Systematic investigation of factors influencing trap stiffness in an optical tweezers setup

Abstract Single-beam gradient traps are produced by focusing light using high numerical aperture (NA) microscope objectives, usually with values between 1.2 and 1.4. Optical tweezers have long been used as photonic force microscopes, which probe the mechanics of biological systems that operate in the regime of picoNewtons. This study aims to address and showcase optical tweezers generated with low NA air objectives and compare them directly to traditional high NA objectives. The variation in the force in the presence of spherical aberration and defocus is also investigated along with the refractive index (RI) difference between the sample and the trapped particle, which will also influence the trap stiffness. All of these effects are shown to play a significant role in the lateral stiffness.