Microsphere-coupled optical tweezers

In this Letter, we study the optical trapping of particles in a focal spot engineered by a combination of a dielectric microsphere and the conventional optical tweezers setup. The dielectric microsphere is placed in the laser path before the focal spot, and its impact on the trapping stiffness is theoretically and experimentally studied in detail. The suggested method for considering the geometric phase shift, due to the presence of the microsphere, into the Debye diffraction integral shows a good agreement with the experiment. We stably trap particles as small as 350 nm in the microsphere-coupled optical trap using a low numerical aperture objective (NA=0.7), while in a conventional optical tweezers setup, it requires at least an NA=1.1. Moreover, the results show that choosing an appropriate microsphere and depth of trapping can enhance the trapping efficiency in the axial direction by a factor of ∼3.8.