Nonlinear optical phenomena are at the basis of powerful imaging techniques with wide applications in biology and medicine (Zipfel et al. 2003; Campagnola 2003; Oron et al. 2004; Cheng and Xie 2004). Coherent anti-Stokes Raman scattering (CARS) microscopy (Cheng and Xie 2004) is a high sensitivity nonlinear technique that is noninvasive and has chemical selectivity. In the CARS process, light is coherently scattered from Raman active vibrational resonances of a sample, resulting in a strong signal that allows for label-free, high-speed imaging. CARS is a third-order nonlinear optical process that requires at least two laser beams, called “pump” and “Stokes,” with optical frequencies lp and ls, respectively. When the frequency difference wp - ws is tuned to the frequency of a Raman-active molecular vibration (W), the anti-Stokes signal is generated at frequency w as = 2w p - ws. In recent years, CARS microscopy has matured as a biomedical imaging modality.