| In this thesis, the mechanism of excited-state absorption-enhanced transient thermal refraction (excited-state thermal refraction, ESTR) in cluster and organic materials'solution media was investigated.Based on the beam propagation equation in nonlinear media, coupled with the excited-state five-level rate equation, the heat conduction equation and photoacoustic equation, the effects of pulse duration, waist size at focus, input energy of single pulse and thermodynamic parameters of solvent on the excited-state thermal refraction were studied numerically. For excited-state absorption (ESA) media, the main source of heat accumulation, related to the thermally induced optical nonlinearities, was found to be the fast relaxation from the higher excited-state levels of singlet and triplet states to the first levels of them. In transient regime, because the density fluctuation is inconsistent with the temperature change, the ESTR with nanosecond pulses irradiance will be overrated greatly by using the quasi-steady thermal approximation method. The effect of pulse duration was found to act on the acoustic wave propagation, i.e. less distance of sound propagation within pulse duration weakens the effect of ESTR, even with the nearly same temperature change. And the waist size affects the propagation of acoustic wave in another way: with same pulse duration and fluence, the temperature change is nearly same for different waist sizes, but the ESTR is reduced for the bigger waist size that will take sound wave more time to propagate through it; with the same pulse duration and input energy, the temperature change is smaller and ESTR is weakened much more. The abnormal phenomenon was found that ESTR decreases near the focus when the absorption cross section of the first triplet excited-state is enlarged. The reason is attributed to the reverse saturable absorption's effect on the refraction curve of the Z-scan experiment is overcoming the effect of nonlinear refraction itself. In addition, the ESTR effect of same material is much different in solvents for different thermodynamic parameters.Because the temperature change has vanished, the heat accumulation between adjacent two pulses is negligible with long interval. The competitions of ESTR with excited-state refraction (ESR) or optical Kerr effect (OKE) were discussed. For different temporal and spatial distributions, the ESTR was found to be incapable to counteract with nonlinear refractions resulting from other mechanisms. With different input energies and waist sizes, the total nonlinear refractions under the conditions of coexistence of ESTR and ESR, and coexistence of ESTR and OKE vary in nearly same way, and the both total nonlinear refractions can transform from self-focusing effect to self-defocusing effect. When the pulse duration is shorter than the intersystem crossing time, the nonlinearities of singlet state exceed those of triplet state; the OKE is enlarged due to its proportional relationship to the intensity yet; so the transformations of total nonlinear refraction under the condition of coexistence of ESTR and ESR obviously fluctuate in a different way compared with those of coexistence of ESTR and OKE.In experiment, the self-defocusing phenomenon of a hexagonal-prism-shaped metallic cluster of Mo2Cu4S8(PPh3)4 dissolved in DMF was studied with different input energies. The mechanism was attributed to the linear absorption induced transient thermal nonlinear refraction. Then the ESTR in C60/toluene solution with different input energies and the competitions of ESTR with ESR in CuPcTsNa4/distilled-water solution with different input energies and pulse durations were also analyzed, the transformations from self-focusing effect to self-defocusing effect associated with the pulse duration were found. The experimental data fit the theoretical results well.
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