The ways in which light interacts with matter has been of fundamental and applied interest for millennia. Some of these interactions can create vibrant patterns of color through well-known mechanisms including wavelength-selective absorption of dyes, dispersion events in prisms, and structural coloration caused by interference when light interacts with films, gratings, or photonic crystals. Discovering new ways light can interact with matter and linking coloration events to the object that gives rise to it can provide fundamental information about its chemical and physical properties. Recently, a previously unrecognized structural coloration mechanism was reported which entails interference between light rays undergoing multiple total internal reflections along a concave interface. Unlike most structural coloration that requires periodic nanostructures, this mechanism is applicable to microscale interfaces, opening new possibilities for fundamental exploration in optics and surface science to other practical applications such as functional displays and sensors. Here, we fabricated and investigated non-hemispherical geometries and how various structural parameters, and illumination conditions impact the iridescent colors produced, and utilized ray-tracing simulations to evaluate how these changes influence the interfering trajectories. We determined that identifying the range of unique families of trajectories a structure supported, and the respective path length differences generated between them, was critical for rationalizing the iridescent colors it produced. We then used this framework to directly link how these trajectory families evolved under rotating illumination conditions to form the rapidly changing iridescent colors observed. Lastly, we used these findings to design structures for specific scattering and iridescent behaviors. We anticipate that these findings could be used to extract chemical and physical information of the structure that gives rise to it, which could be useful to better understand the scattering behavior of irregular particulates and aerosols in atmospheric studies.

Structural coloration behavior of non-hemispherical microstructure surfaces

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ACS Fall 2022

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