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Against the background of the current healthcare and climate emergencies, surface enhanced Raman scattering (SERS) is becoming a highly topical technique for identifying and fingerprinting molecules, e.g., within viruses, bacteria, drugs, and atmospheric aerosols. Crucial for SERS is the need for substrates with strong and reproducible enhancements of the Raman signal over large areas and with a low fabrication cost. Here, dense arrays of plasmonic nanohelices (≈100 nm in length), which are of interest for many advanced nanophotonics applications, are investigated, and they are shown to present excellent SERS properties. As an illustration, two new ways to probe near-field enhancement generated with circular polarization at chiral metasurfaces are presented, first using the Raman spectra of achiral molecules (crystal violet) and second using a single, element-specific, achiral molecular vibrational mode (i.e., a single Raman peak). The nanohelices can be fabricated over large areas at a low cost and they provide strong, robust and uniform Raman enhancement. It is anticipated that these advanced materials will find broad applications in surface enhanced Raman spectroscopies and material science.

People of all ages consume salt every day, but is it really just salt? Plastic nanoparticles (nanoplastics) polluting the oceans and the atmosphere are posing an increasing environmental threat and have begun to contaminate everyday salt in consumer goods. Herein, we developed a combined Surface Enhanced Raman Scattering (SERS) and Stimulated Raman Scattering (SRS) approach that can simultaneously realize the filtration, enrichment, and detection of nanoplastics in commercial salt. SERS was used to explore the potential types of nanoplastic contaminants in salts. SRS was used to conduct imaging and to quantify the presence of nanoplastics. Nanoplastics in sea salts produced from different sources were studied, with the following nanoplastics detected: polystyrene (PS), polyethylene (PE), polypropylene (PP). We estimate that, depending on location, an average person could be ingesting as many as 63,100 nanoplastics per year through the consumption of sea salt alone. Nanoplastics can readily adsorb organic matter and heavy metal particles, so the potential harm to human health should not be underestimated.

In recent years, bioanalytical surface-enhanced Raman spectroscopy (SERS) has blossomed into a fast-growing research area. Owing to its high sensitivity and outstanding multiplexing ability, SERS is an effective analytical technique that has excellent potential in bioanalysis and diagnosis, as demonstrated by its increasing applications in vivo. SERS allows the rapid detection of molecular species based on direct and indirect strategies. Because it benefits from the tunable surface properties of nanostructures, it finds a broad range of applications with clinical relevance, such as biological sensing, drug delivery and live cell imaging assays. Of particular interest are early-stage-cancer detection and the fast detection of pathogens. Here, we present a comprehensive survey of SERS-based assays, from basic considerations to bioanalytical applications. Our main focus is on SERS-based pathogen detection methods as point-of-care solutions for early bacterial infection detection and chronic disease diagnosis. Additionally, various promising in vivo applications of SERS are surveyed. Furthermore, we provide a brief outlook of recent endeavours and we discuss future prospects and limitations for SERS, as a reliable approach for rapid and sensitive bioanalysis and diagnosis.

Linear optical methods of determining the chirality of organic and inorganic materials have relied on weak chiral optical (chiroptical) effects. Nonlinear chiroptical characterization holds the potential of much greater sensitivity and smaller interaction volumes. However, suitable materials on which to perform measurements have been lacking for decades. Here, we present the first nonlinear chiroptical characterization of crystallographic chirality in gold helicoids (≈150 nm size) and core/shell helicoids with the newly discovered hyper-Rayleigh scattering optical activity (HRS OA) technique. The observed chiroptical signal is, on average, originating from between ≈0.05 and ≈0.13 helicoids, i.e., less than a single nanoparticle. The measured HRS OA ellipticities reach ≈3°, for a concentration ≈109 times smaller than that of chiral molecules with similar nonlinear chiroptical response. These huge values indicate that the helicoids are excellent candidates for future nonlinear chiroptical materials and applications.

