerically demonstrate a chirality-distinguishing beamsplitter that can address this challenge. It consists of an integrated polarization rotator and a linear polarization beamsplitter, which together can fulfill the task of distinguishing and splitting left- and right-handed quasi-circularly polarized modes on a chip with an ultra-broadband operation range from 1.45 μm to 1.65 μm. Owning to the reciprocity, the device can emit photons with selectable spin angular momentum depending on the chosen feeding waveguide. The device is compatible with complementary metal-oxide semiconductor technology and it may open up new avenues in the fields of on-chip nano-photonics, bio-photonics and quantum information science.

Metasurfaces and Colloidal Suspensions Composed of 3D Chiral Si Nanoresonators

High-refractive-index silicon nanoresonators are promising low-loss alternatives to plasmonic particles in CMOS-compatible nanophotonics applications. However, complex 3D particle morphologies are challenging to realize in practice, thus limiting the range of achievable optical functionalities. Using 3D film structuring and a novel gradient mask transfer technique, the first intrinsically chiral dielectric metasurface is fabricated in the form of a monolayer of twisted silicon nanocrescents that can be easily detached and dissolved into colloidal suspension. The metasurfaces exhibit selective handedness and a circular dichroism as large as 160° µm^-1 due to pronounced differences in induced current loops for left-handed and right-handed polarization. The detailed morphology of the detached particles is analyzed using high-resolution transmission electron microscopy. Furthermore, it is shown that the particles can be manipulated in solution using optical tweezers. The fabrication and detachment method can be extended to different nanoparticle geometries and paves the way for a wide range of novel nanophotonic experiments and applications of high-index dielectrics.

Plasmon-induced charge separation: chemistry and wide applications

Recent development of nanoplasmonics has stimulated chemists to utilize plasmonic nanomaterials for efficient and distinctive photochemical applications, and physicists to boldly go inside the "wet" chemistry world. The discovery of plasmon-induced charge separation (PICS) has even accelerated these trends. On the other hand, some confusion is found in discussions about PICS. In this perspective, we focus on differences between PICS and some other phenomena such as co-catalysis effect and plasmonic nanoantenna effect. In addition, materials and nanostructures suitable for PICS are shown, and characteristics and features unique to PICS are documented. Although it is well known that PICS has been applied to photovoltaics and photocatalysis, here light is shed on other applications that take better advantage of PICS, such as chemical sensing and biosensing, various photochromisms, photoswitchable functionalities and nanoscale photofabrication.