I am in the middle of focusing a blue (400nm) laser beam through a sapphire substrate, of thickness 430 microns. To do this, I am using a LD-Plan Neofluar 63x objective lens, which is equipped with a coverslip correction collar.
If this were glass, I would merely adjust the coverslip correction to 430 microns, and the thickness would be compensated for.
However, how should I adjust the correction for a substrate of different refractive index? (Assuming for simplicity sapphire is optically isotropic)
Is there any equation which allows you to find the optimum correction for such a system?
I have tried using a correction of 430 microns, and the optical spot was outside the sample completely. In contrast, a correction of 170 microns (which I tried first by mistake) gives a relatively good focus, so the change in refractive index is obviously very important!
Many thanks in advance!