Second-Harmonic Enhancement with Mie Resonances in Perovskite Nanoparticles

Second-harmonic generation (SHG) in nanostructures gives rise to many applications such as lab-on-a-chip and imaging by frequency doubling. However, the SHG signal decreases with volume, and the conversion efficiency is limited. Thus, means to enhance nonlinear signals at the nanoscale are needed. For instance, while plasmonic nanostructures offer a high enhancement due to the strong confinement of the electromagnetic field, they have high losses and the fabrication methods are difficult. In this work, we propose to enhance the SHG by using the intrinsic scattering properties of an all-dielectric perovskite nanostructure. We demonstrate the Mie scattering resonances of individual barium titanate (BaTiO₃) nanoparticles with diameters between 200 and 250 nm. We distinguish contributions of the magnetic dipole and magnetic quadrupole. Then, we use the Mie resonances to achieve an SHG enhancement of 4 orders of magnitude within the same nanoparticle. Our results suggest that a strong increase of the SHG signal can be obtained without using plasmonic or hybrid nanostructures. We show a straightforward way of enhancing low optical signals within a single material, which will facilitate the study of other nonlinear phenomena at the nanoscale

Direct Patterning of Robust One-Dimensional, Two-Dimensional, and Three-Dimensional Crystalline Metal Oxide Nanostructures Using Imprint Lithography and Nanoparticle Dispersion Inks

Dimensionally stable one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) high aspect ratio crystalline metal oxide nanostructures are fabricated using soft nanoimprint lithography with inks comprised of nanoparticle (NP) dispersions in solvent or in sol–gel precursors for the metal oxide. Crystalline TiO₂ and indium tin oxide (ITO) NP dispersions in solvent are imprinted using a solvent permeable patterned poly­(dimethylsiloxane) (PDMS) stamp to yield robust crystalline nanostructures that are dimensionally stable to calcination (less than 8% linear shrinkage in imprinted feature heights upon heat treatment at 500 °C). Inks comprised of 80% crystalline NPs dispersed in 20% sol–gel binder are patterned using thermal- or UV-assisted imprinting with a PDMS stamp. The composition and physical properties of the dimensionally stable imprinted metal oxides (TiO₂ and ITO) can be altered by varying the composition of the ink. Rapid printing of high aspect ratio nanostructures and sub-100 nm features are easily realized. Residual layer free, direct imprinting of isolated features is achieved by using an ink with the appropriate surface energy to ensure dewetting at the stamp–substrate interface. The technique is extended to create 3-D mesh nanostructures by deploying a simple layer-by-layer imprint strategy. TiO₂ 3-D mesh nanostructures are robust and mechanically stable to calcination at temperatures of 1000 °C, which results in an anatase to rutile transition. The direct fabrication of high quality dimensionally stable metal oxide nanostructures opens the door to solution based and roll-to-roll processing of robust and efficient inorganic electronic, optical, and energy generation and storage devices

Enhanced Second-Harmonic Generation from Sequential Capillarity-Assisted Particle Assembly of Hybrid Nanodimers

We show enhanced second-harmonic generation (SHG) from a hybrid metal–dielectric nanodimer consisting of an inorganic perovskite nanoparticle of barium titanate (BaTiO₃) coupled to a metallic gold (Au) nanoparticle. BaTiO₃–Au nanodimers of 100 nm/80 nm sizes are fabricated by sequential capillarity-assisted particle assembly. The BaTiO₃ nanoparticle has a noncentrosymmetric crystalline structure and generates bulk SHG. We use the localized surface plasmon resonance of the gold nanoparticle to enhance the SHG from the BaTiO₃ nanoparticle. We experimentally measure the nonlinear signal from assembled nanodimers and demonstrate an up to 15-fold enhancement compared to a single BaTiO₃ nanoparticle. We further perform numerical simulations of the linear and SHG spectra of the BaTiO₃–Au nanodimer and show that the gold nanoparticle acts as a nanoantenna at the SHG wavelength