Single-beam trapping using laser beams focused by low and high numerical apertures: angular spectrum approach

摘要

Single beam optical tweezers hold particles behind the focal plane due to the highly focused laser beam. However, even moderately focused beams may stably confine particles of high refractive index contrast and sizes up to half wavelength. This is caused by the fact that the light structure near focus is complex even for such moderately focused beams. The intensity profile consists of several axial oscillations as well as off-axis lobes or rings which enhance trapping. Moreover, side lobes provide another multiple trapping sites for small sized particles. The properties of these traps will be analyzed theoretically by means of Generalized Lorenz-Mie Scattering theory.