Abstract
Spectral depth-profiling of optically turbid samples is of high interest to a broad range of applications. We present a method for measuring spatially-offset Raman spectroscopy (SORS) over a range of length scales by incorporating a digital micro-mirror device (DMD) into a sample-conjugate plane in the detection optical path. The DMD can be arbitrarily programmed to collect/reject light at spatial positions in the 2D sample-conjugate plane, allowing spatially offset Raman measurements. We demonstrate several detection geometries, including annular and simultaneous multi-offset modalities, for both macro- and micro-SORS measurements, all on the same instrument. Compared to other SORS modalities, DMD-based SORS provides more flexibility with only minimal additional experimental complexity for subsurface Raman collection.
Highlights
Non-invasive chemical depth-profiling of materials is of high interest in a broad variety of applications
Each point was distant from the point O, which corresponds to the conjugated point on the sample illuminated by the laser
In order to prevent crosstalk of the Raman spectra, only half of the points in the concentric circle were employed as active areas, as spectra from collection points at the same height of the digital micro-mirror device (DMD) would be imaged to the same horizontal tracks of the CCD
Summary
Non-invasive chemical depth-profiling of materials is of high interest in a broad variety of applications. A range of vibrational spectroscopy techniques has been developed for this purpose, including infrared and Raman spectroscopy. Infrared spectroscopy techniques, such as attenuated total reflectance infrared spectroscopy (ATR) [1,2], photoacoustic Fourier transform infrared (FT-IR) spectroscopy [3,4] and thermal emission decay–Fourier transform infrared spectroscopy (TED-FTIR) [5,6] offer depth discrimination in the 0.1-100 μm range. Confocal Raman micro-spectroscopy allows chemical depthprofiling with micrometric resolution for depths up to ~100 μm [7,8,9]. Spatial offset Raman spectroscopy (SORS) is an emerging technique for measuring the subsurface chemical information in diffusely scattering samples, typically from depths of 20 μm – 5 mm range, which is beyond the reach of the above mentioned techniques. The accessible depth of the former technique is in the range of 1-5 mm, while micro-SORS provides a higher depth resolution in the range of
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