Figure 3

(a) Raman mapping of SRS responses from the regions of interest (ROI) within a sample of carbonatite rare earth ore shown in image (b). For reference, the non-degenerate two-photon excited fluorescence from fluorescein solution (collected separately) maps the cross correlation of the pump and Stokes pulses at the sample and provides a normalizing factor for the Raman peak response (note this signal is not collected from the lock-in and is on a different scale). The sulfate Raman response (~1005 cm⁻¹) is shown in red. The response of calcite (~1090 cm⁻¹) is also indicated. The original hyperspectral data set is 256 × 256 image pixels with 94 spectral data points. Images across the peak regions are averaged and used to make the red (around 1005 cm⁻¹) and green-blue (cyan-1090 cm⁻¹) color merged image. Note the dynamic range of this image has been narrowed so as to show the spatial extent of the SRS signals. Very strong signals exhibit as bright points (grey/white) as well as black areas of the image. These signals are generally higher in amplitude than SRS and show different responses to the delay stage position.