Higher Order Bragg Reflection Colors in Polymer Stabilized Cholesteric Liquid Crystals

: We have previously reported on dynamic electro-optic (EO) response of polymer stabilized cholesteric liquid crystals prepared using unpolarized UV light (U-PSCLC), such as reflection bandwidth broadening and either red or blue tuning of the reflection peak. Here, we describe recent efforts to use a polarized single argon ion laser beam to create PSCLCs (L-PSCLCs) with higher order reflections. The L-PSCLCs exhibit a primary reflection peak in the near infrared (NIR) regime and a second order reflection band with a narrow bandwidth in the visible regime that results from a deformed in-plane CLC helical structure. The initial positions of the reflection bands are adjusted mixture, and red, green and blue reflection colors from the second order Bragg reflection are demonstrated. The primary and the second order reflection bands can be shifted to longer wavelengths by application of a direct current (DC) electric field. The reflection efficiency of the higher order reflection notch increases with polymer concentration, which affects the degree of in-plane deformation and fixation of the CLC helix. Modeling is used to further explain the formation of the higher order reflection bands of PSCLCs observed experimentally. The table of contents entry should be 50−60 words long , and the first phrase should be bold. An electrically tunable second order reflection color is demonstrated in the positive  PSCLCs. The second order reflection peak shifts from 570 nm to 740 nm as the DC voltage increases to 150V and a color change from green to red is observed.


Introduction
Cholesteric liquid crystals (CLC) are one-dimensional, photonic materials that exhibit selective Bragg-type reflection due to the self-organized, helicoidal superstructure. [1,2] The periodic CLC phase has a pitch ( ) expressed as and , where  0 is the center of the wavelength,  is bandwidth, is the average refractive index of the liquid crystal, is the cholesteric pitch length, and n is the birefringence of the liquid crystal.
CLCs can be prepared by mixing chiral dopants with a nematic medium, and the spectral position (pitch length) of the CLC can be adjusted by the chiral dopant concentration. The supramolecular chirality of the CLC is determined by the handedness of the chiral dopant, and circularly polarized (CP) light having the same handedness is reflected 100% and CP with the opposite handedness is transmitted 100% (unpolarized light -50% transmitted, 50% reflected).
For a left handed helix and left handed light, 100% is reflected.

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This article is protected by copyright. All rights reserved induces movement of ions, especially positive ions, trapped on and in the polymer network, causing the polymer network to deform towards the negative electrode. The small molecule LC host consequently moves with the polymer network which results in a pitch variation across the cell. [6,8,18] Recently, we also demonstrated that reversible switching and redshifting tuning responses in a single positive  PSCLC sample. [21] AC field application induces switching from reflective to transparent states by rotating the positive  LC in the normal direction of the substrate, while the pitch gradient is induced by the deformation of the polymer network by an increase in the DC field and the position of the reflection notch is controlled.
Higher order reflections in CLCs with positive dielectric anisotropy (> 0) have been previously observed in planar cells fabricated with interdigitated electrodes. In this case, the pitch of the CLC increases when an electric field is applied perpendicular to the helical axis of the CLC, but the planar texture is maintained. [22][23][24] The increase in pitch causes a red shift of the reflection bands. At an electric field higher than the critical electric field, higher order reflection bands were induced in the CLCs due to the helix unwinding of the CLC. [25,26] While Blinov et al. observed a second order reflection band using this technique, Rumi et al.
observed second and third order reflection bands of CLCs with > 0 in planar cells with interdigitated electrodes. This report described high reflection efficiency of the second order reflection band (~80% of the main reflection band) whose position could be tuned by the magnitude of the applied electric field.
Broer and colleagues also reported higher order Bragg reflections in CLC polymers containing dichroic initiators photopolymerized by linearly polarized (LP) UV light. [27][28][29][30] The higher absorption of the dichroic photoinitiator in the direction parallel to the direction of the light polarization leads to faster initiation and photopolymerization of the LC monomers in the CLC mixture. This selective, non-uniform photopolymerization causes inhomogeneous director rotation in the CLC helical structure through the cell thickness which leads to higher

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This article is protected by copyright. All rights reserved order Bragg reflections. The reflection efficiency of the higher order reflection bands is proportional to the degree of deformation of the helical structure in the CLC polymers.
In this manuscript, we report on the formation of higher order reflections of PSCLCs

Results and Discussion
Two PSCLCs were prepared by irradiating with either unpolarized UV light (U-PSCLC) or a linearly polarized argon laser beam (L-PSCLC) and the transmission spectra were collected using right-handed circularly polarized light as the probe beam (Figure 1). The U-PSCLC and L-PSCLC samples were prepared from the same LC mixture containing 1% photoinitiator The chemical structures for the component compounds are shown in Figure S1. As can be seen in Figure 1(a, b), both U-PSCLC and L-PSCLC exhibit a reflection peak at 1320 nm, but the L-PSCLC sample displays a second order reflection peak at 660 nm. The spectral position of this reflection notch is half of the position of the main reflection band. While the U-PSCLC

