A focal spot with variable intensity distribution for optical tweezers

An optical setup that can generate a focused light spot, with its shape tunable from a very sharp peak-centered beam to a donut-shaped dark-centered beam, is always desired in optical tweezing. This setup is now realized by incorporating an image inverting interferometer (III) and a helical phase element into a radially polarized beam generator, which enables the trapping and control of a large variety of particles with a single optical tweezing system. The performance of this system is investigated theoretically. (letter)