Triple Modal Coherent Nonlinear Imaging With Vibrational Contrast

By combining spectroscopic contrast with imaging speed, coherent Raman scattering (CRS) microscopy techniques have reiterated the usefulness of vibrational microscopy. In the nonlinear optical (NLO) microscope, CRS is often combined with other nonlinear optical signals, such as two-photon excited fluorescence or second-harmonic generation, providing images with multimodal contrast. However, it is also possible to expand the vibrational capabilities of the NLO microscope, namely through incorporation of other vibrationally selective nonlinear optical signals beyond CRS. Recently, it has been shown that vibrationally resonant sum-frequency generation (SFG) microscopy can be integrated into the imaging platform as well, offering additional vibrational spectroscopic probing capabilities. In this contribution, we highlight the concept of triple-modal vibrational microscopy, which combines CRS, SFG, and third-order sum-frequency generation on a single laser-scanning microscope. These three vibrationally sensitive NLO imaging techniques provide complementary information about the sample, allowing for more detailed molecular spectroscopic studies of biological specimens. We review the principles of the three light-matter interactions that underlie the techniques and discuss some of the imaging properties of the triple-modal vibrational microscope. We provide several imaging examples and conclude with discussing some of the outstanding challenges.