Detecting atmospheric bioaerosols in a quantitative way is highly desirable for public health and safety. This work demonstrates that surface-enhanced Raman spectroscopy (SERS) is a simple and rapid analytical technique for the detection of atmospheric bioaerosols, on a Klarite substrate. For both simulated and ambient bioaerosols, this detection assay results in an increase in the enhancement factor of the Raman signal. We report a strong SERS signal generated by bioaerosols containing living Escherichia coli deposited on Klarite. Furthermore, we demonstrate that SERS mapping can be used to estimate the percentage of airborne, living Escherichia coli. Moreover, Klarite provides differently distinct SERS spectra at different bacterial growth phases, indicating its potential to identify changes occurring in the bacterial envelope. Finally, we applied SERS for the rapid detection of Escherichia coli in ambient bioaerosols without using time-consuming and laborious culture processes. Our results represent rapid, culture-free and label-free detection of airborne bacteria in the real-world environment.

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. ispartof: NANOSCALE vol:14 issue:10 pages:3888-3898 ispartof: location:England status: published

Chiroptical spectroscopies provide structural analyses of molecules and nanoparticles but they require sample volumes that are incompatible with generating large chemical libraries. New optical tools are needed to characterize chirality for the ultrasmall (−5 µl.

Emerging antibiotic resistant bacteria constitute one of the biggest threats to public health. Surface-enhanced Raman scattering (SERS) is highly promising for detecting such bacteria and for antibiotic susceptibility testing (AST). SERS is fast, non-destructive (can probe living cells) and it is technologically flexible (readily integrated with robotics and machine learning algorithms). However, in order to integrate into efficient point-of-care (PoC) devices and to effectively replace the current culture-based methods, it needs to overcome the challenges of reliability, cost and complexity. Recently, significant progress has been made with the emergence of both new questions and new promising directions of research and technological development. This article brings together insights from several representative SERS-based AST studies and approaches oriented towards clinical PoC biosensing. It aims to serve as a reference source that can guide progress towards PoC routines for identifying antibiotic resistant pathogens. In turn, such identification would help to trace the origin of sporadic infections, in order to prevent outbreaks and to design effective medical treatment and preventive procedures.

Surface enhanced Raman spectroscopy (SERS) shows great promise in studying individual atmospheric aerosol. However, the lack of efficient, stable, uniform, large-array, and low-cost SERS substrates constitutes a major roadblock. Herein, a new SERS substrate is proposed for detecting individual atmospheric aerosol particles. It is based on the sphere segment void (SSV) structure of copper and silver (Cu/Ag) alloy. The SSV structure is prepared by an electrodeposition method and presents a uniform distribution, over large 2 cm 2 arrays and at low cost. The substrate offers a high SERS enhancement factor (due to Ag) combined with lasting stability (due to Cu). The SSV structure of the arrays generates a high density of SERS hotspots (1.3 × 10 14/cm 2), making it an excellent substrate for atmospheric aerosol detection. For stimulated sulfate aerosols, the Raman signal is greatly enhanced (>50 times), an order of magnitude more than previously reported substrates for the same purpose. For ambient particles, collected and studied on a heavy haze day, the enhanced Raman signal allows ready observation of morphology and identification of chemical components, such as nitrates and sulfates. This work provides an efficient strategy for developing SERS substrate for detecting individual atmospheric aerosol.

The ability of plasmonic nanosurfaces to produce strong electromagnetic fields in their vicinity upon illumination can be used to enhance effects, such as those originating from chirality (lack of mirror symmetry) of molecules. We numerically investigate chiral nanosurfaces composed of plasmonic nanobars with varying packing densities. We identify the optimum illumination conditions for maximal field enhancement. Under these illumination conditions, the optical chirality near the surface exceeds the optical chirality of the incident light by almost an order of magnitude in a large area (200 nm × 200 nm) near the surface. Our simulations prove the nanosurfaces to be promising candidates for enhancement of chiral-optical effects.

