Phototunable Selective Reflection of Cholesteric Liquid Crystals

Abstract

The dependence of the helical pitch of a cholesteric liquid crystal based on a composite photosensitive chiral dopant (cChD) on the intensity of light irradiation was studied. The transmission spectra and the selective reflection spectra of cholesteric liquid crystal cells were measured. The concentration of the cChD additive is calculated, so that the peak of selective reflection and its rearrangement occurs in the visible range of the electromagnetic radiation spectrum (380-780 nm). The possibility of photo-control by shifting the peak of the selective reflection of the cChD additive was studied, when exposed to LEDs with wavelengths of 365 nm and 450 nm, while reducing or increasing the intensity, a change in the spiral pitch was observed. Depending on the light intensity, part of the molecules of the chiral additive containing the azo group underwent isomerization, i.e. molecules in the trans-form passed to the cis-form, which led to a spectral shift of the selective reflection peak. If a small intensity of the 365 nm LED was applied, then some of the molecules were forced to transition from the trans- to the cis-form, and then, when illuminated with a 450 nm LED, from the cisto the trans-form. After reversible rearrangement, the properties of cholesteric liquid crystal changes due to interaction with light, because under the influence of light, the equilibrium ratio of the trans- and cis-isomers of the molecules of the substance changed, which macroscopically changed the torsion force of the chiral additive. Using two LEDs of 365 nm and 450 nm with different emission spectra, a reversible control of the cChD selective reflection peak in the visible range was obtained. The maximum displacement occurred at approximately 145 nm.

E-Wave Interaction with the One-Dimensional Photonic Crystal with Weak Conductive and Transparent Materials

“…Here, the results obtained in the framework of the quantum electron plasma with permittivities ( 9) and ( 10) coincide with the data evaluated in the case of the classical electron gas having the permittivity (14). Similar results were observed in experimental studies of 1D photonic crystals [2,3,5,[12][13][14][15][16] as well as in the case of the cholesteric liquid crystals as 1D photonic crystals [8][9][10][11]. Let us consider a case where the graphite layer thickness is d 1 ∼ 25-150 nm and the thickness of the quartz layers d 2 is unchanged.…”

“…In the case of a weak conductor and layer thickness d 1 ∼ d 2 ∼ c/ω p , quartz permittivity ε α ∼ 1, and graphite permittivity (14), the frequency domains of the power coefficients appear, caused by the interference of the waves. The waves are either reflected many times by the layers of 1D photonic crystal or penetrate through them [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Note that the quantity c/ω p is the skin depth of the conductor which characterizes the penetration depth of an alternating electromagnetic field into a plasma [19]. For the conductive layer thickness d 1 , a leading (at small n) contribution of the value k n by ( 8) into the permittivities ( 9) and (10) of the quantum electron plasma, taking into account (11) and (12), has the order πnv T c 1. Therefore, the influence of the quantum permittivities ( 9) and ( 10) on the surface impedance ( 7) is almost the same as that for the classical permittivity ( 14) [19].…”

“…The alternation of layers results in the appearance of allowed and forbidden frequency areas of radiation transmission and reflection. One of the most well-known examples of 1D photonic crystal is the cholesteric liquid crystal and its effect of selective reflection [8][9][10][11]. As to the structure investigated, its potential materials and experimental realization have been presented in papers [12][13][14][15][16].…”

Photoinduced Transformation of the Orientational Structure of a Chiral Nematic under Planar-Conical Anchoring

“…Нематическая смесь ЛН-396 (Белорусский государственный университет) допировалась левозакручивающей хиральной добавкой S811 (TCI) и правозакручивающим хиральным допантом cChD (Институт химии новых материалов НАН Беларуси). Закручивающая способность добавки cChD уменьшается под действием ультрафиолетового (360-380 нм) излучения и увеличивается при облучении синим светом (420-460 нм) [10]. Для нематика ЛН-396 закручивающие способности добавок измерялись спектральным методом.…”