Analytic Model for the Determination of Excited State Parameters from Pump-probe Measurements

摘要

In order to completely and accurately characterize the materials whose nonlinear optical response makes them good candidates for use in eye and sensor protection applications, one must measure not only the absorption and refraction cross-sections of the various quantum energy levels involved in the optical absorption process, but also the decay rates of these states. This can be accomplished in a single experiment using time resolved pump-probe techniques. In a pump-probe experiment, a strong pump pulse excites a sample of the material under investigation, and one measures the transmittance of a weak probe as a function of time. In a double pump-probe experiment, two pump pulses excite the sample sequentially. The first pulse populates various molecular excited states, while the second induces transitions whose rates depend on the various populations and cross-sections. As in a single pump-probe experiment, one measures the sample transmittance of a weak probe beam or pulse as a function of time, from which one can infer the relevant excitation and decay rates. Parameter values are obtained by fitting a theoretical model to the experimental data, a procedure that may be carried out with particular ease if the model has an analytic solution. We describe the most complete model for the normalized probe transmittance that may still be solved analytically.