and then impinging on our eyes. Our brain, rightly or wrongly, interprets these and other sensed patterns as reality. Hence, the saying, "seeing is believing."Given that much of our perceived reality is the result of light-matter interactions, it is no surprise that humans have been actively pursuing methods to understand and control the appearance of materials for thousands of years. Through mixing natural dyes of primary colors, people were able to tailor the color of everyday objects by changing their spectral absorption. The science of absorption-based colored materials evolved to the point of creating materials with light-responsive absorption (photochromics) that, when combined with a camera, were used to capture scenes, thereby allowing us to recreate and share moments of reality. Despite huge advances in the ability to control objects' absorptive color over the last 200 years through chemistry, controlling objects' structural color is still quite challenging. Structural color often refers to spectral interference effects associated with periodic spatial changes in the refractive index of materials (sometimes referred to as photonic crystals). These socalled photonic materials are a unique class of materials that have the ability to strongly affect the light-matter interactions for wavelengths on the scale of the refractive index periodicity. Photonic materials have particularly vivid colors that emerge from their angularly sensitive spectral reflections. Some of the early studies in this area were performed on natural photonic materials that caught the eye of curious scientists. [1] There are many examples of naturally occurring self-assembled photonic materials, including opals, [2] Pollia fruit, [3] jeweled beetles, [1,4] chameleons, [5] and others. [6] These natural examples of structurally derived color can be emulated, altered, or reproduced by researchers. [7][8][9] The ability of biology to self-assemble materials with complex structures and functionality is still unrivaled by engineered systems, but unique opportunities now exist for major advances in autonomously adaptive materials, as scientists are deciphering the molecular mechanisms that biology uses to self-assemble amazingly complex adaptive systems. [10] Particularly relevant to this review, some of the photonic structures found in nature are able to change periodicity in Photoresponsive liquid crystalline photonic materials are a medium in which the self-regulation of light can occur. Liquid crystals are macroscopically selfassembling, optically anisotropic materials capable of amplifying photochemical changes and thus are well suited to demonstrate complex light-driven behavior. This review presents recent progress in liquid crystalline systems exhibiting photoresponsive structural color. More specifically, it surveys progress on liquid crystalline materials and structures with coloration that is the result of the constituents' spatial arrangement (not of presence of dyes or pigments) and for which this arrangement can be externally cont...
