The present work deals with the theoretical and experimental study of the radiation pressure exerted on a homogeneous sphere situated in the passage of the Hermite-Gaussian mode laser beams.
The formulae for the three vector components of the radiation pressure of a TEMoo mode laser beam exerted on a homogeneous sphere are derived by solving the associated scattering problem both in scalar and in vector formalisms, and their analytic properties are examined. Whereas the radiation pressure formulae derived from the scalar scattering theory exhibit complete rotation symmetry about the beam axis, those derived from the vector theory shows reflection symmetry with respect to the beam axis. In the limit of very large beam radius all the scattering formulae reduce to those of a plane wave scattering theory. Also the formulae are generalized for the case of TEMμν complex Idermite-Gaussian mode laser beam scattering by a homogeneous sphere.
Based on the radiation pressure formulae derived in vector scattering theory of a TEMoo mode laser beam by a homogeneous sphere, numerical calculation is made. It is shown that for a sphere with relative refractive index greater that one the direction of the radiation pressure is to attract the sphere toward the beam axis and push it in the direction of beam propagation. On the other hand a sphere with relative refractive index less than one is pushed away from the laser beam. The distribution of the radiation pressure is asymmetric about the beam focal plane, and this can be attributed to the effect of wavefront curvature. When the relative size of the sphere to the beam radius increases, the distribution of the axial component of the radiation pressure shows a dip on the beam axis, and this is explained by the competitive process between the spherical particle size effect and that of the Gaussian amplitude distribution of the laser beam to the scattering efficiency.
Experiments are conducted to accelerate, trap, and levitate microscopically small dielectric spheres suspended in water by the radiation pressure of focused TEMoo mode laser beams. By using a single focused laser beam the spheres, whose refractive index (1.59) is greater than that of water (1.33), are attracted into the laser beam and then accelerated in the direction of the beam propagation. The spheres are trapped and accumulated on the wall of the glass container by the radiation pressure of the laser beam whose focal plane is coincident with the wall. A three dimensional stable optical potential well is established in water by the two colliding laser beams, and particles are trapped stably in the well in the presence of significant external disturbance due to water convection with velocity as large as 100 ㎛/sec.
The radiation pressure formulae derived are in good agreement with the experimental results and predict several new features which are believed to be worth of further study. The formulae can be used to design and analyze the radiation presure experiments. They can be also applied to the radiometric force calculation which is important in radiometric levitation of absorbing spheres.
