Effect of refractive index mismatch aberration in arsenic trisulfide

We demonstrate compensation for the spherical aberration due to the refractive index mismatch that occurs when a laser beam is focused into a thick arsenic trisulfide (As $_2$ S $_3$ ) film with a high numerical aperture objective. The effects of the aberration at different focal depths on the point spread function have been calculated numerically and the axial response method shown to be a useful measure for compensating the spherical aberration. We show that with the addition of adaptive optics based on a spatial light modulator, the aberration can be significantly reduced, resulting in an increase in peak intensity by a factor of 2.4 and a decrease in axial elongation by a factor of 2.2.