Reshaping the Second-Order Polar Response of Hybrid Metal-Dielectric Nanodimers.

We combine the field confinement of plasmonics with the flexibility of multiple Mie resonances by bottom-up assembly of hybrid metal-dielectric nanodimers. We investigate the electromagnetic coupling between nanoparticles in heterodimers consisting of gold and barium titanate (BaTiO3 or BTO) nanoparticles through nonlinear second-harmonic spectroscopy and polarimetry. The overlap of the localized surface plasmon resonant dipole mode of the gold nanoparticle with the dipole and higher-order Mie resonant modes in the BTO nanoparticle lead to the formation of hybridized modes in the visible spectral range. We employ the pick-and-place technique to construct the hybrid nanodimers with controlled diameters by positioning the nanoparticles of different types next to each other under a scanning electron microscope. Through linear scattering spectroscopy, we observe the formation of hybrid modes in the nanodimers. We show that the modes can be directly accessed by measuring the dependence of the second-harmonic generation (SHG) signal on the polarization and wavelength of the pump. We reveal both experimentally and theoretically that the hybridization of plasmonic and Mie-resonant modes leads to a strong reshaping of the SHG polarization dependence in the nanodimers, which depends on the pump wavelength. We compare the SHG signal of each hybrid nanodimer with the SHG signal of single BTO nanoparticles to estimate the enhancement factor due to the resonant mode coupling within the nanodimers. We report up to 2 orders of magnitude for the SHG signal enhancement compared with isolated BTO nanoparticles.

Image-based autofocusing system for nonlinear optical microscopy with broad spectral tuning.

We present an image-based autofocusing system applied in nonlinear microscopy and spectroscopy with a wide range of excitation wavelengths. The core of the developed autofocusing system consists of an adapted two-step procedure maximizing an image score with six different image scorings algorithms implemented to cover different types of focusing scenarios in automated regime for broad wavelength region. The developed approach is combined with an automated multi-axis alignment procedure. We demonstrate the key abilities of the autofocusing procedure on different types of structures: single nanoparticles, nanowires and complex 3D nanostructures. Based on these experiments, we determine the optimal autofocusing algorithms for different types of structures and applications.

Anapoles in Free-Standing III-V Nanodisks Enhancing Second-Harmonic Generation.

Nonradiating electromagnetic configurations in nanostructures open new horizons for applications due to two essential features: a lack of energy losses and invisibility to the propagating electromagnetic field. Such radiationless configurations form a basis for new types of nanophotonic devices, in which a strong electromagnetic field confinement can be achieved together with lossless interactions between nearby components. In our work, we present a new design of free-standing disk nanoantennas with nonradiating current distributions for the optical near-infrared range. We show a novel approach to creating nanoantennas by slicing III-V nanowires into standing disks using focused ion-beam milling. We experimentally demonstrate the suppression of the far-field radiation and the associated strong enhancement of the second-harmonic generation from the disk nanoantennas. With a theoretical analysis of the electromagnetic field distribution using multipole expansions in both spherical and Cartesian coordinates, we confirm that the demonstrated nonradiating configurations are anapoles. We expect that the presented procedure of designing and producing disk nanoantennas from nanowires becomes one of the standard approaches to fabricating controlled chains of standing nanodisks with different designs and configurations. These chains can be essential building blocks for new types of lasers and sensors with low power consumption.

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.

Crystalline heterogeneity in single ferroelectric nanocrystals revealed by polarized nonlinear microscopy.

Ferroelectric nanocrystals have considerable interest for applications in nanophotonics, optical memories and bio-imaging. Their crystalline nature at the nanoscale remains however poorly known, mostly because structural investigation tools on single nanocrystals are lacking. In this work we apply polarization resolved second harmonic generation (P-SHG) imaging on isolated Barium Titanate (BaTiO3) nanocrystals to unravel their crystalline nature, exploiting the sensitivity of polarized SHG to local non-centrosymmetry and nanocrystals surface responses. We evidence crystalline heterogeneities in BaTiO3 nanocrystals manifested by a centrosymmetric shell around the tetragonal core of the crystals, corroborating hypotheses from previous ensemble structural investigations. This study shows that in contrast to bulk materials, nanocrystals exhibit a complex composition, which is seen to be reproducible among nanocrystals. P-SHG appears furthermore as a powerful methodology that reports structural behaviors in nanoscale dielectrics materials, at the individual nanoparticle scale.

Tunable 2D binary colloidal alloys for soft nanotemplating.

The realization of non-close-packed nanoscale patterns with multiple feature sizes and length scales via colloidal self-assembly is a highly challenging task. We demonstrate here the creation of a variety of tunable particle arrays by harnessing the sequential self-assembly and deposition of two differently sized microgel particles at the fluid-fluid interface. The two-step process is essential to achieve a library of 2D binary colloidal alloys, which are kinetically inaccessible by direct co-assembly. These versatile binary patterns can be exploited for a range of end-uses. Here we show that they can for instance be transferred to silicon substrates, where they act as masks for the metal-assisted chemical etching of binary arrays of vertically aligned silicon nanowires (VA-SiNWs) with fine geometrical control. In particular, continuous binary gradients in both NW spacing and height can be achieved. Notably, these binary VA-SiNW platforms exhibit interesting anti-reflective properties in the visible range, in agreement with simulations. The proposed strategy can also be used for the precise placement of metallic nanoparticles in non-close-packed arrays. Sequential depositions of soft particles enable therefore the exploration of complex binary patterns, e.g. for the future development of substrates for biointerfaces, catalysis and controlled wetting.