Simulation of photochemically induced motion of matter in gradient light fields

Abstract

The motion of matter containing photoreactive units, such as azo dyes, occurs when the latter undergo cyclic photoisomerization in gradients of light intensity; an effect referred to by photochemical tweezing. Matter motion is polarization sensitive owing to photo-selection of the azo dyes, and it has been described in detail by the recently developed theory of photoinduced vectorial motion of matter (PVMM). Indeed, motion occurs in the direction of the vector gradient of the actinic light with an efficiency that depends on the respective orientations of light polarization and gradient vectors. This paper uses rigorous numerical simulations to reproduce the motion of matter by photochemical tweezing in gradients of intensities produced by a Gaussian beam as well as by an interference pattern resulting from two coherent beams. The simulations are based on the PVMM theory and reproduce very well experimental observations. In particular, and in agreement with the published literature, the parameters used in our simulations impose mobility enhancement of solid azo-polymers by photoisomerization to the viscous flow level, and matter motion is due to the concomitant effects of the enhancement of molecular mobility and the photoisomerization force in the gradients of light fields.