Photoisomerization-induced orientational wave generation in two-dimensional liquid crystals at the air-water interface

Certain class of amphiphilic derivatives of azobenzene form a stable monolayer analog of single-layer smectic-C (SmC) phase at the air-water interface. Due to the broken up-down symmetry, this two-dimensional liquid crystal shows intriguing static orientation structures such as the stripe and higher-order point defects in equilibrium. We found that weak excitation of isomerization reaction of azobenzene core results in a persistent generation of orientational waves and solitons propagating in the plane of the monolayer. This is the first example of purely light-driven nonequilibrium pattern formation. Analysis of the static structures has revealed the essential significance of the coupling between the orientational order and molecular density. Although no definite explanation as yet exists for the dynamic behavior, we believe the density-orientation coupling should play an important role.