Study of CW Wave Induced Thermal Diffusion in Organic Solutions

• Main Authors: CHIH-CHIEH CHEN, 陳志杰
• Other Authors: Tai-Huei Wei
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/21670582066306024795

碩士 === 國立中正大學 === 物理學系暨研究所 === 101 === In the past few years, we have investigated, using 632.8 nm CW He-Ne laser light, change of transmittance (transmitted power divided by the incident power) with time induced by electronic transition, thermal diffusion and convection in an ethanolic solution of chloroaluminum phthalocyanine, shortened as ClAlPc-ethanol. In the experiments, the He-Ne laser light was normally incident on the sample with the entrance surface normal and parallel to the optical surface respectively. We refer these two experimental geometries to the vertical and horizontal ones respectively. In both experimental geometries, we focused the laser beam to the waist of radius of 20 m half-width at e2 maximum (HWe2M). In the transmittance measurement conducted in the vertical experimental geometry, we fixed the sample at the beam waist. As a result, we found that the transmittance monotonously increased with time. In the first 80 μs after light exposure, the transmittance increased rapidly. Afterwards, it kept increasing at a rate turning lower and eventually stopping in a few seconds. Through quantitative theoretical analysis, we came up with the conclusion that this rapid transmittance increase results from electronic transition, and the slow increase arises from thermal diffusion alone. Since the sample absorption is weakened with time and hence the generalized fluence, defined as time integral of the intensity, absorption of the sample is categorized into saturable absorption (SA) rendered by two mechanisms. From the transmittance measurement conducted in the horizontal geometry, we found the transmittance varied with time in like fashion in the first 300 ms; afterwards, it gradually decreased with time, converting SA of the sample to reverse saturable absorption (RSA). Via qualitative theoretical analysis, we concluded that convection came into play and caused the SARSA transition. In this thesis, we conducted Z-scan and transmittance measurements at each sample position relative to the beam waist on the same samples in the same geometries. Combining the results of Z-scan and transmittance measurements, we looked forward to obtaining more information of electronic transition, thermal diffusion and convection than before. Moreover, we applied a temperature gradient in the sample, either parallel or antiparallel to light propagation directions, to explore the influence of convection on thermal diffusion.