Advantages and disadvantages in using oil immersed microscope objectives for optical trapping

Since the first demonstrations of optical trapping, both theoretical and experimental parts of this technique evolved. With all this, the main problem when trapping in the Mie regime is due to the limited numerical aperture a microscope objective has. In literature one finds characterizations of "classical" microscope lens or, at most, water immersed ones. In this paper we are analysing the forces generated in an optical tweezers setup using oil immersed microscope objective and having as entrapped particles water-immersed silica beads. Using such a set-up, we can take advantage of the numerical aperture an oil-immersed objective can reach. This numerical aperture can have a value as high as 1.4. From Roosen's ¹ and Ashkin's ² formulas, we calculated the forces involved in our experiment. We observed that the entrapping range on the optical path axis is larger and asymmetric. This generates the possibility to build optical catapult and optical tweezers in the same time, changing only the distance from the sample to the entrapment point. One of the disadvantages of optical trapping in these conditions is that the focus point and the entrapment one can be different. This fact generates the need of using a second microscope for inspecting the entrapped particle so the optical setup is more complicated. To our belief, this set-up for optical tweezers can have big advantages in the field of optical trapping mainly due to the not so strict trapping spatial conditions.