Subsurface probing in diffusely scattering media using spatially offset Raman spectroscopy

Area of Science:

• Spectroscopy
• Materials Science
• Optics

Background:

• Retrieving subsurface information from diffusely scattering media is challenging.
• Conventional methods like confocal microscopy have limited depth penetration.
• Surface signals often obscure valuable subsurface data in Raman spectroscopy.

Purpose of the Study:

• To develop a simple methodology for effective retrieval of Raman spectra from subsurface layers.
• To eliminate surface Raman scatter and enhance subsurface signal detection.
• To demonstrate the applicability of the technique for depths beyond conventional confocal microscopy.

Main Methods:

• Collecting Raman scattered light from surface regions laterally offset from the excitation laser spot.
• Utilizing multivariate data analysis to process spectral data and isolate pure layer spectra.
• Testing the methodology on a two-layer sample: trans-stilbene powder beneath a poly(methyl methacrylate) over-layer.

Main Results:

• The developed method yields Raman spectra with variations in relative intensities of surface and subsurface layers.
• Pure Raman spectra of individual sample layers were successfully obtained, effectively eliminating surface scatter.
• A 19-fold improvement in contrast for the subsurface trans-stilbene layer was achieved without numerical processing.

Conclusions:

• The described methodology enables effective retrieval of subsurface Raman spectra from diffusely scattering media.
• This technique offers a significant advancement for non-invasive subsurface probing in various fields.
• Potential applications include biomedical diagnostics (bone quality, dermatology) and materials/catalyst research.