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We study numerically and experimentally the second-harmonic generation (SHG) from rectangular metagratings of V-shaped gold nanoantennas. We show that by carefully engineering the array pitch to steer the diffraction orders toward the single antenna emission, the extracted signal is maximized. This enhancement is attributed to the angular overlap between the radiation pattern and array factor and is comparable to the improvement yielded by the coupling of surface lattice resonances (SLRs) with local modes. Moreover, we demonstrate a simple technique to experimentally reconstruct the emission diagram of an antenna from measurements of the collective grating response as a function of the excitation angle. Excellent agreement is obtained with simulations when the sample is immersed either in air or in water, which is crucial in view of future sensing application. Thanks to the high signal-to-noise ratio and low dependence on the statistical particle dispersity, this method constitutes an effective alternative to back-focal plane imaging when very weak signals such as SHG are involved.

Hybrid dielectric–metal colloid nanoparticle heterostructures have recently attracted attention in both fundamentals and applications. In this paper, we developed a new approach based on mechanical grinding and chemical synthesis in order to obtain hybrid dielectric–metal nanoparticles. By increasing the amounts of the stabilizing agent (polyvinylpyrrolidone (PVP)), three different types of hybrid colloidal structures were obtained, Si/Au core–island, core–shell, and core–shell oligomers heterostructures, respectively. The obtained colloidal heterostructures were analyzed by dynamic light scattering (DLS) and scanning electron microscope (SEM) measurements. Moreover, the optical properties of hybrid nanostructure have been carefully studied. A splitting was observed in the extinction spectra of colloidal solutions for different Si/Au colloidal heterostructures compared to Si colloidal nanoparticles, attributed to the hybridization of plasmon. Thanks to photolitholographic and drop-cast deposition techniques on a substrate, we were able to locate the colloidal nanoparticles heterostructures and to measure the backward scattering spectra using single-particle dark-field scattering spectroscopy. To better understand scattering results, FDTD simulations of scattering cross-section and electric near-field distributions were performed. As is well-known, the scattering spectrum of the pristine Si core is dominated by electric and magnetic dipolar modes in the visible region. More importantly, the scattering spectra of Si/Au core–island and core–shell support several phenomena such as collective magnetic dipolar modes and multiple Fano lineshapes. Our finding highlights the potential of our dielectric–metal heterostructures as a platform for studying light–matter interactions at the nanoscale.

The plasmon resonances (modes) of a metal nanostructure can be defined as a dipole, a quadrupole, or high-order modes depending on the surface charge distribution induced by the incident field. In a nonsymmetrical environment or clusters, the modes can hybridize and exhibit different behavior and properties. In this work, we study experimentally and numerically the substrate-induced hybridization of plasmonic modes of a silver nanocylinder. The applications of plasmonic nanoparticles such as refractive index sensing and enhanced spectroscopies often rely on the sustained mode spectral position and specific spatial near-field distribution. However, we show that the implementation of such plasmonic nanoparticles in a sensing system can result in a change of the mode nature and its hybridization or dehybridization. These changes are not clearly pronounced in the far-field spectra and then may result in unexpected modifications of the sensor behavior. We show that the hybridization between the dipolar and quadrupolar modes of the plasmonic nanoparticle on the substrate results from quadrupolar mode excitation because of the superposition of the reflected and incoming light and, then, depends on the reflection of the substrate. The existence of the hybridized modes strongly depends on the surrounding environment of nanoparticles, and after the deposition of the nanometric polymer layer on top of the nanoparticle, the hybridized modes vanish and are replaced by uncoupled multipolar resonances.

Multi-resonant plasmonic simple geometries like nanocylinders and nanorods are highly interesting for two-photon photoluminescence and second harmonic generation applications, due to their easy fabrication and reproducibility in comparison with complex multi-resonant systems like dimers or nanoclusters. We demonstrate experimentally that by using a simple gold nanocylinder we can achieve a double resonantly enhanced two-photon photoluminescence of quantum dots, by matching the excitation wavelength of the quantum dots with a dipolar plasmon mode, while the emission is coupled with a radiative quadrupolar mode. We establish a method to separate experimentally the enhancement factor at the excitation and at the emission wavelengths for this double resonant system. The sensitivity of the spectral positions of the dipolar and quadrupolar plasmon resonances to the ellipticity of the nanocylinders and its impact on the two-photon photoluminescence enhancement are discussed.

We evaluate experimentally and theoretically the role of the residual ligands and ambient environment refractive index in the optical response of a single spherical gold nanoparticle on a substrate and demonstrate the changes in the near- and far-field properties of its hybridized modes in the presence of the cetyltrimethylammonium bromide (CTAB) layer. Particularly, we show that the conventional bilayer scheme for CTAB is not relevant for colloidal nanoparticles deposited on a substrate. We show that this CTAB layer considerably changes the amplitude and localization of the confinement of the electric field, which is of prime importance in the design of plasmonic complex systems coupled to emitters. Moreover, we numerically study the influence of the CTAB layer on the modification of sensitivity of plasmonic resonances of a gold nanopshere to local refractive index changes.

