Polarized second-harmonic generation optical microscopy for laser-directed assembly of ZnO nanowires.

Two-photon polymerization (TPP) based on laser direct writing is currently one of the most prevailing 3D micro/nano fabrication techniques. Nanomaterials can be doped in resins and assembled by TPP for developing advanced 3D functional devices. However, there lacks an effective visualization tool to determine the distribution and orientation of the nanomaterials as-doped in the composite resins. Herein, we present a nondestructive, in situ, and rapid characterization method to determine the orientation and distribution of the nanomaterials within cured resins using polarized second-harmonic generation (p-SHG). The directional assembly of the ZnO nanowires within micro/nanostructures fabricated by TPP is, for the first time to the best of our knowledge, characterized by p-SHG optical microscopy with a fast imaging speed by two orders of magnitude higher than that of the Raman mapping technique. Our method opens a window for nondestructive, rapid, in situ, and polarization-resolved characterization of functional devices made by TPP micro/nanofabrication.

Full-color enhanced second harmonic generation using rainbow trapping in ultrathin hyperbolic metamaterials.

Metasurfaces have provided a promising approach to enhance the nonlinearity at subwavelength scale, but usually suffer from a narrow bandwidth as imposed by sharp resonant features. Here, we counterintuitively report a broadband, enhanced second-harmonic generation, in nanopatterned hyperbolic metamaterials. The nanopatterning allows the direct access of the mode with large momentum, rendering the rainbow light trapping, i.e. slow light in a broad frequency, and thus enhancing the local field intensity for boosted nonlinear light-matter interactions. For a proof-of-concept demonstration, we fabricated a nanostructured Au/ZnO multilayer, and enhanced second harmonic generation can be observed within the visible wavelength range (400-650 nm). The enhancement factor is over 50 within the wavelength range of 470-650 nm, and a maximum conversion efficiency of 1.13×10-6 is obtained with a pump power of only 8.80 mW. Our results herein offer an effective and robust approach towards the broadband metasurface-based nonlinear devices for various important technologies.

Nonlinear Optical Characterization of 2D Materials.

Characterizing the physical and chemical properties of two-dimensional (2D) materials is of great significance for performance analysis and functional device applications. As a powerful characterization method, nonlinear optics (NLO) spectroscopy has been widely used in the characterization of 2D materials. Here, we summarize the research progress of NLO in 2D materials characterization. First, we introduce the principles of NLO and common detection methods. Second, we introduce the recent research progress on the NLO characterization of several important properties of 2D materials, including the number of layers, crystal orientation, crystal phase, defects, chemical specificity, strain, chemical dynamics, and ultrafast dynamics of excitons and phonons, aiming to provide a comprehensive review on laser-based characterization for exploring 2D material properties. Finally, the future development trends, challenges of advanced equipment construction, and issues of signal modulation are discussed. In particular, we also discuss the machine learning and stimulated Raman scattering (SRS) technologies which are expected to provide promising opportunities for 2D material characterization.