Chemical imaging with a confocal scanning Fourier-transform–Raman microscope

Traditional approaches in confocal microscopy have focused on techniques to generate volumetric intensity or phase images of an object. In these different imaging modes the scattered optical-field properties depend on local refractive index and absorption, properties not unique to a given material. We report here on a confocal microscope that uses Raman scattered light to generate volumetric chemical images of a material. We designed and built a prototype instrument, called a confocal scanning laser Raman microscope, that combines a confocal scanning laser microscope with a Fourier-transform–Raman spectrometer. The high depth and lateral spatial resolution of the confocal optics design define a volume element from which the Raman scattered light is collected, and the spectrometer analyzes its spectral content. The sample is scanned through the microscope probe volume, and a chemical image is generated based on the content of the Raman spectrum extracted from each scan position in the sample. The results include instrument characterization measurements and examples of confocal chemical imaging.