| Summary: | 碩士 === 國立中正大學 === 物理所 === 95 === Thermal lensing effect and saturable absorption (SA) have been observed in chloroaluminum phthalocyanine (ClAlPc) dissolved in ethanol using Z-scan technique with a 632.8 nm He-Ne laser. In this thesis, we particularly verify if mass transport of the solute molecules contributes to SA. To fulfill this goal, we use, as the light source, a 10 mW He-Ne laser sliced into pulses of different durations ranging from 0.3 to 3.5s and of different pulse-to-pulse separations with respect to the mass diffusion time. Also, we prepare the sample to have concentrations of and M.
Based on the five energy band model, SA may result from cascading ground state and first excited state absorptions given the absorption cross section of the first excited state smaller than that of the ground state for ClAlPc/ethanol at 632.8 nm. Also, it may arise from solute migration from lateral center of the laser beam to its peripheral.
Since cascading one photon absorptions rise and end well within an individual pulse, SA induced by it is an individual pulse effect, thus being irrelevant to the pulse-to-pulse separations. On the other hand, mass diffusion time is comparable with the pulse-to-pulse separations, SA induced by it accumulates across pulses and thus varies with the pulse-to-pulse separations.
Keeping the pulse duration approximately equal to the mass diffusion time but changing the pulse-to-pulse separations with respect to the mass diffusion time, we recognized, via Z-scan measurements, that SA becomes more sever with the pulse-to-pulse separations. We therefore conclude SA in ClAlPC/ ethanol may result from outward migration of the solute molecules.
In the future, we will further verify this solute migration by measuring the negative lensing effect induced by it.
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