結合連續波與飛秒脈衝雷射在時間與空間上控制甘胺酸單一結晶的形成

Abstract

光線照射誘導結晶，作為新的現象和潛在的控制單晶形成方法，吸引了研究者的高度重視。文獻指出增幅脈衝雷射在過飽和溶液中誘發水分解，而產生冒泡導致結晶；和連續波雷射捕陷於飽和溶液中，誘發局部高濃度並於空間上控制結晶化。近來的飛秒脈衝雷射捕陷分子研究，促使我們使用飛秒雷射來改善雷射捕陷結晶化技術。相較連續波雷射，飛秒脈衝雷射在單一脈衝的時間內有更高的光子數，因此我們可以期待飛秒雷射可以帶來更好的結晶效率。這裡我們第一個示範以低能量飛秒脈衝雷射執行雷射捕陷結晶化，並且過程中沒有觀察到冒泡現象。 甘胺酸重水溶液(2.4~2.6 M，飽和度：0.9~1.0)滴入一個密封的玻璃容器並形成一個薄的液層(~120 □m)。Ti:sapphire雷射(波長=800 nm)可以控制輸出為脈衝(80 MHz, 150 fs)及連續波模式，此雷射光經由物鏡(60X, NA 0.90)聚焦到空氣與溶液的介面。 我們到在飛秒雷射的焦點上觀察到結晶的產生，結晶過程中並沒有觀察到冒泡及其他非線性光學現象，得知雷射誘發結晶的低限能低於冒泡的低限能。相較於同樣波長的連續波雷射捕陷結晶化技術，飛秒脈衝雷射誘發結晶是更有效率的，我們認為飛秒雷射以更強的光壓收集分子，在焦點形成局部高濃度，並誘導分子叢的變動及從新排列，進一步使結晶化產生。 然而，飛秒雷射能量太強，熔蝕了結晶，導致多核結晶的形成，因此我們必須改善結晶的方法使飛秒雷射應用於單一結晶的製備。我們驗證了只使用飛秒雷射的條件，或結合連續波與飛秒脈衝雷射，並在這些條件下成功的控制單一結晶的形成。最重要的方法是結合短時間的飛秒雷射照射，及長時間的連續波雷射照射，形成單一結晶，我們可以使用這個方法達成高度的時間及空間上的控制能力。

Light irradiation-induced crystallization is attracting much attention as novel phenomena and as potential methods to control single crystal formation. It has been reported that focusing of an amplified fs pulse (microJ ~ mJ/pulse) into a supersaturated solution induces the bubbling due to fs laser-induced break-down of water and eventual crystallization and CW laser trapping with saturated solution induces local concentration increase and spatially controlled crystallization. Recent report on fs laser trapping of particles stimulating us to use fs pulses to improve laser trapping crystallization technique. We can expect higher crystallization efficiency with higher photon density of fs laser in a short time compare to CW laser. Thus here we report first demonstration of low energy fs pulse utilized laser trapping crystallization of glycine without conventional bubbling. Glycine/D2O solution (2.4~2.6 M, supersaturation degree: 0.9~1.0) was prepared and a portion of the solution was dropped in a sealed glass sample chamber to form a thin liquid layer (~120 micron). An output from Ti:sapphire laser (800 nm), which can be operated with pulse (80 MHz, 150 fs) and CW modes, was focused to the air/solution interface through an objective lens (60X, NA 0.90). We observed crystal generation from a focal spot of fs laser. Laser fluence threshold of crystallization is below than that of the bubbling. No bubbling and no other apparent nonlinear behavior was observed during crystallization. Crystallization was effectively induced compared to 800 nm CW laser utilized trapping crystallization. We consider that repetitively exerted photon pressure induced by irradiated fs laser pulse collects molecules, forms high concentration area locally around the laser spot, and induces fluctuation and re-orientation of the clusters, leading to crystallization. However frequently observed fs laser ablation on generated crystal resulted in polycrystal formation. Thus we need to improve the crystallization method to make the application of femtosecond laser in single crystal formation. We have examined a single crystal generation which was induced by fs only or a fs/CW combination. Finally we succeeded to make single crystal by a combination of fs and CW laser trapping. The most important achievement in this study is the success of single crystal generation by combining short fs irradiation and CW laser irradiation. By this method, we can generate one single crystal with high spatial and temporal controllability.