This talk will focus on three absorbing lines of research into nanophotonics. It will begin with the first experimental observation of a chiroptical effect that was predicted 40 years ago.[1,2] Our team recently demonstrated that in chiral metal nanoparticles (those that lack mirror symmetry) the intensity of light, scattered at the secondharmonic frequency, is proportional to the chirality. This effect was predicted 40 years ago, it is >10,000 more sensitive than corresponding linear optical effects and it could enable safer pharmaceuticals. Next, it will consider the smallest backjets (‘nanojets’) ever created and will discuss how they can serve to assemble novel metamaterials based on nano metalworking.[3,4] Nano metalworking is an exciting, emerging field that is largely unexplored. Finally, it will illustrate how a vapour stabilization technique can greatly enhance quantum sensors.[5] REFERENCES [1] Phys Rev. X 9, 011024 (2019). [2] J. Chem. Phys. 70, 1027 (1979). [3] Adv. Mater. 24, OP29-OP35 (2012). [4] Nat. Commun. 5, 4568 (2014). [5] Nat. Commun. 10, 2328 (2019).

Despite the ubiquity of Raman spectroscopy, fluorescence, poor signal strength and photobleaching pose a significant challenge to researchers in the biomedical field. Here, we demonstrate a 17-fold signal enhancement in Raman spectra of crystal violet via surface-enhanced Raman scattering (SERS). The SERS substrate was fabricated by electron beam lithography (EBL); the nanostructured surface was an array of G-shaped elements made of Au on SiO2/Si. In addition to the SERS spectra, finite-difference time-domain simulations were performed to illustrate the distribution of electric-field hot-spots on the SERS substrate. The electric-field hot-spots were prominent at the vertices and edges of the nanostructured G-shaped motifs. The results presented here demonstrate that EBL is a high-end choice for SERS substrate fabrication that opens the way for more complex Raman spectroscopies, for instance involving nonlinear optics or chiral analytes.

Micro- and nanoplastics are considered one of the top pollutants that threaten the environment, aquatic life and mammalian (including human) health. Unfortunately, the development of uncomplicated but reliable analytical methods that are sensitive to individual microplastic particles, with sizes smaller than 1 μm, remains incomplete. Here, we demonstrate the detection and identification of (single) micro- and nanoplastics, by using surface-enhanced Raman spectroscopy (SERS), with Klarite substrates. Klarite is an exceptional SERS substrate; it is shaped as a dense grid of inverted pyramidal cavities, made of gold. Numerical simulations demonstrate that these cavities (or pits) strongly focus incident light into intense hotspots. We show that Klarite has the potential to facilitate the detection and identification of synthesized and atmospheric/aquatic microplastic (single) particles, with sizes down to 360 nm. We find enhancement factors of up to two orders of magnitude for polystyrene analytes. In addition, we detect and identify microplastics with sizes down to 450 nm on Klarite, with samples extracted from ambient, airborne particles. Moreover, we demonstrate Raman mapping as a fast detection technique for sub-micron microplastic particles. The results show that SERS with Klarite is a facile technique that has the potential to detect and systematically measure nanoplastics in the environment. This research is an important step towards detecting nanoscale plastic particles that may cause toxic effects to mammalian and aquatic life when present in high concentrations.

Microdroplets present unique physiochemical properties, which can influence chemical reactions and atmospheric aerosol processes. The microdroplets’ pH is one of the most important factors that dictate chemical processes, such as heterogeneous and condensed reactions. However, currently, there is no well-established method for directly measuring the pH in microdroplets. Herein, we develop a straightforward method, for the first time, to directly measure the pH of size-resolved, micron-sized single droplets with Surface Enhanced Raman Scattering (SERS)-activated pH-indicator paper; realized by embedded Au nanoparticles (20, 40 and 60 nm). We establish the key parameters for optimizing the method, and demonstrate its accuracy by benchmarking various acids and solutions with different ionic strength. With the established method, we report the pH dependence on the size of microdroplets. Microdroplets of sizes down to 230 nm can be analyzed by our method. We also showcase our method’s applicability to ambient samples, establishing its promising potential beyond the laboratory environment.

