Chemically Selective Imaging of Overlapping C-H Stretching Vibrations with Time-Resolved Coherent Anti-Stokes Raman Scattering (CARS) Microscopy

Chemically selective imaging of spectrally overlapping compounds is studied with a time-resolved, femtosecond approach on coherent anti-Stokes Raman scattering (CARS) microscopy taking advantage of time-dependent oscillating CARS amplitude which is sensitive to different chemical components at different time points. Chemically selective imaging is demonstrated for composite material of polypropylene (PP) matrix and om-POSS (octamethyl polyhedral oligomeric silsesquioxane) microparticles having partly overlapping CH stretching vibrations. Inverse Fourier transformation (IFT) was applied to Raman spectra of PP and om-POSS, indicating that the oscillatory structures of the vibrational decays differ markedly and allow selective imaging, with minimally using one time point per spatial point, which is also confirmed by the CARS measurements. CARS decays measured additionally for lipid films of cholesterol and DOPC (1,2-dioleyl-sn-glycero-3-phosphocholine) indicate selective detection of cholesterol at a specific probe delay time. The results of this study show that the tr-CARS technique has potential for chemically selective, nonresonant background free imaging using overlapping vibrations.