Analysis of radiation forces in laser trapping and laser-guided direct writing applications

Radiation forces allow the remote manipulation of physical objects. Two radiation force-based systems have been used, in particular: 1) laser trapping, which uses a strongly convergent beam to form a micrometer-sized focal point in which particles can be trapped and manipulated in three dimensions, and 2) laser guidance, which uses a weakly convergent beam to achieve radial confinement of particles about the beam axis coupled with pushing along the beam axis, which allows the high precision delivery of particles over hundreds of micrometers. Biological applications of laser trapping include high-precision molecular motor force measurement, and those of laser guidance include the direct writing of living cells in two and three dimensions for tissue engineering applications. The results presented here show that a general electromagnetic theory, the Generalized Lorenz-Mie Theory, is able to accurately predict experimental results for both schemes without any assumptions regarding the size of the particle relative to the wavelength of the radiation. In addition, radial forces are found to be directly correlated to the dimensionless particle size (a/w) where the particle radius (a) is normalized over the beam radius (w). The dimensionless particle size can be used to theoretically estimate radiation forces in any arbitrary setup, and facilitate the design of radiation force-based systems for a variety of applications in biology and medicine.