A stable, rapid switching process between two or more different wavelengths is needed for fast imaging to obtain the CARS images of different structures at various Raman shifts. We observed a sample of colocalized deuterated and nondeuterated stearic acids for a rapid switching experiment using our fast spectral CARS microscopy system. Each fatty acid was separately prepared as a stock solution of 40 mM in methanol. First, 5 μL of the deuterated stearic acid solution was dropped onto a glass-based dish (12 mm, AGC Techno Glass Co. Ltd., Shizuoka, Japan). The solvent easily evaporated in the room temperature and the deuterated stearic acid crystals were formed. Then, 5 μL of the nondeuterated stearic acid solution was added. With such small amount of solution, the solvent of the added solution was inadequate to dissolve the deuterated stearic acid crystals because it quickly evaporated. Therefore, we were able to obtain the colocalized deuterated and nondeuterated stearic acids crystals. This sample enabled us to detect sufficiently high Raman signals of stretching and stretching at 2850 and , respectively [Figs. 8(a) and 8(b)]. For that purpose, the AOTF laser was rapidly switched between 833 and 888 nm, and CARS images were acquired within . The merged CARS image indicated the locations where the deuterated and nondeuterated stearic acids were independently and colocatedly recrystallized [Fig. 8(c)]. Hence, the CD bond serves as a powerful chemical tag in CARS microscopy. For instance, in the detection of lipid uptake metabolism by cells, the exogenous lipid can be prepared using its deuterated isotope. The exogenous lipid should be easily detectable because naturally no deuterated isotope presents inside the cells. Thus, it is possible to observe lipid dynamics and metabolism in cells.