Electrically Induced Splitting of the Selective Reflection in Polymer Stabilized Cholesteric Liquid Crystals

Abstract

axis the wavelengths where the Bragg condition is met are given by Equa tions (2) and (3): nP λ = c (2) nP λ ∆ = ∆ (3) Due to the orientational dependence of the refractive index of the liquid crystal, the central wavelength of the Bragg con dition (λ c) is dependent on the average refractive index (n ‾) while the bandwidth of wavelengths that meet the Bragg con dition (∆λ) is dependent on the birefrin gence (∆n) of the liquid crystal. In a CLC, unpolarized light incident upon the material can be evaluated as two modes with oppositely circularly polarized wavevectors. When the Bragg condition is met, one mode is real and can propagate through the material, while the other is imaginary and cannot propagate and is reflected. Outside of the Bragg condition both modes are real and light is transmitted. Therefore, a CLC acts as a polari zation filter across selective wavelengths. Within the reflection regime the CLC reflects light with a matching handedness to the phase, transmitting light with the opposite handedness. As a result unpolarized light has reflection efficiency of 50% for a singular CLC phase. [3] Dynamic control of the selective reflection of CLCs has been subject to previous examination. Both thermal and optical con trol of the selective reflection have been widely explored. In the case of thermal control, thermochromic effects have been docu mented enabled by distinctive phase transitions (e.g., SmA to CLC), temperature dependence of the HTP of chiral dopants, and solubility. [4-8] Optical control of the selective reflection of CLCs has also been explored through the introduction of photo chromic groups (e.g., azobenzene) in either the LC or the dopant, that affect the periodicity or the refractive index of the medium upon irradiation. [9-13] However, thermal and optical regulation of the selective reflection of the CLC phase can be subject to limitations due to slow response times and environmental sensitivity. Com paratively, electrical control of the selective reflection of the CLC phase has the potential to be both rapidly addressable and robust to changes in environment (e.g., temperature or ambient light). Extensive prior research has document approaches to electrically adjust the optical properties of the CLC phase by using patterned electrodes, physical deformation of substrates, or via frequency dependent dielectric properties in LCs. [14-16] In some sense, the common theme of these efforts is that these approaches circumvent physical limits that prevent the periodic Electrical reconfiguration of the cholesteric liquid crystal phase is subject to extensive prior research. Here, a distinctive composition that allows the typically singular selective reflection to be split into two distinct and tunable selective reflections with an electric field is reported. The distinctive electro-optic phenomena (reflection notch splitting) are shown to be composition dependent and enabled by the introduction of appreciable amounts of side chain liquid crystalline mesogens. Functional opportunities enabled by ...

Electrically Tunable, Fully Solid Reflective Optical Elements

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or left-hand circular polarized (LHP) depending on the enantiomeric handedness of the chiral dopant, resulting in a 50% reflection efficiency.Numerous prior investigations have examined the electro-optic reconfiguration of the CLC phase. [8][9][10][11] Application of an electric field to a CLC phase with positive dielectric anisotropy in the planar alignment (reflective state) causes the material to transition to a homeotropic orientation (clear state). [9] However, the recovery of the CLC phase from the electrically induced homeotropic orientation can take many days and often results in the formation of a so-called focal conic texture (optically scattering).

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Assembly of Nanometer-Sized Hollow Sphere Colloidal Crystals for Applications as Tunable Photonic Materials

“… It is worth noting that the structural color can be preserved, while the reflectance is decreased by less than 5% after 100 reduction/oxidation cycles (Figure S4). The highly reversible and stable electrochromism for a wide range of colors is even comparable with most electrochromic materials. − …”

Electro-optic characteristics of stabilized cholesteric liquid crystals with non-liquid crystalline polymer networks

“…Responses depend on the composition and polymerization conditions and vary from bandwidth broadening, [16][17][18] red 19,20 and blue shift tuning, 21 and notch splitting. 22 Generally, when the polymer network is formed within the CLC in a planar orientation this network adopts a chirality that matches the phase in which the polymerization occurred. 23 The electrical response is a result of deformation of the polymer network under a DC field, which adjusts the structural chirality of the polymer through the depth of the cell, In turn this affects the pitch of the bulk mixture.…”