Direct measurement of asymmetric dynamic motion and cross-correlation of a Rayleigh particle in an optical trap

Summary form only given. An optical tweezers is a 3-D trap that has been used to study the dynamic motion of dielectric particles or biological cells. In a colloidal solution a particle can be trapped at the focus of the beam by the gradient force while the particle is also subject to the scattering force. It is usually believed that for an incident beam propagating in the z direction with a Gaussian intensity profile (TEM/sub 00/ mode) the gradient force is symmetric in x and y directions. The symmetric trapping force applies to both Rayleigh particles and Mie particles. However, if the particle size lies in 0.1 to 5/spl lambda/, the trapping force exerted on the particle by the incident polarized laser cannot be explained by either Rayleigh theory or Mie theory. Alternately, electromagnetic theory is applied to calculate the trapping force that was predicted to be asymmetric in x and y directions. we report on our findings that show a polarization-dependent asymmetric trapping force exerted by a linearly polarized Gaussian beam on a suspended particle whose diameter is smaller than /spl lambda//2. We directly measured the Brownian displacements of the trapped particle in x and y directions so that: (1) the mass center of the particle can be mapped on the transverse plane and (2) the cross-correlation between x displacement and y displacement can be simulated. We found that the random movement of the trapped particle was asymmetric in x and y directions and became correlated as the laser polarization was changed.