Abstract
Obtaining molecular information deeper within optically turbid samples is valuable in many applications. However, in many cases this is challenging, in particular when the sample elicits strong laser-induced fluorescence emission. Here, we investigated the use of time-gated and micro-spatially offset Raman spectroscopy (micro-SORS) based on spectral multiplexing detection to obtain sub-surface molecular analysis and imaging for both fluorescing and non-fluorescing samples. The multiplexed spectral detection achieved with a digital micromirror device (DMD) allowed fast acquisition of the time-gated signals to enable three-dimensional Raman mapping (raster scanning in the lateral x,y plane and using time-of-flight calibration for the axial z-direction). Sub-millimeter resolution molecular depth mapping was achieved with dwell times on the order of seconds per pixel. To suppress fluorescence backgrounds and enhance Raman bands, time-gated Raman spectroscopy was combined with micro-SORS to recover Raman signals of red pigments placed behind a layer of optically turbid material. Using a defocusing micro-SORS approach, both fluorescence and Raman signals from the surface layers were further suppressed, which enhanced the Raman signals from the deeper sublayers containing the pigment. These results demonstrate that time-gated Raman spectroscopy based on spectral multiplexed detection, and in combination with micro-SORS, is a powerful technique for sub-surface molecular analysis and imaging, which may find practical applications in medical imaging, cultural heritage, forensics, and industry.
Highlights
Raman spectroscopy is a powerful non-destructive technique for molecular analysis.[1]
We investigated the ability of using time-gated Raman spectroscopy in spectral multiplexed detection mode to obtain 3-D Raman images of non-fluorescing optically turbid samples
Samples consisting of two layers were used to understand the level of temporal overlap between the Raman photons originating from different depths of the sample
Summary
Raman spectroscopy is a powerful non-destructive technique for molecular analysis.[1]. In this paper we use raster scanning time-gated Raman spectroscopy based on spectral multiplexed detection to obtain 3-D Raman information from optically turbid samples In this approach, the spectral multiplexing works as a compressive sensing technique, that has been used in continuous wave Raman spectroscopy to speed up imaging.[15,31,32,33] This technique has been applied recently to time-gated Raman spectroscopy in order to decrease the acquisition times and enable time-gated Raman mapping of samples eliciting strong fluorescence backgrounds.[34] Using a single pixel detector, such as a single photon avalanche diode (SPAD), operating in TCSPC mode combined with multiplexing the Raman bands of interest, provides a speed advantage and higher signal to noise ratio. Fluorescence is one of the main obstacles often encountered in Cultural Heritage materials, as they exhibit a complex mixture of organic and inorganic compounds, frequently damaged and transformed due to the aging and decay mechanisms
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