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AbstractAn oblique helicoidal state of a cholesteric liquid crystal (ChOH) is capable of continuous change of the pitch$$P$$ in response to an applied electric field. Such a structure reflects 50% of the unpolarized light incident along the ChOHaxis in the electrically tunable band determined by$$P$$ /2. Here, we demonstrate that at an oblique incidence of light, ChOHreflects 100% of light of any polarization. This singlet band of total reflection is associated with the full pitch$$P$$ . We also describe the satellite$$P/2$$ ,$$P/3$$ , and$$P/4$$ bands. The$$P/2$$ and$$P/4$$ bands are triplets, whereas$$P/3$$ band is a singlet caused by multiple scatterings at$$P$$ and$$P/2$$ . A single ChOHcell acted upon by an electric field tunes all these bands in a very broad spectral range, from ultraviolet to infrared and beyond, thus representing a structural color device with enormous potential for optical and photonic applications. Impact statementPigments, inks, and dyes produce colors by partially consuming the energy of light. In contrast, structural colors caused by interference and diffraction of light scattered at submicrometer length scales do not involve energy losses, which explains their widespread in Nature and the interest of researchers to develop mimicking materials. The grand challenge is to produce materials in which the structural colors could be dynamically tuned. Among the oldest known materials producing structural colors are cholesteric liquid crystals. Light causes coloration by selective Bragg reflection at the periodic helicoidal structure formed by cholesteric molecules. The cholesteric pitch and thus the color can be altered by chemical composition or by temperature, but, unfortunately, dynamic tuning by electromagnetic field has been elusive. Here, we demonstrate that a cholesteric material in a new oblique helicoidal ChOHstate could produce total reflection of an obliquely incident light of any polarization. The material reflects 100% of light within a band that is continuously tunable by the electric field through the entire visible spectrum while preserving its maximum efficiency. Broad electric tunability of total reflection makes the ChOHmaterial suitable for applications in energy-saving smart windows, transparent displays, communications, lasers, multispectral imaging, and virtual and augmented reality. Graphical Abstract
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Electromagnetically tunable cholesterics with oblique helicoidal structure [Invited]
Cholesteric liquid crystals form a right-angle helicoidal structure with the pitch in the submicrometer and micrometer range. Because of the periodic modulation of the refractive index, the structure is capable of Bragg and Raman-Nath diffraction and mirrorless lasing. An attractive feature of cholesterics for optical applications is that the pitch and thus the wavelength of diffraction respond to temperature or chemical composition changes. However, the most desired mode of pitch control, by electromagnetic fields, has so far been elusive. Synthesis of bent-shape flexible dimer molecules resulted in an experimental realization of a new cholesteric state with an oblique helicoidal structure, abbreviated as ChOH. The ChOHstate forms when the material is acted upon by the electric or magnetic field and aligns its axis parallel to the field. The principal advantage of ChOHis that the field changes the pitch but preserves the single-harmonic heliconical structure. As a result, the material shows an extraordinarily broad range of electrically or magnetically tunable robust selective reflection of light, from ultraviolet to visible and infrared, and efficient tunable lasing. The ChOHstructure also responds to molecular reorientation at bounding plates and optical torques. This brief review discusses the recently established features of ChOHelectro-optics and problems to solve.
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- Award ID(s):
- 1906104
- PAR ID:
- 10190383
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optical Materials Express
- Volume:
- 10
- Issue:
- 10
- ISSN:
- 2159-3930
- Format(s):
- Medium: X Size: Article No. 2415
- Size(s):
- Article No. 2415
- Sponsoring Org:
- National Science Foundation
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