Optimal in-depth trapping by tuning the correction collar of a dry objective lens

Optical tweezers are proven and indispensable micro-manipulation tools. It is very common to use an immersion-assisted high NA objective for optical trapping of micrometer-sized beads. However, such objectives suffer from low working depth range. Here we show, both by theory and experiment, that a dry objective can be utilized for optimal trapping of even sub-micrometer objects. For the first time, to the best of our knowledge, we were able to stably trap polystyrene beads with radii of 270 and 175 nm in 3D using an objective with numerical aperture of 0.9.