In 3D isotropic liquids, optical third-harmonic generation is forbidden, with circularly polarized light (CPL). Yet the associated nonlinear susceptibility directly influences the optical properties at the fundamental frequency by intensity dependence (Kerr effect). Here, the hidden third-harmonic optical properties upon CPL illumination are revealed by demonstrating a new effect, in hyper-Rayleigh scattering. This effect is succinctly enunciated: the intensity of light scattered at the third-harmonic frequency of the CPL incident light depends on the chirality of the scatterers. It is referred to as third-harmonic (hyper) Rayleigh scattering optical activity (THRS OA) and was observed from Ag nanohelices randomly dispersed in water. The first analytical theory model for the new effect in nanohelices is also provided, highlighting the role of localized transition dipoles along the helical length. THRS OA is remarkably user-friendly. It offers access to intricate optical properties (hyperpolarizabilities) that have so far been more easily accessible by computation and that are essential for the understanding of light−matter interactions. The new effect could find applications in hyper-sensitive characterization of the chirality in molecules and in nanostructures; this chirality plays a fundamental role in the function of bio/nano-machinery, with promising applications in next generation technologies.

A simple and rapid method for detecting chemical components of individual aerosol particles on Klarite substrate with surface-enhanced Raman spectroscopy (SERS) is described. For both single simulated aerosol particles and ambient atmospheric particles, this new analytical method promotes the enhancement factor of the Raman signal. The spectra of ammonium sulfate and naphthalene particles at the microscopic level are enhanced by a factor of 6 and therefore greatly improve the detection of the chemical composition of an individual aerosol particle. When aerosol particles are found over a microscopic domain, a set of Raman spectra with chemical information can be obtained via SERS mapping. The maps illustrate the distribution of organic or inorganic species on the SERS substrate. This constitutes a facile and rapid method to study aerosol particles. This new method allows the analysis of chemical composition in single aerosol particles, demonstrating the power of SERS to probe the ambient atmospheric particles and to study the formation of aerosol particles.

Recent progress in nanofabrication has redrawn the boundaries of the applicability of chiroptical (chiral optical) effects. Chirality, often expressed as a twist in biomolecules, is crucial for pharmaceuticals, where it can result in extremely different chemical properties. Because chiroptical effects are typically very weak in molecules, plasmonic nanomaterials are often proposed as a promising platform to significantly enhance these effects. Unfortunately, the ideal plasmonic nanomaterial has conflicting requirements: its chirality should enhance that of the chiral molecules and yet it should have no chiroptical response on its own. Here, we propose a unique reconciliation to satisfy the requirements: a racemic plasmonic nanomaterial, consisting of equal amounts of opposite chiral unit cells. We show how diffraction spectroscopy can be used to unveil the presence of chirality in such racemic nanogratings in the far-field. Our experiments are supported by numerical simulations and yield a circular intensity difference of up to 15%. The physical origin is demonstrated by full wave simulations in combination with a Green's function – group-theory-based analysis. Contributions from Circular Dichroism in the Angular Distribution of Photoelectrons (CDAD) and pseudo/extrinsic chirality are ruled out. Our findings enable the far-field measurement and tuning of racemic nanomaterials, which is crucial for hyper-sensitive chiral molecular characterization.

Driven by applications in chemical sensing, biological imaging and material characterisation, Raman spectroscopies are attracting growing interest from a variety of scientific disciplines. The Raman effect originates from the inelastic scattering of light, and it can directly probe vibration/rotational-vibration states in molecules and materials. Despite numerous advantages over infrared spectroscopy, spontaneous Raman scattering is very weak, and consequently, a variety of enhanced Raman spectroscopic techniques have emerged. These techniques include stimulated Raman scattering and coherent anti-Stokes Raman scattering, as well as surface- and tip-enhanced Raman scattering spectroscopies. The present review provides the reader with an understanding of the fundamental physics that govern the Raman effect and its advantages, limitations and applications. The review also highlights the key experimental considerations for implementing the main experimental Raman spectroscopic techniques. The relevant data analysis methods and some of the most recent advances related to the Raman effect are finally presented. This review constitutes a practical introduction to the science of Raman spectroscopy; it also highlights recent and promising directions of future research developments.

