A dual function atomic force/near-field scanning optical microscope (AFM/NSOM) with an ultrafast laser excitation source was used to investigate apertureless, tip enhanced second-harmonic generation (SHG) of ZnO nanowires with spatial resolution below the optical diffraction limit. Single-wire SHG spectra show little to no contribution from bandgap or other emission. Polarization data established values for χ<sub>33</sub>/χ<sub>31</sub> close to previous estimates and confirm the SHG process. Experimental results indicate that the SHG signal was reduced for nanowires after exposure to an atmosphere of carbon dioxide and water vapor. An equation was derived for estimating the minimum χ<sup>(2)</sup> detectable using apertureless SHG NSOM.
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