Light-induced deformation in a liquid crystal elastomer photonic crystal

Elastomer materials can undergo large, reversible elastic deformation, and offer novel possibilities for coupled optomechanical behavior when light itself is used to induce that deformation. This phenomenology is especially interesting to consider when photonic bandstructure effects and mechanical instabilities are present over the same length scales. Here we investigate a novel, coupled optomechanical material behavior whereby complex deformation, with the potential to occur cyclically, occurs in a soft photonic crystal structure due to a mechanical instability, as a result of constant, uniform illumination by normally incident light. We suppose that the base material for the structure is a material that responds to light by undergoing a microstructural change. Such a behavior is observed, for example, in a liquid crystal elastomer containing azobenzene moieties attached to the liquid crystal main-chains (Finkelmann et al., 2001) transformational strain generated by the effect of localized light energy on the isomerization of the azobenzene moieties can be calculated from an order-parameter based model (Hogan et al., 2002). Under uniform exposure to constant illumination, the interaction between the light, the material, and the deforming structure lead to a complex, reversible deformation sequence. We analyze the electromagnetic energy distribution inside this photonic crystal structure by solving Maxwells equations for the electromagnetic problem of light transmittance using finite element analysis. First, upon contraction of the structure due to isomerization in the uniformly illuminated material, the photonic bandstructure shifts, thereby significantly reducing the average illumination of material within the structure. The locally reduced illumination allows for a relaxation of the strain in some parts of the structure, due to the reversible isomerization at room temperature. Then, as a result of this relaxation, the structure is subjected to uniaxial stress, leading to a mechanical instability that triggers a geometrical pattern transformation. This in turn produces a second contractile deformation, as a result of the buckling-like deformation in the structure. Finally, the highly nonuniform local strain field that results generates a dramatic change in the photonic bandstructure of the system, leading to a new localization of the light that tends to reverse the effect of pattern transformation. This completes the transformation sequence, demonstrating the potential for cyclical deformation induced simply by uniform illumination. The coupled optomechanical material/structure behavior observed here could lead to applications in optically sensors, energy harvesters, or other reversible optomechanically active structures.