Tip-Enhanced Coherent Anti-Stokes Raman Scattering for Vibrational Nanoimaging

An electric field enhanced by a metallic nanoprobe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in a spatial resolution far beyond the diffraction limit of light. Our tip-enhanced CARS microscope visualized the DNA network structure at a specific vibrational frequency (∼1337 cm^-1) corresponding to the ring-breathing mode of diazole of adenine molecules.