Comparison of key modalities of micro-scale spatially offset Raman spectroscopy

Abstract

We compare several basic embodiments of a recently proposed and demonstrated micrometer-scale Spatially Offset Raman Spectroscopy (micro-SORS). Micro-SORS is a recently introduced analytical method for noninvasive characterisation of the chemical composition of subsurface, micrometre-scale-thick diffusely scattering layers at depths beyond the reach of conventional confocal Raman microscopy. The technique is applicable, for example, in nondestructive subsurface analysis of highly scattering stratified matrices such as painted layers in cultural heritage or in noninvasive analysis of stratified polymer systems or biological samples. Using Monte Carlo simulations, we analysed two defocusing variants of micro-SORS and a variant involving a full separation of illumination and collection zones on the sample surface. Both the penetration depth into the sample and relative enhancement of sublayer Raman signals were studied as a function of layer thickness and type of technique and their parameters. The model predicts that the most effective method by far is the variant with fully spatially separated illumination and collection zones. On the other hand, the defocusing micro-SORS, where both the laser and Raman collection zones are defocussed and overlapped, yielded the lowest performance although its key benefit lies in its simplicity as the concept can be practiced on existing conventional Raman microscopes without any modifications. A basic experimental verification of the theoretical findings contrasting two extreme modalities, the fully separated micro-SORS variant with the defocusing one, is also presented.

Raman spectroscopy: techniques and applications in the life sciences

“…However this is not exactly concordant with conventional SORS, as it involves collection of a full circular region of the sample, not the annulus from a single offset value. Recent work has presented alternatives to this in line with conventional SORS by using an additional offset objective [398]. More simply, a DMD can be used to select the annulus region as mentioned earlier [394].…”

Determination of thickness of thin turbid painted over-layers using micro-scale spatially offset Raman spectroscopy

“…As for micro-SORS, even larger penetration depths can be expected from the deployment of full micro-SORS, a micro-SORS variant consisting of fully separated laser illumination and Raman collection points, which, as demonstrated earlier, can achieve considerably higher penetration depths than defocusing micro-SORS [12,13]. On the other hand, the usable range of depths for full micro-SORS is anticipated to be smaller for a given spatial offset than that for defocusing micro-SORS for a given defocusing distance.…”

“…The micro-SORS technique promises further extension of accessible depths using fully separated collection and illumination zones [12]. Given the wide applicability of the technique and the potential for further improvements, the concept of micro-SORS is expected to find a number of practical application niches in a several areas including art, archaeology, forensics, catalytic research, biology and biomedical sciences.…”

Micro-scale spatially offset Raman spectroscopy for non-invasive subsurface analysis of turbid materials

“…The most basic variant of micro-SORS technique is defocusing micro-SORS 2 derived from early research into defocusing SORS. [13][14][15] Although this is not the most effective concept available, 16 as it does not involve fully separated laser illumination and Raman collection zones leading to lower contrast between individual layers derived from SORS, it is undoubtedly the most practical due to its extreme simplicity and suitability for use with existing conventional Raman microscopes without any modifications. Consequently, most research to date in micro-SORS has been based around this modality.…”