Geophotonics: Multimodal Nonlinear Optical Microscopy in Geology

Nonlinear optical microscopy offers label-free contrast and enhanced depth penetration in heterogeneous media compared to more traditional optical microscopy techniques. These advantages make it a natural fit for biological imaging where optical microscopy is frequently the tool of choice. For geologic materials, where electron microscopy is much more prevalent than optical imaging, the utility of newly developed nonlinear optical techniques was less obvious. Nonetheless, we show that these techniques, such as second harmonic generation, coherent Raman microscopy, and pump-probe microscopy, provide complementary contrast mechanisms for a host of geological systems. For some applications, the ability to image rapidly in 3D reveals previously unseen spatial correlations. Even in opaque systems, nonlinear optical signals can achieve comparable contrast to electron microscopy techniques which are inherently confined to the surface. This allows for much more rapid screening of samples with less sample preparation. We believe that by leveraging ongoing developments in biophotonics, it is possible to offer new and improved imaging tools for many other systems that are not traditionally studied using optical microscopy.