MULTIMODAL COHERENT RAMAN SPECTROSCOPY AND 3D MICROSCOPY: NONLINEAR OPTICAL TOOLS FOR GEOSCIENCE RESEARCH

Nonlinear optical responses of materials have been developed into important methods for spectroscopy and label-free real-time microscopy in biomedical research. During experiments characterizing the C:H content of organic matter in shale gas source rocks, we demonstrated that the third-order nonlinear process coherent anti-Stokes Raman scattering (CARS) permits non-destructive imaging of 3D distributions of hydrocarbon-rich fluid inclusions in minerals, with submicrometer resolution (Burruss et al., in press, Geology). Furthermore, two additional nonlinear responses, two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG) are generated simultaneously with CARS, providing three co-localized images with contrast due to molecular vibrations (CARS), fluorescence (TPEF), and structural discontinuities (SHG). These methods have a number of advantages for the imaging and analysis of geologic materials. The localization of the nonlinear signal within the center of the focus of a high numerical aperture objective results in high contrast and excellent Z-sectioning for 3D imaging. Because CARS is a coherent, stimulated process, the signal is 4 to 5 orders of magnitude more intense than conventional, spontaneous Raman scattering. This has allowed us, for the first time, to separate the Raman C-H stretching mode of aliphatic hydrocarbons from intense fluorescence of aromatic hydrocarbons within petroleum fluid inclusions. Our multimodal CARS microscope (Pegoraro et al. Optics Express 2009; 17: 2984) is based on a modified laser scanning confocal microscope with a single tunable femtosecond Ti:Sapphire laser as the source, and has Raman tunablity from ~900 to 4500 cm^-1. The high signal levels allows short (2 to 8 μs) pixel dwell times, providing rapid imaging of large (350 x 350 x 100 μm) 3D fields of view within minutes. We can routinely generate images or spectra based on the distribution of C-H, N=N, P=O, C=C, C=O or O-H stretching vibrations. We have recently generated CARS images of diamond and carbonate and phosphate minerals at 1333, 1088, and 980 cm^-1, respectively. We believe that CARS microscopy and spectroscopy and related nonlinear imaging modalities will have broad applications across the geosciences.