Trapping Microscopic Particles with Singular Beams N.R. Heckenberg, TA. Nieminen, M.E.J. Friese, and H. Rubinsztein-Dunlop Centre for Laser Science, Physics Department, The University of Queensland, Brisbane 4072, Australia Abstract It has been shown previously that it is possible to two-dimensionally trap a microscopic absorbing particle against a substrate using a focussed doughnut beam. Beam angular momentum associated with the phase singularity is trans- ferred to the particle, causing it to rotate. A detailed consideration of the optical forces acting on a particle shows the importance ofwavefront curvature for stable trapping and lead to a quantitative description ofthe motion ofthe particle in single and multiple beam traps. 1. Introduction The irradiance available in a focussed laser beam of even modest power is sufficient to exert radiation forces capable ofmoving or trapping sinai! particles. Since the ground breaking work ofAshkin[l] most work has been concentrated on optical forces on transparent particles and the many applications in biology where cells and bacteria and their component parts can be manipulated[2J. Nearly all of this woit has involved normal Gaussian beams but beams with phase singularities can also be used to advantage in some situations and produce some interesting results. In particular, doughnut beams have been used by several groups to trap highly absorbing particles which also take up the angular momentum of the light and are set into rotation. Heie we consider a simple but quantitative model of the process. 2. Optical Trapping The most commonly used configuration for trapping is the so-called single beam gradient trap or optical tweezers. A laser beam is focussed by a high numerical aperture lens such as a microscope objective to produce as large as possible an intensity gradient both radially and axially. Transparent particles with refractive index greater than their surroundings (typically irmnerscd in water) experience a gradientforce towards the focus as well as a scattering force along the beam direction[31 . When the axial component of the gradient force is greater than the scattering force plus the weight, the particle will be levitatedby a downward propagating beam and canbe manipulated in three dimensions. We call this 3-D trapping. Typically, the particle is several times larger than the beam waist and if a singularbeam like a TEM1 doughnut is substituted the particle will again centre itselfon the beam. Although, for the sake of simplicity, most workers use Gaussian beams, it has been recognized for some time[4, 5] that singular beams have some advantages, giving more efficient axial trapping because the dark central spot reduces the scattering forces due to back reflections from the top of the particle. This could be a real advantage in work with live biological specimens. High refractive index particles much smaller than the beam waist will be attracted into the bright ring, while small particles with refractive index lower than their surroundings will suffer a negative gradient force, suggesting that they SPIE Vol. 3487 • 0277-786X/98/$1O.OO 46 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/02/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx