Ken Marshall is developing new flakes called polymer cholesteric liquid crystals (pCLC) that reflect visible to near-infrared wavelengths. By altering the composition of the liquid crystal films, the research engineer and his team at the University of Rochester create a range of flakes, each of which reflects a different color. "Most devices that generate color do so either by absorbing or emitting certain wavelengths of light," says Marshall. "Cholesteric liquid crystals function by reflecting only a narrow wavelength bandwidth of light." His findings could form the basis of a new type of color display. "The crystal molecules are organized in the form of a helix," he explains. Each of the components is circularly polarized. One component twists in the same direction as the helix and is completely reflected. The other component twists in the opposite direction and is completely transmitted. This effect only occurs when the wavelength of the incident light equals the helix pitch of the cholesteric LC (the distance along the helix it takes to rotate 360 degrees) times the average refractive index of the CLC material. "So, if the helix of the CLC material is about equal to that of green light around 543 nm, then only circularly polarized green light of the same orientation as the CLC helix is reflected," says Marshall. The pCLC flakes, each about 40 microns across, are dispersed in a host fluid such as silicone oil. When an electric field is applied to the device, the flakes rotate and the bright reflection color is extinguished. When red, green, and blue flakes are combined together in a host fluid—with each color type treated differently so as to respond to a different voltage or frequency of the driving electric field—colors can be displayed individually or in combinations. The applications for this technology are much broader in scope than just information displays, according to Marshall. They include e-paper; color filters; optical retardation or modulation elements for polarized light in military security and camouflage; and document security and anti-counterfeiting. Contact Marshall at email@example.com.
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