| Summary: | 碩士 === 國立交通大學 === 應用化學系分子科學碩博士班 === 106 === Laser trapping can be utilized to accumulate nm-sized matter leading to its structural packing and even crystallization. Also, irradiation of intense laser in solution could generate bubble at the same time. In our experiments, both of them were observed simultaneously or separately. In the following study, femtosecond (Fs) laser trapping and continuous-wave (CW) laser trapping accompanied with bubble generation were examined.
In Fs laser trapping of 50 nm-sized polystyrene (PS) nanoparticles (NPs), polarization dependent ejection of trapped particles in solution was reported previously. Three processes (gathering, assembling and ejection) were proposed so as to explain this phenomenon. In this study, laser trapping of 50 nm-sized silica NPs in different viscosity solution was conducted as a starting model to understand the ejection phenomenon. Ethylene glycol was added to increase the viscosity of solution, and trapping behavior was monitored by backscattering light of additional laser (488 nm in wavelength). As a result, within viscous medium, Fs laser trapping becomes less efficient than CW laser trapping. This may be ascribed to larger friction encountered by NPs within duration of 100 fs for every pulse. Following the gathering of NPs, the ejection was also observed and its frequency was higher with increase in the viscosity and the power of Fs laser. In spite of less trapped NPs compared to CW laser trapping, high viscous surrounding enhanced assembling of NPs leading to ejection in high power of Fs laser. Interestingly, ejection and bubbling were observed during Fs laser trapping of 50 nm-sized PS NPs at solution surface. Through dual objective lens microscope system, polarization dependent directional bubbling was also verified in solution. The dynamics and mechanism of these novel phenomena are explained based on anisotropic assembly perpendicular to linear polarization formed by Fs laser trapping.
Bubble generation outside of the laser focus was observed during CW laser irradiation at solution surface. For pure water, multiple bubbles were generated, and their size and locations were controllable by laser power. They were stable during laser irradiation even over 1000 sec. Though, they disappeared when we turn off the laser and its lifetime was estimated 48 msec measured by EMCCD. It was reported that laser heating was induced by absorption of the 1064 nm by OH vibrational overtone. Therefore, through changing the D2O/H2O ratio, it was confirmed that temperature elevation was one of the main reason for bubble generation. For L-phenylalanine (L-Phe) solution, the generated bubbles moved out during laser irradiation and finally crystallization took place. We explained this dynamics of bubbles through the formation of dense domain of liquid-like clusters which affects the temperature, surface tension and viscosity of solution. Thus, the moving out of bubbles may be an indicator for the dense domain of liquid-like clusters.
Laser induced crystallization of L-Phe at solution surface was successfully demonstrated in previous work, and a highly concentrated domain prepared by laser trapping was proposed. Nevertheless, only 2-D crystal growth was discussed upon this proposed mechanism. Through in-situ reflection spectroscopy and theoretical calculation, we obtained the crystal thickness and its increasing rate during backscattering imaging. Therefore, the discussion about the crystal growth perpendicular to the solution surface is feasible. The results showed that longer crystallization time did not contribute to faster increasing rate of crystal thickness. This is probably due to its strong laser heating effect neat the laser focus. In 0.06 W, dissolution of crystal was significant near the its edge not its central part. This may indicate diffusion of trapped liquid-like clusters was efficient at the place far from the laser focus. Also, controllability of increasing rate of crystal thickness by laser power is dependent on its initial thickness, which may give the information of depth of the liquid-like cluster domain.
In summary, combining both laser trapping and laser induced bubble generation may open a new way to have manipulation of small object in nm scale.
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