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. The fluorescence signal is proportional to the square of the peak intensity at the laser focus and thus provides a sensitive measure for the trap quality. It can also be used to provide a feedback signal to (at least partly) correct the aberrations using an adaptive deformable membrane mirror (DMM). The maximum trapping depth inside a sample for a given laser power can be roughly doubled using the DMM, even though full aberration correction seems only possible for quite a limited range (about 7 um).

We also investigate the effect of the numerical aperture on the trap quality when trapping deep inside a sample. It was found that as expected the highest NA yields the strongest trap close to the cover glass. However, when the trap is moved deeper inside the sample the highest NA trap also degrades most rapidly. For trap positions of about 10 um inside the sample it is found that a lower NA objective actually achieves better results.

This work has been published in Optics Communications 236, 145 (2004).