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

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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.