Optical trapping in secondary maxima of focused laser beam

- Secondary trapping sites in optical tweezers are revealed while using vectorial diffraction theory.
- Secondary trapping sites exist both on the optical axis and the off-axis.
- Trap stiffnesses in such locations are calculated and compared to trapping site behind beam focus.

Single beam optical tweezers hold particles behind the focal plane due to the high gradients of optical intensity present in a focused laser beam. However, description of this optical field based on a vectorial theory of diffraction reveals that the high intensity focal area is accompanied by several secondary maxima on the optical axis as well as by a structure of rings away of the optical axis. Such a structure can be found in beams exhibiting spherical aberrations as well as in beams where aberration is corrected. Here, we discuss possibility to use these secondary maxima of aberration-corrected beams as the optical traps. We present the properties of such traps created by objective lenses of various numerical apertures that are focusing plane waves.