Dynamics and light-scattering properties of Brownian particles in suspensions under laser radiation pressure

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Dynamics and light-scattering properties of Brownian particles in colloidal suspensions under the influence of the radiation pressure force of an illuminating laser beam are investigated theoretically and experimentally.Spatial and temporal properties of coherent light multiply scattered backward by the highly concentrated system of particles with aggregations have also been investigated theoretically and experimentally. The main results are summarized in the following:1. Dynamic behavior of the Brownian particles: Suppression of the Brownian motion of the particles in radial direction of the laser beam and induction of translational motions in the beam propagation direction are found to have dependencies on power and beam-waist radius of the laser and particle size. It is clarified from computer simulations using the Fokker-Planck equation and the generalized Lorenz-Mie theory that the changes in the dynamics of the particles in the radial and beam propagation directions are due to a … More gradient force and scattering force components of the radiation pressure of the laser beam, respectively. It is also found that, if the particle radius is larger than the beam-waist radius, velocity of the translational motion of the particle in the beam propagation direction has two maxima along the beam propagation direction and their positions are directly related to the particle size.2. Statistical properties of the scattered light: As a result of the changes of dynamic behavior of the particles, a temporal correlation function of intensity fluctuations of the scattered light deviates from theoretical one. Degree of the deviations in the temporal correlation function is strongly dependent on the the size of the particles and the power and beam-waist radius of the laser beam. It is demonstrated that a peak intensity distribution and a temporal correlation time of the light multiply scattered backward change suddenly due to aggregation of particles in suspensions. These changes in the peak intensity distribution and the correlation time are closely related to spatial properties of the particles in both microscopic and macroscopic scales. Less