When one thinks of moving microscopic objects, the most immediate thought is to use a mechanical tweezer or probe. The radiation pressure from a focused laser beam is strong enough to trap small particles. In the biological sciences, these instruments have been used to apply forces in the pico-Newton range and to measure displacements in the nm range of objects ranging in size from 10 nm to over 100 mm. Nobel prizes in physics have been given out those who pioneered the use of this phenomenon. The challenge remains in how to harness and adapt this innovation for various applications.

• Push-pulling using focus positioning (NEW)

  

  The ability to hold single matter in place using a laser tweezer or trap is essentially gained through the gradient force component of the beam which is strongest at the waist. That this is also the location of highest intensity of the beam presents a problem in manipulating cells, where there have been reports of photodamage. We advance a scheme here that locates the focus of the beam either above or below the matter to pull and push it relative to the beam axis in a predominant lateral sense based on the resultant action of scattering and gradient forces. Switching to a laser that acts oppositely serves to restore the axial position of the matter. Although an exact value could not be derived due to the statistical nature of Brownian perturbations and time frame considerations, we simulated the optical force fields to visualize the effective force envelope. The lateral optical push-pull operation was conducted experimentally on polystyrene beads in which the motion manipulation efficacy was characterized.Watch video.

  M. Muradoglu, W.S.-Y. Chiu, T.W. Ng Optical force lateral push-pulling using focus positioning. Journal of the Optical Society of America B. Accepted 24/12/2011.