Optical Trapping and Binding of Colloidal Microparticles in Evanescent Waves

Optical trapping of colloidal microparticle arrays in evanescent fields is a relatively new area of study. Optically driven array formation is a complex and fascinating area of study, since light mediated interactions have been shown to cause significantly different behaviour for multiple particles when compared with the behaviour of a single particle in an optical field. Array formation was studied with interference fringes in the counterpropagating evanescent fields so as to investigate the effect of a periodic trapping potential. A subtle balance between optical trapping and optical binding forces is shown to produce modulated lines and arrays. Optically trapped colloidal arrays were also studied in the absence of interference fringes, by using either orthogonally polarised laser beams or a beam delay line. When interference fringes were absent, the formation of arrays was mainly due to gradient forces and optical binding. The experimental studies presented here include the optical trapping of dielectric soft and hard spheres, Au colloids, and Janus particles.