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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. [ABSTRACT FROM AUTHOR]

Although transition metal dichalcogenides (TMDs, e.g., WS2, WSe2, MoS2, MoSe2) have emerged as highly promising 2D materials for nonlinear optics, they are limited by intrinsically small light‐matter interaction length and (typically) flat‐lying geometries. The first hyperspectral multiphoton analysis of a tridimensional webbed network of densely‐packed stacks (of 1–5 layers) of twisted and/or fused 2D nanosheets of WS2, referred to as "nanomesh", is presented here. The optical second harmonic generation (SHG) is mapped across the three characteristic spectral features (A, B, and C) and two‐photon luminescence and third harmonic generation signatures are established. Compared to flat‐lying WS2 layers, the nanomesh is highly efficient, broadband, and robust against degradation (with main enhancement originating from the C feature, spreading from 850 to 1100 nm), and scalable in terms of growth. The origin of these spectral differences is assigned to hotspots, whose location changes depending on the wavelength of illumination. The main SHG enhancements result from double resonances in an energy landscape modified by in‐built defects (e.g., vacancies and their passivated variants, or grain boundaries) that induce intra‐bandgap energy levels. These characteristics establish the nanomesh as a prime candidate for integration into quantum optical technologies, such as miniaturized devices on chip. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Recent advances in nonlinear optics, hot electrons for renewable energy (e.g., solar cells and water‐splitting), acousto‐optics, nanometalworking, nanorobotics, steam generation, and photothermal cancer therapy are reviewed here. In all these areas, one of the key enabling properties is the ability of metallic nanoparticles to harvest and control light at the subwavelength scale by supporting coherent electronic oscillations, called localized surface plasmon resonances (LSPRs). Various physical properties and potential areas of application emerge depending on the decay mechanism of the LSPR and, especially, depending on the considered timescale. The field of plasmonics has mainly been associated with manipulating electromagnetic near‐fields at the nanoscale, where absorption is an obstacle. However, plasmonic absorption leads to a stream of temperature‐related phenomena that have only recently attracted significant attention. The goal of this review is to highlight exciting new areas of research (such as nanorobotics, nanometalworking, or acousto‐optical techniques) and to survey the most recent progress in more established areas (such as hot electrons, photothermal therapy, and plasmonic steam generation). To set each research area in context, the text is organized around the thermal cycle of the nanoparticles. [ABSTRACT FROM AUTHOR]