Improving optical trapping in the axial direction and a continuous change of the optimal trapping depth

摘要

Oil immersion objectives have higher numerical aperture than water immersion objectives thus providing higher optical resolution. This is important for confocal microscopy as well as for the strength of an optical trap created by such an objective, because the efficiency of an optical trap is limited by its axial strength. However, light focused by oil immersion objectives suffer from spherical aberrations caused e.g. by a mismatch between the refractive index of the immersion and sample media. Such aberrations widen the intensity profile in the focal region thus restricting the axial resolution of the objective and decreasing the axial optical trapping strength. Objectives are typically designed such that the spherical aberrations are minimized for visible wavelengths and a few microns away from the surface. However, often optical traps are based on infrared lasers or are used further away from the surface thus introducing considerable spherical aberrations. We have shown that a tuning of the immersion refractive index can minimize the total spherical aberrations at any desired depth, thus maximizing the trapping efficiency and giving rise to optical trapping strengths twice as large as previously reported. Changing the immersion media, however, is a discrete way of tuning the optimal trapping depth: An increment (decrement) of 0.01 in the refractive index of the immersion media gives rise to an increase (decrease) of approx = 4μm and approx =10μm of the most efficient trapping depth for infinity-tube length and finite-tube length objectives, respectively. Here, we show that combining a change of immersion media with changing tube length provides a continuous way of changing the optimal trapping depth. Also, we show how trapping conditions change with polarization.