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Illumination & Displays

Tunable cholesteric color

Planar chiral-nematic liquid crystals show promise for use as switchable-bandpass filters and bistable reflective displays.
3 February 2009, SPIE Newsroom. DOI: 10.1117/2.1200901.1482

Cholesteric liquid crystals (CLCs), characterized by helical structures, can uniquely separate incident light into its left- and right-handed circular components by selective reflection and transmission.1,2 Planar-aligned CLCs with preselected helical pitch can only generate light at a single Bragg-reflected wavelength (i.e., monochrome). Multiple-wavelength/color-reflecting CLCs can be produced by stacking multiple CLC layers with different cholesteric pitches. The layers are normally arranged such that CLCs reflecting shorter (longer) wavelengths are placed on top (at the bottom). Alternatively, multiple-wavelength-reflecting CLCs can be fabricated using a sequential liquid-crystal arrangement in a single layer with each CLC reflecting different wavelengths.

It was recognized early on that electric-field-induced color change in CLCs can be achieved by applying a voltage, either parallel or perpendicular to the helix axis. Bragg-reflection tuning is done by either extending or compressing the helical pitches or through inducing helix tilts.3,4 The color-tuning mechanism (Helfrich deformation5) is based on uniform periodic CLC-layer deformations in response to external stimuli. Tilting the helix leads to a shortening of the pitch as observed from a normal direction. Helix unwinding or dilation leads to redshifted Bragg-reflected wavelengths. Both actions are usually followed by an increase in reflection bandwidth.

Alternatively, one can use interdigitized electrodes to untwist the helix, assisted by the in-plane electric field. The reported redshift in CLCs with positive dielectric anisotropy is caused by the field-induced pitch dilation.6, 7 However, these methods require a high switching voltage because of the inhomogeneous field distribution within the cell and the electric torque applied to different layers of the CLC. A dramatic loss in reflectivity with increasing voltage is observed because of the reduction in both the number of layers and the pitch elongation.

Using a polymer network to create liquid-crystal gels leads to a blueshift and broadening of the reflected wavelength with increasing voltage.8 Polymer-stabilized CLCs (PSCLCs) enable reversible color switching from blue to red with the helix axes oriented in the device plane.9 An electric field is applied perpendicularly to the helix axis, which leads to unwinding of the helixes as the field strength is increased.

We have developed a number of approaches to cholesteric-color tuning, including linking the photo-induced pitch change to a tunable chiral material, thus enabling reflection of different colors (see Figure 1) through isomerization or racemization.10 We also proposed using electric-field-induced color changes.10 CLCs display pixels of different colors at zero voltage and reflect black when a voltage pulse is applied (i.e., they are bistable). Using light control of the cholesteric pitch can also be achieved by CLC doping with an azobenzene chiral dye.11

Figure 1. Bistable color-reflective cholesteric display.

Here, we present an electrically switched color-cholesteric display with transmissive and reflective properties in a PSCLC cell that exhibits a color reflection on one side emdash see Figure 2(a) emdash and transmits light on the other: see Figure 2(b).12 The reflectivity remains at a certain value while the reflected wavelength is electrically switchable. Instead of broadening, the reflection bandwidth decreases slightly with increasing voltage. CLC cells using electrically switched reflective wavelengths have a threshold of approximately 2.0V/μm and the electrically tuned color is blueshifted by a maximum of 140nm as the voltage is increased: see Figure 2(c). The inserts in this panel show the color tuning of a cell in response to the applied voltage. The tuning range and critical voltage of the initial reflectivity decrease are controllable by either varying the film thickness or localization of the cell's polymer network.

Figure 2. Reflectivity and wavelength as a function of applied voltage (indicated) of a polymer-stabilized cholesteric-liquid-crystal cell.

We have thus demonstrated field-induced color tuning in a PSCLC with dual-mode (transmissive and reflective) properties. The cell shows good thermal stability and the electrically induced color tuning is reversible. Electrically tunable wavelength filters may be applied to other spectral ranges as well. Several related projects are currently in progress. One of the emerging areas in CLC research centers on liquid-crystal colloidal suspensions forming novel structures, which is aimed at enhancing the speed of electrically induced color. This can be achieved because of modifications in the anchoring and elastic energy of the anisotropic liquid-crystal medium. As a second potential future research direction, we will explore employing optical isotropic and polymer-stabilized blue-phase media, which can lead to a new way of observing field-induced colors.

Liang-Chy Chien, Shin-Ying Lu
Liquid Crystal Institute and Chemical Physics Interdisciplinary Program,
Kent State University
Kent, OH

Liang-Chy Chien is a professor.