Case study of the effect of spherical aberration on optical tweezers with a long-working-distance objective lens

The working range of optical tweezers deep inside a suspension medium is severely limited by the refractive-index-mismatch spherical aberration (RIMSA) due to the presence of media with different indices of refraction, e.g., an immersion medium, a coverslip, and a suspension medium. In this case study, the feasibility of versatile optical tweezers with a moderate NA objective is investigated with a wavefront correction method. The effect of RIMSA on a tightly-focused beam through planar interfaces is described with the angular spectrum representation based on the rigorous vectorial diffraction theory, and the associated effects on optical tweezers are presented by evaluating the light-matter interaction in the Rayleigh region. Finally, I show that a stable optical trap for 70-nm silica spheres 100 μm deep inside a suspension medium can be realized with a 780-nm laser beam of a few tens of mW by using a moderate NA objective lens having a centimeter-long working distance.