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.

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 2 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 submicron 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 toward detecting nanoscale plastic particles that may cause toxic effects to mammalian and aquatic life when present in high concentrations.

Experiments were performed to investigate the aerodynamic characteristics of two-wing configurations at a low Reynolds number of 100,000. The wing models were rectangular flat plates with a semi-aspect ratio of two. The stagger between the wings was varied from ΔX/c = 0 to 1.5, the gap was varied from ΔY/c = 0 to 2 and ΔY/c = -1.5 to 1.5 for biplane and tandem configurations respectively with the decalage angle fixed at 0°. Lift, drag, aerodynamic efficiency and power efficiency ratios show that for small incidence angles, performance compared to the single wing is degraded. However, for single wing post-stall angles of attack, lift performance improves and stall is delayed significantly for many configurations with non-zero gap, i.e., ΔY/c ≥ 0. For a fixed angle of attack there are optimal gaps between the wings for which total lift becomes maximum. Particle image velocimetry measurements show that performance improvement relies heavily on the strength of the inter-wing flow and the interaction of the separated shear layers from the leading-edge and trailing-edge of the leading-wing with the trailing-wing. Unsteady forces are found to intensify for certain two-wing configurations. A switching between the stalled and unstalled states for the trailing-wing as well as a switching between the merged and distinct wakes are shown to have high flow unsteadiness and large lift fluctuations. Nomenclature b Semi-span c Chord length CD Time-averaged drag coefficient CD1 Time-averaged drag coefficient of wing 1 CD2 Time-averaged drag coefficient of wing 2 CDm Time-averaged monoplane drag coefficient CDt Time-averaged total drag coefficient, (CD1 + CD2) / 2 CL Time-averaged lift coefficient CL1 Time-averaged lift coefficient of wing 1 CL2 Time-averaged lift coefficient of wing 2 CLm Time-averaged monoplane lift coefficient CLt Time-averaged total lift coefficient, (CL1 + CL2) / 2 q Dynamic pressure

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.

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