Optical trapping core formation and general trapping mechanism in single-beam optical tweezers

The working mechanism of single-beam optical tweezers is revisited using a recently established method. The optical force is split into conservative and nonconservative components, and these components are explicitly calculated for particles in the Rayleigh, Mie and geometrical optics regimes. The results indicate that optical trapping is attributable to the formation of an 'optical trapping core'. Stable trapping is achieved when the conservative forces are larger than the nonconservative forces in the core region centered at the beam centers for all particle sizes. According to the conventional understanding, stability is a result of the conservative force overcoming the nonconservative force. In comparison, the concept of the optical trapping core more accurately illustrates the physical mechanism of optical trapping, for not only single-beam optical tweezers but also optical trapping settings.