Color selectivity lent to a cholesteric liquid crystal by monitoring interface-induced deformations
Résumé
The cholesteric liquid crystalline structure is omnipresent in living matter and concerns many applications in optics because of its property of selective light reflection. The color reflected by this structure depends on material parameters such as the molecular chirality or the concentration of chiral dopant, the helical pitch of the twisted structure and the optical indices. In the present publication, we show that the color may be selected simply by varying the annealing time of a cholesteric oligomer film with hybrid anchoring. Experimental and simulated transmittance spectra reveal the color shift. The three-dimensional representation of the structure is provided from the macroscopic scale to the mesoscale by combining complementary imaging techniques (optical microscopy, transmission and scanning electron microscopies, atomic force microscopy) to image the texture in different directions. We show that the color selectivity is due to controlled changes of the orientation of the helical axis with respect to the air–material interface. The optical behavior is described with a single parameter, which is the angle between the film surface normal and the orientation within the film of the helical axis. Potential applications in the field of chiro-optical devices concern self-assembled chiral microreflectors fabricated via a low-cost controllable thermal process. Additionally, the texture formation provides insight to aid the understanding of the exoskeleton morphogenesis of beetles like Chrysina Gloriosa.
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