Solid-liquid phase transition activated by optically driven molecular motors

We report on a nearly isothermal, reversible transition of a polymer film from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. The phase transition phenomenon and the related unidirectional mass-transport effect are caused by optically driven molecular motion of azobenzene-functionalised molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also show that the selective excitation of chromophores by linear polarized light induces an anisotropic expanding force, which can be used for polarization-selective optomechanical actuators and sensors. Moreover, since the molecular motions can be induced also by spatially confined non-propagating optical fields, we are able to manipulate the state and position of nanoscopic elements of matter using optical near-field approaches.