The light interacting on asymmetric nanostructures can give rise to the phenomenon called extrinsic chirality [1–3], even if the nanostructures are not chiral by themselves. In this cases, light can be manipulated in order to obtain interesting behaviour such as anomalous refraction [4] or circular dichroism [5].

Alkali metal vapors enable access to single electron systems, suitable for demonstrating fundamental light-matter interactions and promising for quantum logic operations, storage and sensing. However, progress is hampered by the need for robust and repeatable control over the atomic vapor density and over the associated optical depth. Until now, a moderate improvement of the optical depth was attainable through bulk heating or laser desorption – both time-consuming techniques. Here, we use plasmonic nanoparticles to convert light into localized thermal energy and to achieve optical depths in warm vapors, corresponding to a ~16 times increase in vapor pressure in less than 20 ms, with possible reload times much shorter than an hour. Our results enable robust and compact light-matter devices, such as efficient quantum memories and photon-photon logic gates, in which strong optical nonlinearities are crucial., Robust and fast control of the atomic vapor pressure in alkali vapor cells would greatly extend their use for many quantum technologies. Here, the authors exploit plasmonic absorption in a cell coating containing gold nanoparticles to control the vapor pressure with milliseconds response time.

Asymmetric nanostructures can mimic a chiral response when circular polarized light interacts with the structures under particular angle of incidence [1]. This phenomenon is called ‘extrinsic chirality’ and usually is present under linear optical investigation with low visibility. Due to the fact that optical second harmonic generation is possible only in samples with some degree of asymmetry, this can be used in order to investigate the extrinsic chirality with a background free technique, thus inducing a high visibility of the artificial circular dichroism [2,3]. Here we present the second harmonic generation (SHG) measurements obtained on samples composed by GaAs nanowires grown on silicon. The wires present resonant leaky modes around 800nm and at 400nm due to the high refractive index contrast ratio between wires and air, even if these wavelengths lie on the absorption band of GaAs [4]. The measurements performed on this sample present good SHG signal due to the second order nonlinear term of GaAs, but did not present any circular dichroism (SHG-CD). By coating the sample with a 20nm thin layer of gold deposited asymmetrically, by evaporating the metal only from one side of the nanowires, the symmetry of the structure is broken, thus induced high SHG-CD. The SHG-CD is measured by shining the sample with circular polarized pump light at the fundamental wavelength of 800nm and by revealing the second harmonic signal at 400nm in s or p polarization, as a function of sample rotation. Four samples were measured with GaAs wires of about 5 micron in length with different diameters ranging from 140nm to 200nm. In this case it is possible to explore different resonance conditions and different SHG-CD is revealed. For each sample, the measured were carried out before and after the asymmetric gold layer deposition, thus allowing direct comparison of the results. References [1] A. Belardini, M. C. Larciprete, M. Centini, E. Fazio, C. Sibilia, D. Chiappe, C. Martella, A. Toma, M. Giordano, and F. Buatier de Mongeot, Phys. Rev. Lett. 107, 257401 (2011). [2] A. Belardini, M. Centini, G. Leahu, D. C. Hooper, R. Li Voti, E. Fazio, J. W. Haus, A. Sarangan, V. K. Valev, C. Sibilia, Sci. Rep. 2016, 6, 31796. [3] G. Leahu, E. Petronijevic, A. Belardini et al., Adv. Optical Mater. 2017, 1601063 (2017). [4] G. Leahu, E. Petronijevic, A. Belardini, M. Centini, R. Li Voti, T. Hakkarainen, E. Koivusalo, M. Guina, C. Sibilia. Sci. Rep. 7, 2833 (2017).

