Characterization of 2PA Chromophores

We review several experimental methodologies for the determination of nonlinear absorption coefficients of organic chromophores in either solution or thin film form. The goal here is to also be able to identify and quantify the nonlinear absorption process or processes, e.g. the essentially instantaneous two-photon absorption (2PA) or the cumulative excited-state absorption (ESA). These methods include the direct methods of nonlinear transmission and Z-Scan, along with the indirect methods of two-photon-induced fluorescence, excite–probe approaches, and Degenerate Four-Wave Mixing. We also discuss both frequency degenerate and frequency nondegenerate processes with emphasis on 2PA.