Self-organized array of regularly spaced microbeads in a fiber-optical trap

The behavior of several simultaneously trapped, micrometer-sized particles in a fiber-optical trap consisting of two opposing single-mode fibers delivering counterpropagating, near-IR laser beams strongly depends on the size of the particles. Whereas beads that are considerably larger than the laser wavelength are pressed against each other in an axial line, smaller beads spontaneously arrange themselves into regular chains of equidistantly separated particles suspended in space with increasing separation for increasing bead diameter. A simple model based on self-organization by means of diffraction from the particles is capable of explaining the basic features of our experimental observations in the investigated range of bead diameters and refractive indices.