DMD-based software-configurable spatially-offset Raman spectroscopy for spectral depth-profiling of optically turbid samples

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.

Raman spectroscopy: techniques and applications in the life sciences

“…Following this, a popular approach to SORS was to use an annular collection scheme, whereby one fixed offset could be collected from all points in the circle around the excitation point with the radius being the desired offset. This is usually implemented by using ring-fibers with a fixed value of offset chosen for a particular application [391,392], though other approaches such as line-based SORS [393] or using a digital micro-mirror device (DMD) placed at a sample-conjugate plane [394] …”

“…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]. The latter approach is especially flexible due to the software-control of offset, and compatibility with microscopes using only a single objective lens, with the maximum offset determined by the field-of-view of the objective being used.…”

Feasibility of Spatially Offset Raman Spectroscopy for in Vitro and in Vivo Monitoring Mineralization of Bone Tissue Engineering Scaffolds

“…Semi-annulus collection geometry was employed for all SORS measurements as it provides the high collection efficiency for the Raman photons. 27 The SORS spectra measured for the IEIC16 scaffold/HA layered samples are presented in Figure 2 (b) and (c), and compared to the Raman spectra of pure IEIC16 scaffold and HA powder. Figure 2 shows that the most intense band in the Raman spectrum of HA corresponds to the PO4 3-symmetric stretching at 962 cm -1 .…”

“…S1). The area of the Raman collection region at the sample surface represented ~25% of the total area of the full annulus corresponding to a spatial offset value 27 .…”

“…The former is a representative of bioactive materials able to bond to human tissue, while the latter represents the synthetic polymer family which exhibit tailorable and reproducible physical, chemical and biodegradation properties. Here, a SORS instrument based on a digital-mirror-device (DMD) 27 was used to measure Raman spectral depth-profiles of the scaffold/HA layered samples, demonstrating the potential of SORS to monitor the sub-surface mineralization of the scaffolds in-situ. The detection of HA is important in bone tissue engineering because the formation of hydroxyapatite (HA) within the scaffold is a step occurring to bone bonding, thus it is an indication of the scaffold bioactivity 26 .…”