Hyper-reflective cholesteric liquid crystal fabricated using surface-initiated photopolymerization
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Hyper-reflective cholesteric liquid crystal fabricated using surface-initiated photopolymerization
a
b
Polymer + RH
+ CLC
C
holesteric liquid crystals (CLCs) are promising candidates for a variety of photonic applications because they selectively reflect incident radiation. The central wavelength of the Bragg reflection is a function of the helical pitch, which can typically be controlled by a chiral dopant. A key limitation of a single CLC film is a maximum reflectivity of 50% due to the circular polarization of the reflection, which matches the handedness of the CLC’s helical structure. Increased reflectivity can be achieved with designs employing multiple CLC cells but such devices suffer drawbacks related to the increased number of interfaces. Fabrication approaches of socalled hyper-reflective CLC devices, which simultaneously reflect both rightand left-handed (RH and LH) circularly polarized light from a single film, have heretofore relied on local differences in handedness in the film’s x-y plane (where the pitch axis is in the z-direction). Resulting devices exhibit >50% reflectivity at a fixed wavelength. Now, T.J. Bunning, T.J. White, and co-researchers at Wright-Patterson Air Force Base have fabricated hyper-reflective CLC films where spatial segregation of LH and RH domains is in the z-direction—through the cell thickness. In addition, the researchers incorporated photosensitive liquid crystals (LCs) to create dynamically induced hyper-reflective cells. As reported recently in Chemical Communications (DOI: 10.1039/c0cc02215b; p. 505), Bunning, White, and coresearchers fabricated a CLC cell—a central CLC layer (containing an achiral nematic LC, a RH chiral dopant, an achiral diacrylate monomer, and a RH chiral monomer that reflects at about 720 nm) sandwiched by two polyimide layers, all sandwiched by two glass layers (see Figure 1)—all fairly typical except that one of the polyimide layers contained a photoinitiator. Polymerization therefore proceeded from only one side of the cell,
c
d
hiral NLC
Polymer + Ac
Figure 1. (a–c) Fabrication steps are represented schematically, along with corresponding transmission spectra; (d) a cartoon illustrating the two distinct regions in the cell separated across the z-direction. RH CLC is right-handed cholesteric liquid crystal; NLC is nematic LC. Reproduced with permission from Chemical Communications 47 (2011) DOI: 10.1039/c0cc02215b; p. 505. © 2011 Royal Society of Chemistry.
creating a RH helical polymer scaffold that extends about two-thirds into the cell, about 19 μm (see Figure 1d). The LC mixture and unreacted monomer were subsequently leached from the cell. A Bragg reflection of 670 nm was observed after the cell was refilled with the same nematic LC (see Figure 1c), confirming that the polymer scaffold is RH helical, and maintains the original pitch after the leaching/refilling process. The researchers said that structural chirality, imposed by the chiral polymer scaffold, induces the reflection, as opposed to induction by chiral dopants, whereas
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