Efficient in-depth trapping with an oil-immersion objective lens

Maximum trapping efficiency in optical tweezers occurs close to the coverslip because spherical aberration owing to a mismatch in the refractive indices of the specimen (water) and the immersion oil dramatically decreases the trap efficiency as the trap depth increases. Measuring the axial trap efficiency at various tube lengths by use of an oil-immersion objective has shown that such an aberration can be balanced by another source of spherical aberration, leading to a shift in the position of the maximum efficiency in the Z direction. For a 1.1 mu m polystyrene bead we could achieve the maximal efficiency at a depth of 70 mu m, whereas the trap was stable up to a depth of 100 mu m. (c) 2006 Optical Society of America.