Polarization effects in super resolution STED microscopy

Abstract

The invention of the microscope made it possible for people to study the structure of matter below the millimeter scale. Unfortunately the smallest details which can be resolved by conventional techniques, such as confocal microscopy, are limited due to the diffraction limit. Therefore people have been trying to develop techniques to circumvent this limit and increase the resolution. I worked on such a method called Stimulated Emission Depletion, or STED. In STED one uses two different beams, one which is called the excitation beam and one which is called the depletion beam. The excitation beam is used to cause a fluorescence signal. The depletion beam has the shape of a ring with a central dark spot. When both beams are combined the depletion beam will deplete (i.e. getting rid of the fluorescence signal) the outer part of the excitation beam. This will result in a focal spot, which is smaller than the diffraction limit. The quality of this beam determines the resolution which can be achieved.

For this thesis I have built a setup with which I have looked into some of the possible effects that can influence the shape of this depletion beam. I have used a Spatial Light Modulator, which can modulate phase, to see if it can be used effectively to correct for certain aberrations. I have also looked at how different polarizations of the light will alter the shape of the focal spot. It was found that the SLM was not able to get rid of all aberrations, although it performed well on average. The polarization is found to have a big impact on the shape of the beam, but it was experienced that misalignment issues play a bigger role on the shape of the doughnut than was expected beforehand.