Red, Green, and Blue Reflections Enabled in an Electrically Tunable Helical Superstructure

Abstract

reflection of light. The structure of a CLC possesses self-assembled periodicity, with the orientation of the CLC mole cules rotating around a helical axis. Such a configuration can separate light traveling along the helical axis into right-and left-handed circularly polarized components. Circularly polarized light with the same handedness as that of the cholesteric molecule is reflected, whereas the other is transmitted. The selective reflection, also known as the Bragg reflection, is produced in the spectral range in terms of wavelengths λ: Δλ = (n e − n o )P, which is determined by the typically temperature-dependent pitch length P of the CLC helix as well as the birefringence defined as the difference between the extraordinary refractive index n e and the ordinary refractive index n o . The Bragg reflection band is centered at λ 0 = (n e + n o )⋅P/2 for incident light propagating along the cholesteric helix. If the selective reflection band is located in the visible range, the CLC will reflect colors, rendering its iridescent look. The pitch length and, in turn, the reflective wavelengths of light can be varied by stimuli such as electric or magnetic field, heat, [9] and even light, [1,10] facilitating CLC's applications as temperature indicators, [3] biosensors, [11] and general optical devices. [12,13] Recently, light-driven Bragg reflections of CLC materials have been extensively explored. [14,15] Such light-sensitive systems can be achieved by doping photoresponsive chiral or achiral azobenzenes into a CLC host so that the resultant cholesteric phase behavior can be modulated by trans-cis photo isomerization of the azo dopant. Nevertheless, the most desirable and least arduous approach to controlling the reflected light is by applying an electric field to the optical component. Recently, Xiang et al. have shown electrical tuning of selective reflection light from ultraviolet to visible based on an oblique helicoidal cholesteric cell. [16] The dielectric heating effect is the process in which a highfrequency electric field heats a dielectric substance. At high frequencies, this effect is caused by molecular dipole rotation within the dielectric material. Some special liquid crystalline materials such as dual-frequency liquid crystals (DFLCs) exhibit such pronounced effect. As a physical property of DFLCs, a relaxation of the component of dielectric constant parallel to the molecular axis, ε || , takes place at some point (typically of the order of 10 kHz) in the frequency domain, causing ε || to drop and become equal to the component of dielectric constant perpendicular to the molecular axis, ε ⊥ . Consequently, the dielectric anisotropy Δε (ε || − ε ⊥ ) changes its sign from positive to negative beyond the so-called crossover frequency f c . This phenomenon Materials with tunable optical properties in the visible spectrum are intriguingly important in science and technology. Such desired tunability can be conveniently accomplished by controlling optical materials with electric field. Now an organic film...

Nature‐Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self‐Assembly to DNA Mesophase and Nanocolloids

“…[151,152] Electrically tunable reflection of cholesteric LCs have been achieved by applying different methodologies such as polymer stabilization, interdigitated electrodes, and negative dielectric anisotropy LCs. [153][154][155][156][157][158] However, the wavelength range of electric tuning is rather limited in the cholesterics with helicoidal organization due to the low stability and easy formation of light scattering fingerprint structures. Recently, Lavrentovich et al developed the cholesterics with unique heliconical arrangement, and achieved the electrically tunable-selective reflection covering the spectral ranges from UV to visible and infrared by applying a low electric field.…”

Chiral-Selective Tamm Plasmon Polaritons

“…The other advantage of using CLC to generate chiral-selective TPP is that CLC could be easily controlled by external stimuli such as electric field [ 44 ] and ambient temperature [ 45 ]. By utilizing the temperature dependence of the CLC stopband, the wavelength of resonance can be effectively controlled to achieve a wide-range tunability [ 46 ].…”

59‐4: Fast‐response polarization volume gratings for AR/VR displays

“…The grating's turn-off time (when switching to low-frequency setting) was measured to be 0.36 ms, while the turn-on time (when switching to high-frequency setting) was measured to be 5.54 ms. While it is desirable to drive at higher voltage to further accelerate the turn-on process, the dielectric heating effect may be more severe such that the temperature became unstable [16,17].…”