Dear Colleagues,

Nonlinear optics is currently a promising and growing research field due to its many applications, ranging from laser science to quantum technologies. Though nonlinear optical effects have been known for a long time (e.g., the Kerr effect was discovered in 1875 and the Pockels effect in 1893), the starting of the field is associated with the discovery of the second harmonic generation phenomenon in 1961, just one year after the first laser was switched on. Ever since, the field has diversified to into a number of important and interesting new phenomena, such as harmonic generation, self-focusing, plasmons, or parametric down conversion, which are currently very active research fields on their own.

At a more fundamental level, the efforts being made to find underlying symmetries in Nature are also widely known. This search has always fascinated us, not only because of the inherent beauty at a mathematical level, but because of far-reaching implications—particularly regarding conservation laws—which in turn has led to the discovery and classification of the different families of elementary particles, or the  prediction and search for new ones. Nonetheless, within a wider scenario, the behavior exhibited by physical systems is also connected with the concept of symmetry—for example, when we talk about pattern formation (fractality, percolation, etc.), which in the realm of optics is typically connected to phase-dependent (interference and diffraction) phenomena.

The underlying idea behind this Special Issue of Symmetry is to feature articles and reviews that are of tremendous interest to scientists who study linear and nonlinear optics, all oriented around the common leitmotiv of symmetry. We thus seek contributions across the entire breadth of the field, which may deal (but not exclusively) with the notions of symmetry-breaking, nonlinear optics, quantum optics, Fourier optics, pattern formation, interference, diffraction, coherence, entanglement, matter-wave interferometry, or open quantum systems.

Prof. Dr. Ángel S. Sanz
Guest Editor

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• second harmonic generation
• high-harmonic generation
• solitons and plasmons
• parametric down conversion
• self-focusing
• entanglement
• quantum technologies