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO3- x suboxides leading to a wide range of substrate-supported 2H-WS2 polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS2, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO3 + 7S → 2WS2 + 3SO2 reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO2.92/WO2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. The continuous transition of WS2 from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS2 nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO3- x) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS2 domains grow via "self-seeding and feeding" where short WO2.92/WO2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W-O-S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.

Conversion of CO 2 into fuels and chemicals via electroreduction has attracted significant interest. Via mesostructure design to tune the electric field distribution in the electrode, it is demonstrated that the Cu-In alloy with an inverse opal (CI-1-IO) structure provides efficient electrochemical CO 2 reduction and allows for sensitive detection of the CO 2 reduction intermediates via surface-enhanced Raman scattering. The significant enhancement of Raman signals of the intermediates on the CI-1-IO surface can be attributed to electric field enhancement on the "hot edges" of the inverse opal structure. Additionally, a highest CO 2 reduction faradaic efficiency (FE) of 92% (sum of formate and CO) is achieved at-0.6 V vs. RHE on the CI-1-IO electrode. The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results show that the Cu-In alloy with an inverse opal structure has faster adsorption kinetics and higher adsorption capacity for CO 2. The "hot edges" of the bowl-like structure concentrate electric fields, due to the high curvature, and also concentrate K + on the active sites, which can lower the energy barrier of the CO 2 reduction reaction. This research provides new insight into the design of materials for efficient CO 2 conversion and the detection of intermediates during the CO 2 reduction process.

There is now crucial medical importance placed on understanding the role of early stage, subvisible protein aggregation, particularly in neurodegenerative disease. While there are strategies for detecting such aggregates in vitro, there is no approach at present that can detect these toxic species associated with cells and specific subcellular compartments. We have exploited excitation-energy-dependent fluorescence edge-shift of recombinant protein labeled with a molecular beacon, to provide a sensitive read out for the presence of subvisible protein aggregates. To demonstrate the potential utility of the approach, we examine the major peptide associated with the initiation of Alzheimer's disease, amyloid β-protein (Aβ) at a patho-physiologically relevant concentration in mouse cortical neurons. Using our approach, we find preliminary evidence that subvisible Aβ aggregates are detected at specific subcellular regions and that neurons drive the formation of specific Aβ aggregate conformations. These findings therefore demonstrate the potential of a novel fluorescence-based approach for detecting and imaging protein aggregates in a cellular context, which can be used to sensitively probe the association of early stage toxic protein aggregates within subcellular compartments.

We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1’-Bi-2-naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.

Correction for ‘Surface-enhanced Raman spectroscopy for bioanalysis and diagnosis’ by Muhammad Ali Tahir et al., Nanoscale, 2021, 13, 11593–11634, DOI: 10.1039/D1NR00708D.

Because of their large figures of merit, surface lattice resonances (SLRs) in metal nanoparticle arrays are very promising for chemical and biomolecular sensing in both liquid and gas media. SLRs are sensitive to refractive index changes both near the surface of the nanoparticles (surface sensitivity) and in the volume between them (bulk sensitivity). Because of its intrinsic surface-sensitivity and a power law dependence on electric fields, second harmonic generation (SHG) spectroscopy can improve upon both the surface and volume sensitivities of SLRs. In this report on SHG spectroscopy of plasmonic nanoparticles, we show that the SHG signal is greatly increased (up to 450 times) by the SLRs. We also demonstrate very narrow resonances in SHG intensity (∼5 nm fwhm). We illustrate how the SHG resonances are highly sensitive to SLRs by varying the fundamental wavelength, angle of incidence, nanoparticle material, and lattice constant of the arrays. Finally, we identify an SHG resonance (10 nm fwhm) that is electric dipole forbidden and can be attributed to higher-order multipoles, enhanced by the strong near-fields of SLRs. Our results open up new and very promising avenues for chemical and biomolecular sensing based on SHG spectroscopy of SLRs.

