Objectives
The purpose of this study is to investigate the fundamental characteristics of particle behavior under laser radiation pressure from the view point of particle separation. The laser radiation pressure exerted on a small sphere and its behavior in the laser beam were calculated using a geometrical optics model. In order to confirm the results of the calculation, the behavior of small spherical particles was observed. Three kinds of suspensions, that mono-sized polymer micro-spheres were dispersed in water, benzene, and 1-bromonaphthalene, were used as the sample liquid. The laser beam (488nm wavelength,TEM00 mode) was focused in a glass cell filled with the sample liquid.
Results
The particle trajectory in water under laser radiation pressure could be calculated using the 2 dimensional equations of motion by considering radiation pressure, viscous drag, gravity and buoyancy as the forces acting on a particle (see Fig. 1) : where the beam focus is located at (z, y)=(0,0). The calculated particle trajectory almost agreed with the observed motion. Fig. 2 shows the calculated trajectories of the particle in three kinds of surrounding mediums. The vector of laser radiation pressure exerted on the particle strongly depended on the relative refractive index between the particle and medium(n), even if the particle size and optical system were constant. The possibility of particle separation based on the refractive index was suggested since the effect of laser radiation pressure was sufficiently large to characterize the particle motion. In water (n=1.20) and benzene (n=1.06), the particle is accelerated in the incidental direction of the laser beam, and it can pass through the focus. However, in 1-bromo-naphthalene (n=0.96), the particle can not pass it because it is immediately pushed out from the beam's inside.
Symbols, Dp : particle diameter, P: laser beam intensity, v :
particle velocity, u : medium flow velocity
