Appl. Phys. B 45, 137-144 (1988) Applied "'*~ physics Physics B and Laser Chemistry 9 Springer-Verlag 1988 Optical Bistability in Fluorescein Dyes S. Speiser* and F. L. Chisena Allied-Signal Incorporated, Engineered Materials Sector P.O. Box 1087R, Morristown, NJ 07960, USA Received 30 September 1987/Accepted 26 November 1987 Abstract. Optical bistability has been observed in highly concentrated fluorescein dye solutions and in thin (~ 1 gm) doped polymeric films. At concentrations larger than 10-5 mole/1 dye dimers are formed. For fluorescein dye, the dimer-monomer equilibrium constant is 105 1/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10-18 cm2/molecule, while that for the dye monomer molecule is 7.6 x 10 -17 cm2/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. This high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system. Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10 .4 mole/l fluorescein than in 10 -6 mole/1 fluorescein in which dimer formation is not that efficient. In disodium fluorescein no significant dimer formation is observed even at 10- 3 mole/1 dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems. PACS" 33, 42.65, 42.70 Optical bistability is by now a widely studied subject, mainly because of its potential use in optical data processors and optical computers [1, 2]. Optical bista- bility is characterized by two different light trans- mission states of an optical system for a given input light intensity [-1,2]. In order to observe optical bistability, a nonlinear optical medium and an optical feedback are required. As is the case with other nonlinear optical devices, good optical materials suit- able for bistable devices are needed [3]. In recent years organic and polymer media have been inves- tigated as potentially promising new optically non- linear materials [4, 5]. The fluorescein dye was used as a nonlinear optical medium for a variety of applic- ations [6, 7] which make this molecule a natural choice for optical bistability studies in nonlinearly absorbing media. * On sabbatical leave. To whom correspondence should be addressed at Department of Chemistry, Technion-Israel In- stitute of Technology, Haifa 32000, Israel In order to analyze optical bistability for various systems, we have developed [8, 9] the nonlinear com- plex eikonal approximation. Our goal was to set a standard mathematical treatment for the analysis of the propagation of light waves through nonlinear media. The lack of such a general treatment had complicated both the engineering modeling of optical systems incorporating nonlinear elements and better understanding of the related physical phenomena (since most of the other methods yield only numerical solutions for complicated cases). The nonlinear eikonal approximation can be sum- marized in the following equation: Z ~b(z) = (2rC/2o)f n[I(z')]dz', (1) o where ~b(z) is the complex accumulated phase, z the distance of propagation in the medium, n the non- linear complex index of refraction, and I(z) the local light intensity. This integral equation can be applied to