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) elasto polymer host. Depending on the ratio between the absorption cross sect ion from the ground to the first excited state (sigma(01)) and from th e first to the higher excited states (sigma(12)) and on the pump wavel ength, they demonstrate absorption that either increases or decreases with increasing incident energy density. Our interest is directed at s tudying the optical Limiting behavior of the dyes for sensor protectio n applications, which requires that sigma(12) be greater than sigma(01 ). We performed a detailed investigation of the dynamics of these proc esses, using Z scan and pump-probe techniques at the single wavelength of 532 nm. We obtained sigma(12)/sigma(01) approximate to 80 for one of the dyes, which is significantly higher than any previously reporte d value of which we are aware. The Linear absorption remained signific ant, a requirement for good optical limiting. However, at high inciden t energy density for most of the dyes the increasing absorption conver ted to a bleaching effect with repeated irradiation. This effect was r eversed after several seconds in liquid solutions and several minutes in the polymer host, possibly because of replenishment of the dye by d iffusion in both media. (C) 1998 Optical Society of America.