Probing the photonic properties of a cholesteric elastomer under biaxial stress

We have investigated the photonic properties of a free-standing cholesteric elastomer film under biaxial mechanical load. The sample, which was obtained by UV cross-linking of a cholesteric side-chain polymer, shows substantially improved optical properties compared to cholesteric elastomers obtained by the anisotropic deswelling method. The selective reflection of circularly polarized light and the modified spontaneous emission of dispersed fluorescent guest molecules show that the cholesteric order is well preserved in the cross-linked film. Application of biaxial stress leads to a shift of the photonic stop band to shorter wavelengths due to a contraction of the cholesteric helix; the change of the cholesteric pitch is affine to the film thickness. Pulsed excitation of the fluorescent guest molecules gives rise to photonic band edge laser emission, which is mechanically tunable in a wavelength range of almost 100 nm. The lasing performance is found to be comparable to that of conventional cholesteric systems.