We propose to dynamically control the reflective color of a cholesteric liquid crystal (CLC) by electrically tuning the center wavelength (λc) of the bandgap. The CLC, sandwiched in a planar-aligned cell with indium-tin-oxide electrodes, possesses negative dielectric anisotropy and thermo-responsive spectral properties. The helix in the Grandjean planar state subject to a vertically applied voltage should be undisturbed in that the long molecular axis is initially perpendicular to the direction of the electric field. Surprisingly, when the frequency of the applied voltage is higher than a critical value, λc of the CLC cell varies as a function of the voltage. The underlying mechanism is the voltage-induced temperature change through dielectric heating in the frequency regime of pseudo-dielectric relaxation, attributable to the significant equivalent resistance-capacitance circuit of the cell due to the use of electrode layers with finite conductance. The driving voltage enabling the tuning of λc in the entire visible spectrum is as low as 12 Vrms in a 5-μm-thick cell at a frequency of 2 MHz. The proposed CLC cell exhibiting a broad electrically tunable spectral range from near infrared to ultraviolet holds great promise for developing tunable photonic devices such as multicolor reflectors, filters, and sensors.

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Many published papers bear flaw information caused by misinterpreting the pseudo-dielectric relaxation in liquid crystal (LC) confined in thin cells. The pseudo-dielectric relaxation stems from the cell geometries or parameters, which should not be misrecognized as an inherent material property of the LC. In this work, we explored the pseudo-dielectric relaxation and demonstrated an unconventional tuning means to electrically manipulate the reflective color of a typical cholesteric LC cell based on the heating due to the pseudo-dielectric relaxation. A tuning range in the full-color spectrum can be easily achieved by a limited temperature change of merely a few degrees Celsius.

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