Confocal Raman microspectroscopy through a planar interface

We have developed a model to describe the effect of refraction through a pl anar interface on the collection efficiency and depth of focus when perform ing confocal Raman microspectroscopy. The planar interface introduces spher ical aberration, which can substantially degrade the performance of the mic roscope, especially fur large-numerical-aperture microscope objectives. Thi s spherical aberration will increase the range of focal depths spanned by t he paraxial and marginal rays of the illuminating Laser beam within the sam ple. In the collection path, it will also distort the scattering volume def ined by the confocal aperture; this results in a dramatic fall in the colle cted light intensity with increasing depth. We demonstrate that there is an optimum numerical aperture for collected light intensity at a given depth. The prediction of this theoretical model is compared to empirical results obtained by mapping the stress distribution within the diamond anvil of a h igh-pressure cell. Both the collected Raman intensity and the effective dep th of focus are compared to the predictions from the theory.