Plasmonic nanostructures have demonstrated a remarkable ability to control light in ways never observed in nature, as the optical response is closely linked to their flexible geometric design. Due to lack of mirror symmetry, chiral nanostructures allow twisted electric field hotspots to form at the material surface. These hotspots depend strongly on the optical wavelength and nanostructure geometry. Understanding the properties of these chiral hotspots is crucial for their applications; for instance, in enhancing the optical interactions with chiral molecules. Here, the results of an elegant experiment are presented: by designing 35 intermediate geometries, the structure is enantiomorphed from one handedness to the other, passing through an achiral geometry. Nonlinear multiphoton microscopy is used to demonstrate a new kind of double-bisignate circular dichroism due to enantiomorphing, rather than wavelength change. From group theory, a fundamental origin of this plasmonic chiroptical response is proposed. The analysis allows the optimization of plasmonic chiroptical materials. V.K.V. acknowledges support from the Royal Society through the University Research Fellowships. Nuno Braz acknowledges support from the EPSRC grant reference EP/G037256/1. The authors would like to acknowledge the C2 project (C24/15/015) and the PDMK/14/126 project of KU Leuven, the FWO long term stay abroad project grant V405115N, and the Methusalem project funded by the Flemish government. The authors are grateful to Hongjian Tao, Deputy Director of Department of International Cooperation and Exchanges in the Ministry of Education of the People's Republic of China, for facilitating the collaboration between the UK, Chinese, and Belgian teams. All data supporting this study are openly available from the University of Bath data archive at https://doi.org/10.15125/BATH-00483.

Chiral plasmonic nanostructures, those lacking mirror symmetry, can be designed to manipulate the polarization of incident light resulting in chiroptical (chiral optical) effects such as circular dichroism (CD) and optical rotation (OR). Due to high symmetry sensitivity, corresponding effects in second-harmonic generation (SHG-CD and SHG-OR) are typically much stronger in comparison. These nonlinear effects have long been used for chiral molecular analysis and characterization; however both linear and nonlinear optical rotation can occur even in achiral structures, if the structure is birefringent due to anisotropy. Crucially, chiroptical effects resulting from anisotropy typically exhibit a strong dependence on structural orientation. Here we report a large second-harmonic generation optical rotation of ±45°, due to intrinsic chirality in a highly anisotropic helical metamaterial. The SHG intensity is found to strongly relate to the structural anisotropy; however, the angle of SHG-OR is invariant under sample rotation. We show that by tuning the geometry of anisotropic nanostructures, the interaction between anisotropy, chirality, and experimental geometry can allow even greater control over the chiroptical properties of plasmonic metamaterials.

Throughout the 19th and 20th century, chirality has mostly been associated with chemistry. However, while chirality can be very useful for understanding molecules, molecules are not well suited for understanding chirality. Indeed, the size of atoms, the length of molecular bonds and the orientations of orbitals cannot be varied at will. It is therefore difficult to study the emergence and evolution of chirality in molecules, as a function of geometrical parameters. By contrast, chiral metal nanostructures offer an unprecedented flexibility of design. Modern nanofabrication allows chiral metal nanoparticles to tune the geometric and optical chirality parameters, which are key for properties such as negative refractive index and superchiral light. Chiral meta/nano-materials are promising for numerous technological applications, such as chiral molecular sensing, separation and synthesis, super-resolution imaging, nanorobotics, and ultra-thin broadband optical components for chiral light. This review covers some of the fundamentals and highlights recent trends. We begin by discussing linear chiroptical effects. We then survey the design of modern chiral materials. Next, the emergence and use of chirality parameters are summarized. In the following part, we cover the properties of nonlinear chiroptical materials. Finally, in the conclusion section, we point out current limitations and future directions of development.

We present direct experimental mapping of the lateral magnetic near-field distribution in plasmonic nanoantennas using aperture scanning near-field optical microscopy (SNOM). By means of full-field simulations it is demonstrated how the coupling of the hollow-pyramid aperture probe to the nanoantenna induces an effective magnetic dipole which efficiently excites surface plasmon resonances only at lateral magnetic field maxima. This excitation in turn affects the detected light intensity enabling the visualization of the lateral magnetic near-field distribution of multiple odd and even order plasmon modes with subwavelength spatial resolution.

