Spherical aberration correction for optical tweezers

Typical optical tweezers setups use high numerical aperture oil-immersion objectives to trap particles suspended in an aqueous medium. When trapping deep inside a sample or out of the imaging plane the quality of the trap in such a system deteriorates due to optical aberrations caused by the refractive index mismatch at the glass-water interface. We investigate this effect experimentally by monitoring the two-photon fluorescence of trapped dye-stained polystyrene spheres. We consider the effect of the numerical aperture on the trap quality and also partly corrected the aberrations by optimising the fluorescence signal using an adaptive deformable membrane mirror.