A new type of highly sensitive spatially offset Raman spectroscopy (SORS) developed for deep noninvasive probing of stratified turbid media is described. The technique, termed inverse SORS, permits much greater depths to be interrogated than those accessible with the conventional SORS approach. This is achieved by enhancing the sensitivity of the technique through the elimination of spectral distortions inherent to the conventional SORS methodology. The method also permits the use of higher laser powers in applications where intensity limits exist, such as when probing human tissue <i>in vivo</i>. In addition, the new approach possesses a much higher degree of flexibility, enabling on-the-spot tailoring of experimental conditions such as the magnitude and number of spatial offsets to individual samples. The scheme uses a reverse SORS geometry whereby Raman light is collected through fibers at the center of the probe and laser radiation is delivered to the sample through a beam in the shape of a ring. The method is demonstrated on a layered powder sample and several practical examples of its uses, presented for the first time, are also given. Potential applications include disease diagnosis, noninvasive probing of pharmaceutical products and chemicals through packaging, probing of polymers, biofilms or paints, and homeland security screening.
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