Molecular association in solution controlled by photon pressure of a focused laser beam

研究了近红外激光束聚焦后光子压力引起的一种新现象；通过捕获聚合物、超微粒子、胶束等纳米级胶体粒子，形成单μm大小的粒子。这种激光操作可以扩展到在新的光子压力扰动下形成分子组装结构。用激光捕获光谱方法研究了各种胶体粒子，得到了以下有趣的结果。聚焦光束的光子压力克服了带电聚合物胶束之间的静电排斥。确定了聚焦激光束能捕获的胶体的最小尺寸为12 nm，而分子极化率是一个可靠的参数。化学和光学机制可以很好地解释纳米胶体颗粒的聚集和μm大小的颗粒的形成。形成的微粒形状像一个橄榄球，周围的温度梯度是由水分子的有效光热转化产生的。当聚焦激光束关闭时，捕获的微粒消失，因此我们提出了一种利用纳秒紫外激光诱导瞬态熔化将其固定在聚合物衬底上的新方法。

We have studied a new phenomenon induced by photon pressure due to a focused near infrared laser beam; a single μm-sized particle is formed by trapping nm colloid particles such as polymer, ultramicroparticle, and micelles. This laser manipulation can be extended to form molecular assembly structure under a novel perturbation of photon pressure. By investigating various colloid particles with laser trapping-spectroscopy methods, the following interesting results have been obtained. Photon pressure of a focused beam overcomes electrostatical repulsion between charged polymer micelles. The minimum size of colloid which can be trapped by the focused laser beam is determined to be 12 nm, while molecular polarizability was demonstrated to be a responsible parameter. Chemical and optical mechanism can explain well gathering of nm colloid particles and the following formation of a μm-sized particle. The formed microparticle has a shape like a rugby ball, around which temperature gradient is produced by efficient photothermal conversion in water molecules. The trapped microparticles disappear when the focused laser beam is switched off, so that we have proposed a new method to fix it on polymer substrate by nanosecond UV-laser induced transient melting.