  Nanogenerators based on ZnO nanowires (NWs) realize the energy conversion at
nanoscale, which are ascribed to the piezoelectric property caused by the
lattice distortion of the ZnO NWs. The lattice distortion can significantly
tune the electronic and optical properties, and requires a sensitive and
convenient measurement. However, high-resolution transmission electron
microscopy (HRTEM) technique provides a limited sensitivity of 0.01 nm on the
variation of the lattice spacing and requires vacuum conditions. Here we
demonstrate a highly-sensitive detection of the lattice distortion in single
bent ZnO NWs by second-harmonic generation (SHG) microscopy. As the curvature
of the single bent ZnO NW increases to 21 mm-1 (<4% bending distortion), it
shows a significant decrease (~70%) in the SHG intensity ratio between
perpendicular and parallel excitation polarization with respect to c-axis of
ZnO NWs. Importantly, the extraordinary non-axisymmetrical SHG polarimetric
patterns are also observed, indicating the twisting distortion around c-axis of
ZnO NWs. Thus, SHG microscopy provides a sensitive all-optical and non-invasive
method for in situ detecting the lattice distortion under various
circumstances.
