Single Molecule STM Nanospectroscopy using Difference Frequency Generation at the Tunnel Gap

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We have developed a new scheme of single-molecule-resolved nanospectroscopy (STM-DFG) that uses an intense infrared light locally generated at the tunnel gap of tip and sample in a scanning tunneling microscope (STM). The achievements are as follows. (1)Measurements of optical field enhancement at STM probe tips : From a tunneling current arising from a rectification of optical field at the tunnel gap due to the nonlinear I-V relation, we have measured for graphite samples the optical field enhancement factors as large as 10^3 and 5×10^2 even for W tips and Pt-Ir tips, respectively. (2)Proving difference frequency generation at a tunneling gap : For the combination of C_<60> film sample and W tip that was illuminated with two laser lights from a Ti-sapphire laser (wavelength fixed) and a diode laser (wavelength swept), we have successfully acquired spectra in agreement with infrared absorption by C_<60> crystals. (3)Field enhancement effect in carbon nanotubes (CNT) : We have found a remarkable enhancement of Raman scattering from N-Dimethyl formamide (DMF) molecules only when the DMF contained CNT and the excitation light is resonant (633nm) with CNT, a signature of the expected field enhancement effect of CNT. (4)STM observations of optical field enhancement effect : Using Au nano-rods as a model sample, we have developed an STM technique to observe directly the optical enhancement taking place at individual particles by detecting the local rectified current. (5)Optimum conditions for STM-DFG : We have concluded that it is most efficient to more intensely focus the light beam, to use pulsed laser lights, to increase the set tunneling current, and to use special materials (Ag, Au, CNT) for STM tips.