Measuring lateral efficiency of optical traps: The effect of tube length

An optical tweezer setup is made based on a custom-designed inverted microscope, which can work both in finite and infinite tube length microscopy modes. It is shown that the spherical aberration due to the mismatch in the refractive indices of the specimen (water) and the immersion oil as well as the wavelength can be partially compensated by introducing another source for the spherical aberration provided it has the opposite sign. Changing the tube length is shown to be a good candidate for this effect: an improvement of up to a factor of four has been observed in the lateral efficiency of the trap.