Study of the negative lensing effect in organic solutions induced by a CW He-Ne laser
Yu-Ting Kuo1*, Yuu-Sheng Tang1, Yi-Ci Li1, Po-Yuan Huang1, Tai-Huei Wei1
1Department of Physics, National Chung Cheng University, Min-Hsiung, Chia-Yi, Taiwan
* presenting author:郭育挺,
By measuring the total and axial transmittances of an 1 mm thick ethanolic solution of chloroaluminum phthalocyanine (dubbed as ClAlPc-ethanol) at two different concentrations (7Χ10-4 M and 2Χ10-4 M) with a 632.8 nm CW He-Ne laser, we study how convection, thermal and mass diffusions as well as thermal lensing effect contribute to nonlinear absorption and refraction. To minimize the influence of convection, we made the laser beam propagate downward perpendicularly to the optical table.
From the measured total transmittance as a function of time relative to the shutter opening time (denoted by T), we explain how the increase of total transmittance with T in terms of the electronic transition and the subsequent thermal and mass diffusions of the solute molecules. On the other hand, via the measured axial transmittance as a function of T, we interpret how the observed thermal lensing effect strengthens with T within τth (characteristic time for thermal diffusivity) and then turns steady after T exceeds τth in terms of electronic transition, solution convection and thermal and mass diffusions of the solvent and solute.
By theoretical interpretation of the experimental results based on the laws of conservation of mass, momentum and energy in fluids, we learn how the convection of the solution as well as thermal and mass diffusion of both the solute and solvent are induced by the temperature gradient induced in the absorptive solution.

Keywords: chloroaluminum phthalocyanine, Z-scan, CW laser pulses, convection, mass diffusion