The far-field pattern of the focused laser beam scattered by a dielectric particle is investigated experimentally and theoretically. The analytical formulae for the intensity distribution of the far-field pattern are derived from the scattering theory of a Hermit Gaussian mode laser beam. A new method of trapping a particle near the water surface is developed. It employs the balance between the radiation pressure of a focused laser beam and the hydrodynamic pressure upon the particle. A particle can be trapped in a potential well formed near the balanced position stably for about twenty minutes.
The forward and right angled far-field pattern of focused laser beam scattered by a trapped particle is measured experimentally. For comparing and analyzing the results, the numerical calculations for the derived formulae are carried out. Circular forward pattern is formed as the consequence of the interference between the incident laser beam and the scattered light in the region defined by the incident laser beam divergence. Also we have found that the spacings of rings in the forward field pattern depend on the position of a particle on the beam axis of the focused laser beam.
In the right angle scattering experiment, a scattering field pattern composed of the dark and bright strips are observed. The numerically calculated results are in good agreement with the experimental results.
The effect of the optical property and geometrical size of the particle on the scattering pattern is investigated by the numerical calculation of the formula. Variation of the size affects dominantly the fringe spacing of the field pattern in the right angle scattering. The change of the refractive index affects the intensity distribution of the scattering pattern dominantly. From these analysis, particle size($n_1 = 1.5-1.8$) can be determined by measuring the fringe spacing of the right angle scattering pattern.
Laser particle trapping and focused laser beam scattering from trapped particle require further investigation as the have many potential applications. The extension of the present experiment to back scattering is necessary, and the detailed experiment of particles with known controlled velocity is also required.
