UC Irvine

Sum frequency generation microscopy (SFG) is a nonlinear optical technique used to visualize interfaces and noncentrosymmetric structures that lack inversion symmetry. The advantage of SFG, which uses two collinearly coherent lasers over its degenerate counterpart, second harmonic generation (SHG), is that it enables chemical contrast through molecular vibrational absorptions in the infrared known as the fingerprint region. IR-active molecular modes are energized into a vibrationally excited state through a direct dipole excitation, and then probed with higher energy photon(s) before parametrically radiating a photon at the sum of the incident frequencies. Incorporating this technique into a multi-modal microscope is an important development as it allows for high resolution imaging with spectroscopic information without exogenous labels. Following the development of the SFG microscope came the discovery of vibrationally resonant third-order sum frequency generation (TSFG), a four-wave mixing process that probes resonant contributions of the chi_(3) tensor. This technique,comparable in sensitivity to the coherent anti-Stokes Raman scattering (CARS) modality,was applied to label-free imaging of cells rich in lipid droplet content. This work contributes to the study of high resolution IR-microscopy by merging it with the field of nonlinear optical microscopy.