Z-scan theory for material with two- and three-photon absorption

We present a theoretical study on the Z- scan characteristics of thin nonlinear optical media with simultaneous two- and three- photon absorption, a situation that exists, for example, in polydiacetylenes. With the introduction of a coupling function between two- and three- photon absorption, we find a quasi- analytic expression for open aperture Z- scan traces. We make a comparison of the analytic solutions with numerical solutions in detail, showing that they are in good agreement. This theoretical result allows us to easily identify and determine simultaneously the two- and three- photon absorption coefficients from the open aperture Z- scan traces. (c) 2005 Optical Society of America.