Wednesday, 23 January 2019

Light bends it better: Photonic switch and large elasticity changes

P. Lakshmi Madhuri, S. Krishna Prasad, Uma S. Hiremath, and C. V. Yelamaggad, Appl. Phys. Lett. 104, 241111 (2014)
S. Krishna Prasad, P. Lakshmi Madhuri, Uma S. Hiremath, and C. V. Yelamaggad, Appl. Phys. Lett. 104, 111906 (2014)

We have proposed a photonic switch employing a blend of host banana-shaped liquid crystalline molecules and guest photoisomerizable calamitic molecules. The material exhibits a change in the sign of the dielectric anisotropy switching from positive to negative, at a certain crossover frequency of the probing field. The consequent change in electric torque can be used to alter the orientation of the molecules between surface-determined and field-driven optical states resulting in a large change in the optical transmission characteristics. Here we demonstrate the realization of this feature by an unpolarized UV beam, the first of its kind for banana-shaped molecules. The underlying principle of photoisomerization eliminates the need for a second driving frequency. The device also acts as a reversible conductance switch with an order of magnitude increase of conductivity brought about by light. Possible usage of this for optically driven display devices and image storage applications are suggested. Concomitant with the trans-cis isomerization a large decrease of both the Frank elastic splay and bend moduli is observed, and the latter became almost independent of temperature except near the isotropic phase. The observed variations are much larger than can be explained by order parameter changes and are explained using an argument based on the molecular arrangement.

Posted by S. Krishna Prasad