Ultrafast relaxation times of metalloporphyrins by time-resolved degenerate four-wave mixing with incoherent light

We present experimental results on the measurement of the population-relaxation times of the excited states of some metalloporphyrins such as zinc meso-tetra-phenylporphyrin (ZnmTPP), zinc meso-tetra $(p$-methoxyphenyl) tetrabenzporphyrin (ZnmpTBP), tetratollylporphyrin (TTP), cobalt tetratollylporphyrin (CoTTP) and nickel tetratollylporphyrin (NiTTP) with time-resolved degenerate four-wave mixing with the incoherent emission of a nanosecond pumped-dye laser. The experiments are carried out in the counterpropagating geometry $({\mathbf{k}}_4={\mathbf{k}}_1+{\mathbf{k}}_2-{\mathbf{k}}_3),$ where ${\mathbf{k}}_1$ and ${\mathbf{k}}_2$ are counterpropagating to one another and ${\mathbf{k}}_3$ is at angle θ ($<90°$ with respect to ${\mathbf{k}}_1$). Excited-state and ground-state relaxation times are calculated based on the theoretical formalism developed by Hiromi Okamoto [J. Opt. Soc. Am. B , 10, 2353, 1993]. The signals are recorded by delaying the probe beam $({\mathbf{k}}_3)$ for different fixed delays of the backward pump beam $({\mathbf{k}}_2).$ We observe strong signal at a second peak owing to the coherence of beams ${\mathbf{k}}_2$ and ${\mathbf{k}}_3$ for samples exhibiting excited-state absorption and a narrow peak corresponding to the coherence of beams ${\mathbf{k}}_1$ and ${\mathbf{k}}_3$ for samples that do not exhibit excited-state or reverse-saturable absorption. The nonlinear absorption is studied with the Z-scan technique. From the experimental data, we estimate the relaxation times of excited states.