Mechanism of electrically induced photonic band gap broadening in polymer stabilized cholesteric liquid crystals with negative dielectric anisotropies

Nemati, Hossein Mashad; Liu, Shiyi; Zola, Rafael S.; Tondiglia, Vincent P.; Lee, Kyung Min; White, Timothy J.; Bunning, Timothy J.; Yang, Deng‐Ke

doi:10.1039/c4sm02283a

Cited by 67 publications

(90 citation statements)

References 28 publications

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“…For example, the displacement of the polymer network is visually observed in control experiments employing alignment cells with patterned electrodes wherein the polymer network aggregates on the negatively charged electrodes. 20 Spatial pitch variation is induced by the displacement of polymer network which induces linear-pitch variation for symmetrical dynamic broadening and non-linear pitch variation for dynamic tuning of reflection bandwidth.…”

Section: Resultsmentioning

confidence: 99%

“…Simplified examinations of nematic and twisted nematic orientations prepared from related compositions indicate that the polymer stabilizing network is delocalized upon application of a DC bias and that the movement of the network is sensitive to the polarity of the DC bias 18,20 Subsequent examination has confirmed that the polymer stabilizing network formed in the presence of the low-molar mass CLC mixture retains a ''structural chirality'' that can overrule the natural ''chemical chirality'' of the low-molar mass CLC mixture. 19 Commercial liquid crystal mixtures contain ionic impurities (10 9 -10 14 ions per cm 3 ), which might come from synthetic and/or purification steps (catalysts, salts, moisture, and dust), alignment layers, 34 and/or degradation of the LC molecules.…”

Section: Introductionmentioning

confidence: 99%

“…Electrically-induced dynamic optical responses have been examined in CLCs or polymer stabilized CLCs (PSCLCs) including reflection notch switching, [3][4][5][6][7][8][9][10][11][12][13][14][15][16] broadening, [17][18][19][20][21][22][23][24] and tuning. [25][26][27][28][29][30][31][32] Most commonly, the dynamic changes in the reflection of CLCs or PSCLCs are observed by inducing tilt in the pitch in samples formulated with nematic liquid crystals with positive dielectric anisotropy (+De).…”

Section: Introductionmentioning

confidence: 99%

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Large range electrically-induced reflection notch tuning in polymer stabilized cholesteric liquid crystals

et al. 2015

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Large range electrically-induced reflection notch tuning in polymer stabilized cholesteric liquid crystals

et al. 2015

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“…Intermediate control of scatter has been reported in. [4,5,9,10] Recently, we have reported on the ability to electrically induce bandwidth broadening [20][21][22][23] and tuning [24,25] of the reflection notch of PSCLCs formulated with negative dielectric anisotropy (−Δε) nematic liquid crystal hosts. As confirmed in a series of recent reports, [20][21][22] the mechanism for the dynamic response is related to electromechanical displacement of the polymer stabilizing network.…”

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confidence: 99%

Bistable switching of polymer stabilized cholesteric liquid crystals between transparent and scattering modes

2015

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We report on the ability to switch an optical material composed of a polymer stabilized cholesteric liquid crystal (polymer stabilized cholesteric texture, PSCT) between stable transparent (reflective) and scattering modes. The degree of scattering is controllable with the strength of the applied electric field. The mechanism for bistable switching of the PSCT is distinguished from prior examinations by employing electromechanical displacement of a stabilizing polymer network. The stable transparent (reflective) or scattering modes are induced with a variety of driving schemes employing both alternating and direct current fields. The relative degree of scattering can be varied to allow for grayscale control potentially useful in smart window and display applications.Cholesteric liquid crystals (CLCs) are materials that naturally organize into a one-dimensional photonic crystal exhibiting a Bragg reflection which is expressed by λ = nP 0 , where λ, n, and P 0 are notch position, average refractive index, and pitch, respectively. The bandwidth of the reflection is expressed by Δλ = ΔnP 0 , where Δn is the birefringence. [1,2] The position of the reflection notch is controlled by the concentration of chiral dopant mixed with the nematic liquid crystal. The reflection of light from CLCs is naturally circularly polarized. If unpolarized incident light is exposed to the right-handed (RH) CLC, RH circularly polarized light (CPL) is reflected, whereas opposite handedness left-handed CPL is transmitted.A CLC can be prepared to be reflective or scattering [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] when the alignment is planar in the ground state (0 V). When the CLC is prepared from a positive dielectric anisotropy (+Δε) nematic liquid crystal host, the CLC can be switched to optically transparent by an applied electric field that orients the liquid crystal molecules into the homeotropic orientation. After the field is removed, the liquid crystal will exhibit a metastable scattering texture as the material transitions to form focal conic domains. Stabilizing the low molar mass CLC with a polymer network has been shown to allow for bistable switching between reflective and scattering textures typically formulated from +Δϵ nematic liquid crystal hosts. [4][5][6][7][8][9][10][11][12][13][14][15] These materials are generally described as polymer stabilized CLCs (PSCLCs) but historically scattering systems have been referred to as polymer stabilized cholesteric textures (PSCTs). PSCTs exhibit two types of switching modes. Switching from scattering to clear (or reflective) states is defined as "normal mode" [3,16,17] and switching from reflective to scattering state as "reverse mode". [3,18,19] The operation mode (normal or reverse) is dictated primarily by the sample preparation conditions. Application of an electric field during photopolymerization aligns the polymer networks normal direction to the cell in the planar geometry. After the field is removed, the composite forms the scattering focal conic textu...

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“…For PSCLC based IR reflector, electrically regulated reflection bandwidth without change in the effective birefringence of the LC was achieved by spatially varying the pitch throughout the cell. Previous research strongly indicated that the electrically-induced changes to the reflection bandwidth was ionic in nature and attributed to the pitch distortion caused by the displacement of the polymer network [33][34][35][36]. Recent investigations reveal that both preparation conditions and chemical components such as the photoinitiator of the polymers can influence electrically-induced reflection band change, for example, blue-shifting turning, red-shifting tuning and bandwidth broadening [37,38].…”

Section: Introductionmentioning

confidence: 99%

Cell thickness dependence of electrically tunable infrared reflectors based on polymer stabilized cholesteric liquid crystals

Haan

Khandelwal

et al. 2017

Sci. China Mater.

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We reported here the fabrication of the electrically tunable infrared (IR) reflectors based on the polymer stabilized cholesteric liquid crystal (PSCLC) with negative dielectric anisotropy. A systematic study of the influence of cell gap on the electrically tunable reflection bandwidth was performed. When a direct current (DC) electric field was applied, the reflection bandwidth red shifted in the cells with small cell gap, whereas the bandwidth broadening was observed in the cells with large cell gap. It is therefore reasonable to deduct that the reflection is dictated by the pitch gradient steepness which strongly relies on the cell thickness. The results reveal that for making PSCLC based IR reflector windows with electrically induced bandwidth broadening, a minimal cell gap thickness is required. The resulted IR reflectors possess a short native switching time and long-term operation stability, and are potentially applicable as smart energy saving windows in buildings and automobiles.

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Mechanism of electrically induced photonic band gap broadening in polymer stabilized cholesteric liquid crystals with negative dielectric anisotropies

Cited by 67 publications

References 28 publications

Large range electrically-induced reflection notch tuning in polymer stabilized cholesteric liquid crystals

Large range electrically-induced reflection notch tuning in polymer stabilized cholesteric liquid crystals

Bistable switching of polymer stabilized cholesteric liquid crystals between transparent and scattering modes

Cell thickness dependence of electrically tunable infrared reflectors based on polymer stabilized cholesteric liquid crystals

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