Use of a microscope objective corrected for a cover glass to improve confocal spatial resolution inside a sample with a finite index of refraction.

The Raman microscope has gained wide recognition in the last years for its ability to combine molecular/crystalline spectroscopy with the spatial resolution inherent in optical techniques (better than 1 mm). In particular, it is attractive for identifying and/or characterizing small amounts of material included in matrices and for measuring interfaces in laminated structures. Contaminants in manufactured products1 and fluid inclusions2 in minerals are two areas where the technique has shown to be particularly useful. Depth profiling without sample preparation is also an area being pursued. However, it has been shown that the predicted confocal (vertical) spatial resolution is degenerated inside a material with index of refraction greater than 1,3–9 and the loss increases with the depth of examination. The source of this error is the optical system itself. The microscope objective performs best when used in a manner consistent with its design. Typical objectives supplied with Raman microscopes are metallographic objectives that are engineered to correct optical aberrations of light passing through air (medium with index equal to 1). Clearly, when probing inside a material with an index different from 1, these corrections will no longer work, which is the source of the degradation in confocal performance.