Opt. Lett. 44, 2430–2433 (2019)

Super oscillation (SO) beams, which consist of a subwavelength central spot surrounded by a side ring, can be used to implement optical trapping below the diffraction limit. Harel Nagar and co-workers from Israel systematically investigated the effect of particle size and beam waist on the stiffness of an SO-beam-based optical trap. The SO beam was generated by sending a 1,083-nm laser beam to a spatial light modulator, and then focusing it onto a polystyrene bead against a cover glass. Four different bead diameters were prepared from 490 nm to 1,370 nm. The beam waist of the central spot was controlled by changing the radius of a mask aperture at the pupil plane. The bead position was recorded using video microscopy. The trapping quality was characterized by calculating the trap stiffness as a function of the ratio between the bead diameter and the width of the beam waist of the central spot in the range from 0.67 to 2.4. The scientists found that interference from the side ring caused instability in trapping for the larger beads or the narrower beam waist. Consequently, the maximum stiffness was obtained when the ratio was 1.5.