Nonlinear absorption of light: two-photon absorption and optical saturation in metalloporphyrin-doped boric acid glass

The nonlinear absorption of five metal TMPPs (TMPP: tetrakis-(3,4,5-trimethoxyphenyl)porphyrin) doped in boric acid glass are measured with the linear polarized nanosecond laser pulses at different wavelengths by Z-scan; the two-photon absorption (TPA) is dominant in the near infrared, while the characteristic of saturation absorption (SA) is observed close to the Q(0,0) band of porphyrin. The symmetry allowed two-photon π*←π transitions are suggested to be 1B1g*←S0 and 1B2g*←S0, with the cross-sections δ ranging from 25×10−50 to 114×10−50cm4s/photon. We analyze the property of SA with a four-level system, and find that the magnitudes of excited-state absorptivity and saturation intensity are not affected by changing the central metal ion in these experiments.