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This article is protected by copyright. All rights reserved sample is visually transparent because the Bragg reflection is in the NIR, the L-PSCLC sample has exhibits red reflection (see inset in Figure 1(b)). Additional L-PSCLC samples containing 4wt% and 5wt% chiral dopant R1011 were prepared. Figure 1(c, d) depicts the transmission spectra and inset photographs of thin films of these two additional L-PSCLC samples. The spectral positions of the primary and secondary peaks are easily adjusted by the chiral dopant concentration in the CLC mixture, and red, green, and blue reflection colors are observed in L-PSCLC samples containing 3.5 wt%, 4 wt%, and 5 wt% R1011, respectively.
As a control experiment, a PSCLC sample was prepared from a CLC mixture without a photoinitiator exposed to the same argon laser beam under the same exposure conditions. [31] In Figure S2, higher order Bragg reflections are not observed in the PSCLC sample without a photoinitiator, whereas the second order Bragg reflection peak is observed in the PSCLC sample with 1wt% photoinitiator. This indicates that the ratio of reflection of the L-PSCLC sample can be controlled by the curing direction of the CLC mixture. [27,28] Accepted Article This article is protected by copyright. All rights reserved

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This article is protected by copyright. All rights reserved Modeling of higher order Bragg reflections for L-PSCLC was performed and is shown in Figure 3 and Figure S3. The higher order Bragg reflection of L-PSCLC is modeled with an inhomogeneously twisted director through the CLC helix using the various nondimensional products ( ) from 0.05 to 0.5. A good agreement between the experimental data ( Figure   1(d)) and the calculated results is observed in Figure 3.

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This article is protected by copyright. All rights reserved Comparing Figure 1(d) and Figure 3 shows that the proposed mechanism for the in-plane pitch distortion due to inhomogeneous photopolymerization yields qualitatively good agreement between modeling and experimental observations. The slight discrepancy in the position of the second order reflection peak between the experimental and modeled transmittance is probably due to the incomplete normal incidence of the UV laser beam on the CLC sample.   The reflection efficiency of L-PSCLC samples depends on the degree of stabilization of polymer network in the CLC medium. Figure 5 shows the reflection efficiency of the second order reflection band as a function of polymer concentration from 6 wt% to 20 wt%. No higher order reflection peaks are observed in samples with a polymer concentration of less than 5 wt%. The reflection efficiency of the main CLC peak at ~1080 nm is constant regardless of the polymer concentration ( Figure 5(a)), but the reflection efficiency of the

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This article is protected by copyright. All rights reserved second order reflection peak at ~540 nm is strongly dependent on the polymer concentration, as summarized in Figure 5(b and c). Exposure to the laser beam induces the LC in-plane rotation, resulting in the distortion of CLC helix, and which is stabilized by the polymerization process. PSCLC samples with higher polymer concentrations show higher reflection efficiency of the second order reflection band, whereas PSCLCs with a polymer concentration of less than 5wt% does not stabilize the distorted CLC. Samples with polymer concentrations higher than 15wt% show maximum reflection efficiency, indicating that the deformed CLC helix is completely fixed. To investigate the electrooptic response of the higher order reflection peak, the L-PSCLC samples with 15-20 wt% polymer concentrations are used due to their high reflection efficiency. Samples with a polymer concentration higher than 20 wt% require a very high electric field to control the reflection band or form a polymer-dispersed liquid crystal (PDLC) that exhibits switching behaviour. Figure 6 show the dynamic response of the L-PSCLC sample with a polymer concentration of 15 wt% as the DC field increases. [32] The main and the second-order reflection bands shift to longer wave length by increasing the DC voltage to 150V. The main CLC band shows a notch shift of 300 nm from an initial notch position of 1150 nm, while the second-order reflection band exhibits a ~150 nm shift. The initial green reflection color of the second-order reflection band can be tuned to red with 50V. Another L-PSCLC sample with a higher polymer concentration (20 wt% C11M) shows a similar red-

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This article is protected by copyright. All rights reserved tuning response (Figure S4). When the applied DC voltage is removed, both reflection bands return to the initial notch position. The spectral position, tuning range, and normalized tuning range of the reflection bands of L-PSCLC with 20 wt% polymer concentration (C11M) are summarized in Figure S4(c, d, e). The primary reflection band displays twice the tuning range of the second order reflection band ( Figure S4d), but the tuning ranges of the two peaks overlap nicely when the data is normalized ( Figure S4e).

Conclusion
The formation of higher order reflection bands and electro-optic response of L-PSCLCs have been reported. The cholesteric liquid crystal mixture exposed to an argon ion laser beam creates a deformed in-plane helical structure of CLC. The deformed inhomogeneous CLC helical structure is fixed via photopolymerization of LCM and leads to higher order reflection bands. The main and the second order reflection bands of these materials can be shifted to longer wavelengths (red tuning) by applying a DC field. The electrically controllable reflection colors and polarization dependence of the main and higher reflection bands in L-PSCLCs can be used in several optical applications.

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This article is protected by copyright. All rights reserved Experimental Setup and Measurements: Transmission spectra were collected with a fiber optic spectrometer, and unpolarized, linearly polarized, left-handed or right handed circularly polarized light was used as a probe beam. Transmission spectra were collected before, during, and after application of electric fields with the scanning rate of 1 V s -1 or directly applied to the target voltage.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.