Holographic aberration correction: optimising the stiffness of an optical trap deep in the sample.

We investigate the effects of 1(st) order spherical aberration and defocus upon the stiffness of an optical trap tens of μm into the sample. We control both these aberrations with a spatial light modulator. The key to maintain optimum trap stiffness over a range of depths is a specific non-trivial combination of defocus and axial objective position. This optimisation increases the trap stiffness by up to a factor of 3 and allows trapping of 1 μm polystyrene beads up to 50 μm deep in the sample.