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.

Optical microscopes are an essential tool for both the detection of disease in clinics, and for scientific analysis. However, in much of the world access to high-performance microscopy is limited by both the upfront cost and maintenance cost of the equipment. Here we present an open-source, 3D-printed, and fully-automated laboratory microscope, with motorised sample positioning and focus control. The microscope is highly customisable, with a number of options readily available including trans-and epi-illumination, polarisation contrast imaging, and epi-florescence imaging. The OpenFlexure microscope has been designed to enable low-volume manufacturing and maintenance by local personnel, vastly increasing accessibility. We have produced over 100 microscopes in Tanzania and Kenya for educational, scientific, and clinical applications, demonstrating that local manufacturing can be a viable alternative to international supply chains that can often be costly, slow, and unreliable.

Chiral nano-or metamaterials and surfaces enable striking photonic properties, such as negative refractive index and superchiral light, driving promising applications in novel optical components, nanorobotics, and enhanced chiral molecular interactions with light. In characterizing chirality, although nonlinear chiroptical techniques are typically much more sensitive than their linear optical counterparts, separating true chirality from anisotropy is a major challenge. Here, we report the first observation of optical activity in second-harmonic hyper-Rayleigh scattering (HRS). We demonstrate the effect in a 3D isotropic suspension of Ag nanohelices in water. The effect is 5 orders of magnitude stronger than linear optical activity and is well pronounced above the multiphoton luminescence background. Because of its sensitivity, isotropic environment, and straightforward experimental geometry, HRS optical activity constitutes a fundamental experimental breakthrough in chiral photonics for media including nanomaterials, metamaterials, and chemical molecules.

complications as the technique can become too sensitive, making it difficult to separate the desirable and undesirable contributions to the SHG signal. An example of an undesirable complication is extrinsic chirality, [15,16] which affects the nonlinear susceptibility tensor components in SHG. [17,18] Any anisotropy present in the sample can have a similar effect and our work here focuses on highlighting the effects of rotational anisotropy. We begin by selecting the sample geometry.Chirality is intrinsically a 3D property; however, a great number of works have focused on so-called "planar" nanostructures. A few recent examples include S-shaped nanostructures, [19,20] three-and fourfold symmetric propellers, [21] and heptamers. [22] These planar chiral nanostructures are very attractive because of their ease of fabrication with electron beam lithography. The necessary 3D symmetry-breaking arises from a dissymmetry along the axis perpendicular to the sample plane, [23] for instance, due to the presence of a substrate on one side of the sample and air on the other. Although planar meta/nanomaterials are thus 3D, it is clear that their threedimensionality is not very pronounced. At optical frequencies, various 3D structured meta/nanomaterials have been proposed, such as rosettes, [24,25] twisted arcs, [26] 3D shuriken, [27] stacked split rings, [28,29] oligomers, [30,31] gyroids, [32] and helices. [33][34][35][36] Of all these examples, the latter (i.e., the helix) is the archetypical chiral structure. The strong interaction of nanohelices with circularly polarized light (CPL) gives rise to large chiroptical effects, such as circular dichroism. This makes them attractive for applications involving CPL. [6,8,[37][38][39] Thus, the nonlinear optical response of helical metamaterials is of particular interest as they already demonstrate strong linear chiroptical effects. However, until recently it has been very difficult to fabricate high quality helical metamaterials for use at optical frequencies. Herein, we have investigated a chiral metamaterial made of nanohelices, with substantially subwavelength dimensions (<λ/10). As the archetypical chiral geometry, the helical design is particularly suitable because it is pronouncedly 3D, it gives rise directly to superchiral field configurations along the center of the helix, and its structural chirality parameter is straightforward to estimate as a function of varying dimensions. [33,40] Within this metamaterial, we clearly identify three different rotational anisotropies and demonstrate how they can mask the true chiral effect, rendering the SHG-CD signals unreliable. Our experimental results highlight the need for a general method to extract the true chiral contributions to the SHG signal. Here, we use a method for approximating these contributions. Although not fully rigorous, this method yields three measures of the chirality: averaged SHG-CD, direct inspection of the chiral component of the effective susceptibility tensor, and evaluation of the chiral coefficients tha...

The statement "However, it is yet unclear the extent to which the optical chirality of a field influences the dissymmetry of a chiroptical system in a way that enhances measurable quantities such as CD, or plasmon resonance shifts. Indeed, a complete QED theory to study chiroptical radiation matter interactions in the near field has not been developed." on page 29 of the originally published article is not correct, in the sense that a nearfield quantum electrodynamic (QED) theory has indeed been developed in earlier works. [1] The authors regret this mistake in the article and take the chance to add further references for the readers who might want to go deeper in the subject. [2,3] However, to the best of our knowledge, it is yet unclear to which extent the optical chirality of a field influences the dissymmetry of a chiroptical system in a way that enhances measurable quantities such as circular dichroism (CD), or plasmon resonance shifts in QED frameworks. This correction does not affect the conclusions of the original publication.