Third-Order Nonlinear Susceptibilities of Dye Solutions Determined by Third-Harmonic Generation

The third-order nonlinear susceptibilities xxxxx ( — ^ i> w i> &>i) versus concentration are determined for rhodamine 6G, fuchsin, and methylene blue in methanol. Third-harmonic generation of a modelocked Nd-glass laser is applied. The real and imaginary part of the nonlinear susceptibility are measured. A resonance enhancement due to absorption bands is observed. P A C S : 42.65, 42.65 B Phasematched third-harmonic generation in dye solutions was studied in the past as potential process of efficient frequency tripling [1-6]. The conversion efficiency r\ could be increased about a factor of 50 (fuchsin [1-4]) to 4000 (hexamethylindocarbocyanine iodide [5]) compared to the neat solvent. Phasematching was achieved by utilizing the anomalous dispersion of the refractive index of the dye solutions. The absolute conversion efficiency remained weak (rj^2 x l O " 1 1 in fuchsin [2], 7 / ^ 2 x 1 0 " 9 in hexamethylindocarbocyanine iodide [5] at 10 7 W / c m 2 laser intensity) because of light absorption at the thirdharmonic frequency limiting the interaction length. The nonlinear hyperpolarizabilities of dye molecules were estimated and related to conjugated bonds and two-photon-resonances [5]. Here we determine the dependence of third-order nonlinear susceptibilities xx3xxx ( — ^3; col9 cou cox) on dye concentration for rhodamine 6G, fuchsin and methylene blue in methanol. The efficiency of conversion of picosecond light pulses of a mode-locked N d glass laser to the third harmonic is measured. The nonlinear susceptibilities are deduced by comparison with calculations [7]. The absorption coefficients and refractive indices entering the calculations are separately measured [8,9]. The nonlinear susceptibilities comprise contributions from the solvent and the solute. The complex solute hyperpolarizabilities are determined and compared with the real solvent hyperpolarizability. The resonance enhancement is analyzed.