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

我们研究了由聚焦的近红外激光束引起的光子压力引起的一种新现象：单个μm大小的粒子是由捕获纳米胶体粒子如聚合物、超微粒和胶束形成的。这种激光操作可以扩展到在一种新的光子压强微扰下形成分子组装结构。通过用激光捕获光谱方法研究各种胶体粒子，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斥力。确定了聚焦激光能捕获的胶体的最小尺寸为12 nm，而分子的偏振性是一个重要的参数。化学和光学机制可以很好地解释纳米胶体粒子的聚集和随后形成μm大小的粒子。形成的微粒形状像橄榄球，其周围的温度梯度是通过水分子中有效的光热转换而产生的。由于被捕获的微粒在关闭聚焦激光后消失，因此我们提出了一种利用纳秒紫外光诱导的瞬时熔化将其固定在聚合物基片上的新方法。

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.