Trapping Microscopic Particles in an Optical Vortex

Alex Kelser, Martin G. Cohen and John Noé

Laser Teaching Center
Department of Physics and Astronomy
Stony Brook University

Optical tweezers utilize the momentum of light to trap small dielectric particles. In a tweezers setup, light from a laser beam is strongly focused by a microscope objective onto a target particle. Light striking the particle at an angle is refracted and experiences a change in momentum. The opposite momentum change is imparted to the particle, resulting in a net force towards the region of highest intensity in the beam. A relatively new development in tweezer technology is the use of an optical vortex beam to exert a torque on trapped particles, thereby causing them to rotate. An optical vortex beam has a phase factor of e^iℓφ, where ℓ is the topological charge, and φ is the polar angle around the axis of the beam. This phase distribution gives rise to helical wavefronts and a phase singularity in the center of the beam. Since the Poynting vector, and hence linear momentum flow, must be perpendicular to the wavefront of a beam, optical vortices also have an azimuthal component of linear momentum. This property endows the beam with a form of angular momentum, termed orbital angular momentum.

Our experiments used a re-constructed inverted optical tweezers apparatus with a 40 mW 632.8 nm helium-neon laser focused by a 50x, 0.85 NA objective. Optical vortex beams were created by inserting a spiral phase plate into the beam path; it can efficiently create vortex beams of order ℓ = 1 – 8. Trapped particles were contained in uncovered water droplets that rested on No. 0 cover slips. In the initial set of experiments we trapped 5 and 10 micron polystyrene spheres and yeast cells. The particles could be manipulated in both the transverse and axial directions, attesting to trap stability. We subsequently experiemented with trapping copper (II) oxide particles, which have irregular shapes and a range of sizes, in an order ℓ = 2 vortex beam. Multiple small (few microns) particles could be trapped in a ring, but they did not circulate around the ring. Some particles did appear to spin but further investigation showed that this effect was not related to the angular momentum of the light. Recently we observed that trapped copper oxide particles are occasionally strongly repulsed in the transverse direction. We are currently investigating this unexpected phenomenon.

This work was supported by the Simons Summer Research Program and the Laser Teaching Center. We thank RPC Photonics for providing the spiral phase plate.