Femtosecond continuum probe measurements of nonlinearities of organic dyes

We report measurements of the nonlinear transmission spectra of organic dye solutions. The reverse saturable absorption (RSA) of these organic dyes in the visible and near IR was measured using a 200 fs 425 nm excitation and a temporally delayed 200 fs white light continuum probe. These RSA materials have attracted attention due to their potential usefulness in optical limiting devices. Knowledge of the dynamics of the nonlinear response along with the spectral dependence is important, in order to determine the range of operation of a given material and to properly model the level structure and lifetimes. We developed an optical source based on an argon ion pumped, Kerr lens modelocked Ti:sapphire oscillator followed by a Cr:LiSAF regenerative amplifier producing millijoule level, 200 fs pulses (FWHM) around 850 nm. A single pulse is then split to generate a second harmonic (SH) at 425 nm and a femtosecond continuum that are used as the pump and probe respectively in a standard pump-probe geometry. The SH is produced in a thin BBO crystal and the continuum is produced by focusing the 850 nm light into a water cell. This results in up to 50 microjoules of pump and 3 microjoules of probe in the spectral range from 200 nm to 800 nm. These pulses have been used to temporally resolve the nonlinear spectra of several organic solutions including zinc tetra (p-methoxyphenyl) tetrabenzporphyrin, lead phthalocyanine, and silicon naphthalocyanine up to delays of several nanoseconds