High index dielectric nanoparticles have been proposed for many different applications. However, widespread utilization in practice also requires large-scale production methods for crystalline silicon nanoparticles, with engineered optical properties in a low-cost manner. Here, we demonstrate a facile, low-cost, and large-scale fabrication method of crystalline silicon colloidal Mie resonators in water, using a blender. The obtained nanoparticles are polydisperse with an almost spherical shape and the diameters controlled in the range 100–200 nm by a centrifugation process. Then the size distribution of silicon nanoparticles enables broad extinction from UV to near-infrared, confirmed by Mie theory when considering the size distribution in the calculations. Thanks to photolithographic and drop-cast deposition techniques to locate the position on a substrate of the colloidal nanoparticles, we experimentally demonstrate that the individual silicon nanoresonators show strong electric and magnetic Mie resonances in the visible range.

A metallic nanocylinder is one of the most studied simple plasmonic nanostructures. Due to its internal symmetry, it can sustain different modes, but the majority of the studies focus on the horizontal dipolar resonance. Studying the whole variety of the excited modes in an individual nanocylinder is crucial to the development of further applications. Herein, we analyze the modes excited in a single gold nanocylinder on a glass substrate with standard optical characterization techniques and numerical simulations. The analysis of the scattering radiation patterns of the different modes shows that specific collection geometries are required to distinguish them. We propose a new method to record the hidden modes using a conventional dark-field scattering microscope, and for the first time, we show how the vertical dipolar mode of a single nanocylinder can be revealed optically.

It has been recently suggested that the nonlinear optical processes in plasmonic nanoantennas allow for a substantial boost in the sensitivity of plasmonic sensing platforms. Here, we present a sensing device based on an array of non-centrosymmetric plasmonic nanoantennas featuring enhanced second harmonic generation (SHG) integrated in a microfluidic chip. We evaluate its sensitivity both in the linear and nonlinear regime using a figure of merit (FOM=ΔI/IΔn)that accounts for the relative change in the measured intensity, I, against the variation of the environmental refractive index n. While the signal-to-noise ratio achieved in both regimes allows attaining a resolution (i.e., minimum detectable refractive index variation) Δnmin≈ 10–3, the platform operation in the nonlinear regime features a sensitivity (i.e., the FOM) that is at least 3 times higher than the linear one. Thanks to the surface sensitivity of plasmon-enhanced SHG, our results show that the development of such SHG sensing platforms with sensitivity performances exceeding those of their linear counterparts is within reach.

In this article we compare the two-photon photoluminescence and second harmonic generation from single connected gold nanodimers. Analyzing the particle size-dependent nonlinear optical spectra and performing excitation polarization resolved measurements using an experimental setup combining a femtosecond laser source with a parabolic mirror, we show that second harmonic generation and two-photon photoluminescence have different behaviors despite the same expected fundamental intensity-dependence. For further understanding of the observed phenomena, the plasmon resonances of single nanodimers are investigated using dark-field optical microscopy, and calculations are performed with Green’s tensor method. Furthermore, the underlying mechanisms explaining the differences between these two optical processes are investigated using a surface integral equation method for the nonlinear computations. This study reveals that the different trends in the polarization-dependences of two-photon photoluminescence and second harmonic generation with the increasing diameters of the connected discs are due to their distinct physical nature, resulting in specific rules for plasmon enhancement and different coherence properties. Furthermore, this article clearly points out that special care has to be taken when two-photon photoluminescence and second harmonic generation are used to evaluate the amplitudes of electromagnetic hot-spots generated in plasmonic nanostructures.

The plasmonic properties of single Au triangular nanoprisms are investigated using photoemission electron microscopy with particular emphasis on polarization dependence. Two localized surface plasmon resonances (LSPRs) are studied, namely, the in-plane dipolar and quadrupolar plasmon excitations. Experimental maps of the near-field spatial distribution upon polarization and wavelength of the exciting field are interpreted in the framework of a group theory description and finite difference time domain simulations. This work demonstrates the selective excitations, by lifting of degeneracy, of the different LSPR eigenmodes at the single object level and opens ways for the active control of the angular radiation patterns of optical nanoantennas. This approach is general and applies to any nano-object, whatever its initial shape symmetry.

We report on the quantitative characterization of the plasmonic optical near-field of a single silver nanoparticle. Our approach relies on nanoscale molecular molding of the confined electromagnetic field by photoactivated molecules. We were able to directly image the dipolar profile of the near-field distribution with a resolution better than 10 nm and to quantify the near-field depth and its enhancement factor. A single nanoparticle spectral signature was also assessed. This quantitative characterization constitutes a prerequisite for developing nanophotonic applications.

Metallic or dielectric nano-objects change the photon local density of states of closely placed emitters, particularly when plasmon or Mie resonances are present. Depending on the shape and material of these nano-objects, they may induce either a decrease or an increase in decay rates of the excited states of the emitter. In this work, we consider the reduction of the probability of optical transitions in emitters near high-refractive index dielectric (silicon and zinc selenide) nanoparticles. We tune the spectral positions of magnetic and electric modes of nanocylinders to obtain the largest overlap of the valleys in the total decay rate spectra for differently oriented dipoles and, in this way, find the highest inhibition of about 80% for randomly oriented emitters. The spectral positions of these valleys are easy to control since the wavelengths of the modes depend on the height and diameter of nanocylinders. The inhibition value is robust to the distance between the emitter and the nanoparticle in the range of nearly 50 nm, which is crucially important for the applications, such as selective optical transition engineering and photovoltaics.