Artificial chameleon

The colors we see usually arise from the chemistry of materials, dyes, or pigments: A surface might reflect only some wavelengths of light; a filter might transmit only a narrow range of colors. Less common than chemistry, structural materials sometimes come along to split white light into some of its constituent colors (see Physics Today, October 2006, page 82 ). But that is a very low-efficiency process. Now Connie Chang-Hasnain and a team of researchers at the University of California, Berkeley, have made a carefully tailored metastructured surface that can harness almost all of the optical power in the process of color splitting. The key is a regular array of high- and low-optical-index materials. The initial experiments are with lithographically produced silicon bars about 120 nm tall, spaced according to the desired wavelength of light, and embedded in a flexible layer of silicone. The Berkeley group’s calculations show that such a high- contrast metastructure (HCM) is capable of enhancing any one of the many diffraction orders at the expense of the rest. What’s more, the resulting anomalous reflection (or refraction) of the HCM is highly efficient, collecting more than 80% of the desired color’s power from the incident white light. The team’s final twist, as shown here, is that small bends and stretches of the HCM can change the colors in a predictable way by altering the spacing of the high-index nanostructures. Chang-Hasnain and company envision applications that include display technologies, active camouflage, and sensors. (L. Zhu et al., Optica 2, 255, 2015, doi:10.1364/OPTICA.2.000255 .)