Experimental demonstration of optical transport, sorting and self-arrangement using a 'tractor beam'

Following the Keplerian idea of optical forces, one would intuitively expect that an object illuminated by sunlight radiation or a laser beam will be accelerated along the direction of photon flow. Recent theoretical studies(1-5) have shown that small particles can be pulled by light beams against the photon stream, even in beams with uniform optical intensity along the propagation axis. Here, we present a geometry to generate such a 'tractor beam', and experimentally demonstrate its functionality using spherical microparticles of various sizes, as well as its enhancement with optically self-arranged structures of microparticles. In addition to the pulling of the particles, we also demonstrate that their two-dimensional motion and one-dimensional sorting may be controlled conveniently by rotation of the polarization of the linearly polarized incident beam. The relative simplicity of this geometry could serve to encourage its widespread application, and ongoing investigations will broaden the understanding of the light-matter interaction through studies combining more interacting micro-objects with various properties.