Contrast in coherent raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy is becoming a widely used technique for sub-micron, chemically-selective imaging at high rates of speed In this thesis I discuss three methods for increasing the specificity and selectivity of coherent Raman experiments. The first method is the application of a false-color coding of hyperspectral coherent Raman data, which enables the rapid visual analysis of complicated samples. This measurement technique has found extensive use in pharmaceutical and biomedical applications, including the characterization of the polymorphic behaviors of crystalline and semi-crystalline compounds in oral dosage forms. We have further utilized this technique to localize medicinally relevant compounds within whole, unprocessed plant material. The second method relies on the measurement of the full complex vibrational response of the sample in a heterodyne configuration, which we have combined with hyperspectral acquisition and advanced spectral unmixing algorithms to allow quantitative characterization of both heterogeneous and homogeneous mixtures. Vibrational phase contrast CARS, as we refer to it, is a powerful tool for measuring samples that have large non-resonant background signals, as the phase of the resonant vibrational response differs significantly from that of the non-resonant background and so can be easily separated. An upgraded set of infrastructure and fresh understanding the of the underlying physics enables us to acquire full phase-resolved vibrational information at high rates and in thick and/or highly scattering samples. Finally, a third method casts the heterodyne coherent Raman scattering process in a new paradigm wherein energy transfer between the optical fields and the molecule becomes the parameter of interest. Two separate optical processes are identified: parametric processes are those in which energy is merely re-arranged between the optical fields incident on the sample, leaving the molecule in its original state, while dissipative processes are those in which energy is exchanged from the optical fields into the molecule, leaving it in an excited state. In this new paradigm the pure dissipative vibrational signal can be readily measured even in the presence of a large electronic background.