Experimental and theoretical investigations on the validity of the geometrical optics model for calculating the stability of optical traps

Abstract

We have developed a computer program based on the geometrical optics approach proposed by Roosen to calculate the forces on dielectric spheres in focused laser beams. We have explicitly taken into account the polarization of the laser light and thd divergence of the laser beam. The model can be used to evaluate the stability of optical traps in a variety of different optical configurations.Our calculations explain the experimental observation by Ashkin that a stable single-beam optical trap, without the help of the gravitation force, can be obtained with a strongly divergent laser beam. Our calculations also predict a different trap stability in the directions orthogonal and parallel to the polarization direction of the incident light.Different experimental methods were used to test the predictions of the model for the gravity trap. A new method for measuring the radiation force along the beam axis in both the stable and instable regions is presented. Measurements of the radiation force on polystyrene spheres with diameters of 7.5 and 32 pm in a TEM,,-mode laser beam showed a good qualitative correlation with the predictions and a slight quantitative difference.The validity of the geometrical approximations involved in the model will be discussed for spheres of different sizes and refractive indices.

Radiation forces exerted on arbitrarily located sphere by acoustic tweezer

“…In this paper, the former is determined by the radius of the curvature of the incident rays, which differs from a previous approach. 11 In other words, the conventional method regards simply as varied from 0 to / 2 and approximates the case where the beam uniformly illuminates the sphere. But, this is not true for the tightly focused beam and we needed to consider both min and max at the same time.…”

One-Dimensional Optically Bound Arrays of Microscopic Particles

“…Theoretically, we consider the interaction of dielectric spheres of radius a with a Gaussian beam of wavelength when the relation a is satisfied. Our calculations of the axial force follow those elsewhere [15][16][17] with the sphere radius VOLUME 89, NUMBER 28…”

Calculation of radiation forces exerted on a uniaxial anisotropic sphere by an off-axis incident Gaussian beam

“…To verify the correctness of our method, the axial radiation force exerted on a uniaxial anisotropic sphere reduced to an isotropic sphere is calculated and compared with the experimental results (the curves are denoted by Static measurement and Dynamic measurement) obtained by Schut et al [5] and ROT results from [5,8,16] shown in Fig. 2.…”

“…Conversely, for a particle much larger than the incident wavelength ( / 10 d λ > ), many scholars employed the ray optics theory (ROT) as it is most applicable to radiation force calculations. In 1991, Bakker Schut et al [5] investigated the validity of ROT for calculating the stability of optical traps, making comparisons with experimental results on axial radiation forces. In 1995, Gauthier et al [6] demonstrated that the axial and radial forces applied to micrometer-sized spheres could be obtained from ROT.…”