Chiral materials possess no mirror plane and as a consequence exist in two different forms, so-called enantiomers that are mirror images of each other. While these enantiomers usually have very similar chemical and physical properties, their optical properties are very different. For example, they will rotate linearly polarized light in equal amounts but in opposite directions. This is a manifestation of optical activity, which is due to a different interaction of the material with left- and right-hand circularly polarized light. Optical activity in a medium may occur either from chiral molecules or from supramolecular/larger-scale chiral arrangement of chiral or even achiral species. Well-known optical activity (gyrotropic) effects are optical rotation and circular dichroism that have long provided detailed information on internal molecular structure and supramolecular assemblies. The first one to observe optical rotation was Jean-Baptiste Biot in 1815, while in 1848 Louis Pasteur demonstrated that molecular chirality was at the origin of this effect, by showing that tartaric acid from biological origin rotated the polarization of incoming light, while chemically synthesized tartaric acid did not [1].

Chiral materials are usually associated with biology, since many biologically active molecules are chiral. For this reason, chiral-sensitive optical techniques such as circular dichroism spectroscopy, optical rotatory dispersion and, more recently vibrational circular dichroism, are currently used in biology, pharmacology and biochemistry. However, it also became clear that chiral materials could have further additional interesting optical applications. For example, many optical applications of chiral liquid crystals are due to chiral structures [2]. In the beginning of the nineties, it also became clear that chirality can play an important role in the field of nonlinear optics. New characterization tools based on second-harmonic generation became available and where used to characterize chiral surfaces. Furthermore, it was realized that due to their lack of mirror symmetry, chiral materials could be excellent candidates for the design of nonlinear optical devices. Along that line, new chiral optical effects such as magneto-chiral dichroism where demonstrated and could form the basis for entirely new types of optical devices [3]. Chirality may also play an important role in the field of nanotechnology and more specifically for the design of artificial nanostructures with properties usually not found in nature. One of those properties is negative refraction and it was hypothesized that chirality could be the key in realizing negative refraction [4]. Currently, several research groups all over the world are involved with chiral nanostructures.

It is clear, vide supra, that chiral optical materials are of interest in a wide variety of scientific disciplines. In selecting these invited papers at the occasion of this inaugural issue of Optical Materials Express, we tried to focus on some of these disciplines, with papers that illustrate recent advances in that open field of chiral optical materials. In particular, we have contributions focusing on nonlinear optical properties of chiral organic molecules (Persoons et al., Benichou et al.), optical properties of chiral liquid crystals (Araoka et al.) and chiral nanomaterials (Valev et al., Huttunnen et al.), with a link to advanced optical characterization tools (Geiger et al.) for chiral samples.

For an area of research that has been around for almost 200 years, it is surprising that chiral optical materials still continue to fascinate researchers all over the world. And, as this focus issue illustrates, chirality will continue to play a prominent role in the field of optics and materials. Especially the design of new complex and multifunctional chiral materials will stimulate new researches and will definitely lead to the development of new optical techniques and applications. We therefore encourage our colleagues in this exciting field to submit their work to Optical Materials Express.