Manipulation of microparticles with integrated optics

In a dielectric waveguide, the optical power is confined mostly in the core of the waveguide, where the refractive index is highest. Outside of the core the field is evanescent, i.e., the field strength decreases exponentially with the distance from the core. This evanescent field can be used to manipulate microparticles. For a particle with index of refraction higher than that of the surrounding medium (water), the optical forces due to the evanescent field act to guide the particle along the waveguide. The use of waveguides to trap particles combines the possibilities of conventional optical tweezers with the techniques employed in integrated optics, and it has the added advantage of integration of several functions on a single chip. We have experimentally observed size-dependent trapping and propulsion at velocities up to 33µm/s of polystyrene spheres, of diameters between 3 and 12µm, and in propulsion of 0.25µm diameter gold spheres at velocities up to 500µm/s. A Y-junction with a multimode input waveguide has been used to sort particles. By moving the input fibre relative to the input waveguide, the light goes into one of the two output branches. We have shown that this principle can be used to sort polystyrene microbeads. Recently we have used counter-propagating waves to move particles in both directions and also to stop a particle at a precise location. Experimental results and simulations for polystyrene microbeads, yeast cells and gold particles are presented