Nanogenerator can convert nanoscale mechanical energy into electrical power. It is primarily based on the piezoelectric property caused by the bending lattice distortion. Lattice distortion is a universal phenomenon in materials. Besides, it has been proven that lattice distortion in nanostructures can significantly affect their optical properties, electrical properties and mechanical properties. The conventional methods to measure lattice distortion are high-resolution transmission electron microscopy and X-ray diffraction. However, there are still some limitations, such as the size of sample, and test environment. Therefore, an all-optical method based on second-harmonic generation (SHG) can be developed to detect the lattice distortion. SHG is a typical nonlinear optical process. Universally, crystal orientations can be precisely detected by SHG, based on the sensitivity of the polarized response to the crystallographic orientations. However, as far as our knowledge, detection of the lattice distortion in nanostructures by polarization-dependent SHG microscopy has not been addressed yet.

The ultrafast optics group led by Prof. Peixiang Lu presents that the polarization-dependent SHG response varies with different curvatures of single ZnO nanowires. 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. A high detection sensitivity of 10−3 nm on the bending distortion is obtained in our experiment. Importantly, the extraordinary nonaxisymmetrical 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 noninvasive method for in situ detecting the lattice distortion under various circumstances.

    This work is published on ACS Photonics Vol.3, No.7, 1308(2016). This work was supported by the 973 Programs under Grant 2014CB921301, National Natural Science Foundation of China (11204097), the Doctoral fund of Ministry of Education of China under Grant No. 20130142110078, and the Fundamental Research Funds for the Central Universities, HUST: 2016YXMS015. 

Fig1. Schematic diagram of detecting bending distortion and twisting distortion, and corresponding polarization-dependent SHG responses, respectively.