Tunable lasing in cholesteric liquid crystal elastomers with accurate measurements of strain.

Andrii Varanytsia,Peter Palffy-Muhoray,Hama Nagai,Kenji Urayama

doi:10.1038/srep17739

Abstract

We report wide range and reversible tuning of the selective reflection band of a single crystal cholesteric liquid crystal elastomer (CLCE). The tuning is the result of mechanical shortening of the helical pitch achieved by imposing a uniform uniaxial strain along the helical axis. On doping the CLCE sample with a laser dye, we observe lasing from the CLCE in both glassy and rubbery states. By changing the cholesteric pitch, mechanical compression provides tuning of the laser emission from the dye doped CLCE over a significant part of the fluorescence band of the laser dye. In this work we demonstrate for the first time that both the CLCE pitch and the lasing wavelength are linearly dependent on the strain imposed on the CLCE film.

Highlights

CLCEs are soft solids formed by mesogenic molecules incorporated into a polymer network with helical structure similar to low molecular weight Cholesteric liquid crystals (CLC)
Upon transition from the glassy to the rubbery state, the selective reflection band of the CLCE film narrows in width and its position red-shifts indicating the considerable temperature dependence of the refractive indices and of the helical pitch of the CLCE film
The red-shift of the selective reflection band and decrease of its area is a direct evidence of an increase of the helical pitch and of the film thickness as the CLCE film is heated from the glassy to the rubbery state above the glass transition temperature Trg

Summary

Synthesis and manufacturing of the sample

The CLCE film was fabricated using the method described in detail elsewhere[28]. The photopolymerization of the reactive chiral monoacrylate (A*-6OCB; Fig. 1 (a)) and the diacrylate cross-linker (HDDA; Fig. 1(a)) was conducted in the presence of a non-reactive achiral nematic solvent (6OCB; Fig. 1 (a)). The swollen gel was gradually deswollen by adding methanol to the swelling solvent, resulting in a fully dried film. The laser dye was introduced into the sample by swelling the CLCE in its glassy state at room temperature in a toluene followed by a contraction in. Solvent replacement was performed in steps; by adding a small amount of methanol solution into the vial containing the swollen sample with subsequent gentle mixing and removal of a similar amount of the resulting solution of both solvents. The solvent replacement procedure was repeated until only a very small amount of toluene was left in the vial and the sample was completely contracted. Slow contraction of the sample by gradual replacement of solvents is critical to prevent a rapid decrease of volume which can result in cracking or breaking of the CLCE film. The concentration of the dye in the CLCE film after swelling is ~0.02 wt.%

Experimental results

Conclusions

Additional Information