Nonlinear light absorption of polymethine dyes in liquid and solid media

We investigate nonlinear optical absorption of a series of polymethine dyes in liquid solution and in a solid poly(urethane acrylate) elastopolymer host. Depending on the ratio between the absorption cross section from the ground to the first excited state (σ01) and from the first to the higher excited states (σ12) and on the pump wavelength, they demonstrate absorption that either increases or decreases with increasing incident energy density. Our interest is directed at studying the optical limiting behavior of the dyes for sensor protection applications, which requires that σ12 be greater than σ01. We performed a detailed investigation of the dynamics of these processes, using Z-scan and pump–probe techniques at the single wavelength of 532 nm. We obtained σ12/σ01≈80 for one of the dyes, which is significantly higher than any previously reported value of which we are aware. The linear absorption remained significant, a requirement for good optical limiting. However, at high incident energy density for most of the dyes the increasing absorption converted to a bleaching effect with repeated irradiation. This effect was reversed after several seconds in liquid solutions and several minutes in the polymer host, possibly because of replenishment of the dye by diffusion in both media.