Optical Tractor Beam: Force on a Small Microbead

Michael Wolff


Through the electromagnetic fields of light extending outside its boundaries, an optical nanofiber may manipulate microscopic objects. While previous work has involved the HE+11 field mode, we used boundary conditions, circularly polarized unit vectors, and Maxwell's equations to derive general formulas for six guided electromagnetic field modes in a "few mode" optical nanofiber. Plotting these modes, we found they matched their expected forms. Using these field equations, we calculated the general Poynting vector (energy flow) formula for each mode, finding that it had no radial component. Finally, approximating a dielectric nanobead as a dipolar particle, we used the electric fields and Poynting vectors to calculate the force on a bead near the fiber. We found the magnitude of the forces to be on the order of 10−30 N for a 1 mW laser. We have successfully expanded the concept of the "Optical Tractor Beam" to a new set of modes.