Passive signal enhancement in spatially offset Raman spectroscopy

We demonstrate experimentally, for the first time, the feasibility of enhancing signals in Spatially Offset Raman Spectroscopy (SORS) using a dielectric bandpass filter, building on our earlier experimental work on the enhancement of transmission Raman signals. The method is shown to lead to the enhancement of both the surface and subsurface Raman layer signal improving the signal-to-noise ratio of Raman spectra from the deep areas of samples, thus enhancing the technique's sensitivity and penetration depth. The filter is placed over the laser illumination zone, on the sample surface acting as a 'unidirectional' mirror transmitting the collimated laser beam on one side and reflecting photons escaping from the sample back into it. This enhances the degree of coupling of laser radiation into the medium and associated generated Raman signal. The feasibility study was performed on a two-layer sample with the second layer located at the limit of the penetration depth of the method for this sample. The sample consisted of a 2.2-mm over-layer of a thinned paracetamol tablet followed by a 2-mm layer of trans-stilbene powder. The Raman signal was collected from a spatially offset region through a hole fabricated within the filter. The experiments demonstrate the presence of an enhancement of the Raman signal from both the layers by a factor of 4.4-4.5 and the improved signal-to-noise ratio of sublayer signal by a factor of 2.2, in agreement with photon shot noise dominated signal. Copyright (C) 2008 John Wiley & Sons, Ltd.