A color filter is one of the most basic ways to create colors by purposely filtering the white light. Among many candidates for color filters, photonic crystals (PCs) have received much attention due to their distinctive structural colors. Generally, PCs are composed of periodic dielectric structures with a regular spatial distribution of the refractive indices, which determines the photonic bandgap (PBG) of the PCs. Various attempts have been made to construct PC-based color filters because the periodic geometries of PCs are directly related to structural coloration. The structural color can be controlled by modulating its geometry using various stimuli such as light, temperature, electric field, and mechanical force. However, the conventional approaches typically require complex processes or expensive equipment, and thus far, it is still challenging to find color variation applications in real life.<br/>Here, we developed an optical rotation-based color variation using two linear polarizers and cholesteric liquid crystal film, which can control the polarization state of transmitted light as a chiral PC. In chiral PC, the polarization axis of linearly polarized light rotates to a specific angle when it passes through the chiral medium depending on the effective optical activity of the medium. The optical rotatory dispersion, in which the optical rotation drastically varies with wavelength, provides a driving force for color variation. The various colors can be represented within one PC film by simply rotating a polarizer, and the color range can be controlled by the position of the PBG wavelength. Based on this concept, we demonstrated tunable protective goggles, which can attenuate the transmittance of strong laser sources. Our approach, which utilizes the optical rotation of chiral PC for color variation, is expected to be applicable to advanced optical technologies, such as smart windows and security codes.

Engineered optical rotation-based color variation using chiral photonic crystals

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