Masked by rotational anisotropies, xthe nonlinear chiroptical response of a metamaterial is initially completely inaccessible. Upon rotating the sample the chiral information emerges. These results highlight the need for a general method to extract the true chiral contributions to the nonlinear optical signal, which would be hugely valuable in the present context of increasingly complex chiral meta/nanomaterials.

Nanoactuators and nanomachines have long been sought after, but key bottlenecks remain. Forces at submicrometer scales are weak and slow, control is hard to achieve, and power cannot be reliably supplied. Despite the increasing complexity of nanodevices such as DNA origami and molecular machines, rapid mechanical operations are not yet possible. Here, we bind temperature-responsive polymers to charged Au nanoparticles, storing elastic energy that can be rapidly released under light control for repeatable isotropic nanoactuation. Optically heating above a critical temperature [Formula: see text] = 32 °C using plasmonic absorption of an incident laser causes the coatings to expel water and collapse within a microsecond to the nanoscale, millions of times faster than the base polymer. This triggers a controllable number of nanoparticles to tightly bind in clusters. Surprisingly, by cooling below [Formula: see text] their strong van der Waals attraction is overcome as the polymer expands, exerting nanoscale forces of several nN. This large force depends on van der Waals attractions between Au cores being very large in the collapsed polymer state, setting up a tightly compressed polymer spring which can be triggered into the inflated state. Our insights lead toward rational design of diverse colloidal nanomachines.

Extrinsic or pseudo-chiral (meta)surfaces have an achiral structure, yet they can give rise to circular dichroism when the experiment itself becomes chiral. Although these surfaces are known to yield differences in reflected and transmitted circularly polarized light, the exact mechanism of the interaction has never been directly demonstrated. Here we present a comprehensive linear and nonlinear optical investigation of a metasurface composed of tilted gold nanowires. In the linear regime, we directly demonstrate the selective absorption of circularly polarised light depending on the orientation of the metasurface. In the nonlinear regime, we demonstrate for the first time how second harmonic generation circular dichroism in such extrinsic/pseudo-chiral materials can be understood in terms of effective nonlinear susceptibility tensor elements that switch sign depending on the orientation of the metasurface. By providing fundamental understanding of the chiroptical interactions in achiral metasurfaces, our work opens up new perspectives for the optimisation of their properties.

We present experimental results of linear and nonlinear optical measurements from a plasmonic metasurface composed by self-assembled tilted gold nanowires. We investigated the extrinsic chiral response due to the particular symmetry breaking induced by the nanowires geometry by using second harmonic generation, photoacoustic absorption and optical reflectance measurements.

3D chirality is shown to be unnecessary for introducing strong circular dichroism for harmonic generations. Specifically, near-unity circular dichroism for both second-harmonic generation and third-harmonic generations is demonstrated on suitably designed ultrathin plasmonic metasurfaces with only 2D planar chirality. The study opens up new routes for designing chip-type biosensing platform, which may allow for highly sensitive detection of bio- and chemical molecules with weak chirality.

Because of its high surface and interface sensitivity, the nonlinear optical technique of second harmonic generation (SHG) is a designated method for investigating nanostructured metal surfaces. Indeed, the latter present a high surface-to-volume ratio, but even more importantly, they can exhibit strong near-field enhancements or "hot spots". Hot spots often appear as a result of geometric features on the nanoscale or surface plasmon resonances, which are collective electron oscillations on the surface that, on the nanoscale, can readily be excited by light. In the last 10 years, near-field hot spots have been responsible for dramatic developments in the field of nano-optics. In this Feature Article, the influence of hot spots on the SHG response of nanostructured metal surfaces is discussed on both the microscopic and macroscopic levels. On the microscopic level, the nanostructured metal surfaces were characterized by scanning SHG microscopy, complemented by rigorous numerical simulations of the near-field and of the local electric currents at the fundamental frequency. On the macroscopic level, SHG-circular dichroism and magnetization-induced SHG characterization techniques were employed.