Evidence for cascaded third harmonic generation in non-centrosymmetric gold nanoantennas

The optimization of nonlinear optical processes at the nanoscale is a crucial step for the development of nanoscale photon sources for quantum-optical networks. The development of innovative plasmonic nanoantenna designs and hybrid nanostructures to enhance optical nonlinearities in very small volumes represents one of the most promising routes. In such systems, the upconversion of photons can be achieved with high efficiencies via third-order processes, such as third harmonic generation (THG), thanks to the resonantly-enhanced volume currents. Conversely, second-order processes, such as second harmonic generation (SHG), are often inhibited by the symmetry of metal lattices and of common nanoantenna geometries. SHG and THG processes in plasmonic nanostructures are generally treated independently, since they both represent a small perturbation in the light-matter interaction mechanisms. In this work, we demonstrate that this paradigm does not hold in general, by providing evidence of a cascaded process in THG, which is fueled by SHG and sizably contributes to the overall yield. We address this mechanism by unveiling an anomalous fingerprint in the polarization state of the nonlinear emission from non-centrosymmetric gold nanoantennas and point out that such cascaded processes may also appear for structures that exhibit only moderate SHG yields - signifying its general relevance in plasmon-enhanced nonlinear optics. The presence of this peculiar mechanism in THG from plasmonic nanoantennas at telecommunication wavelengths allows gaining further insight on the physics of plasmon-enhanced nonlinear optical processes. This could be crucial in the realization of nanoscale elements for photon conversion and manipulation operating at room-temperature.Comment: 25 pages, 4 figure

Measuring and modelling the absolute optical cross- sections of individual nano-objects

Nanoparticles are ubiquitous in nature, and the number of technological applications exploiting nano-objects, either synthesized chemically or fabricated lithographically, is in steady rise. In particular, metal nano-objects exhibit resonant modes corresponding to an enhanced coupling to electromagnetic radiation. The interaction of light with a nano-object is wholly described by its cross-sections for absorption and elastic scattering. In this thesis we present a method to measure the absolute amplitude of the cross-sections. Differently from currently available techniques, we account for the finite angular collection of the objective via an analytical model of the scattering process, thereby rendering our method accurate also for objects dominated by scattering and high numerical aperture detection. The model of scattering assumes that the nano-object is placed at a planar dielectric interface, representing the substrate, and a homogeneous optical environment is obtained as a limiting case. The accuracy of the quantitative method was tested on several model systems using two widespread experimental techniques: Micro-spectroscopy and widefield imaging, which are both implemented with a simple experimental set-up, constituted by a commercial microscope equipped with an imaging spectrometer or a camera. In order to quantitatively simulate microscopy experiments, a realistic description of the excitation must be included in numerical models. In this thesis we describe novel modelling practices which reproduce typical coherent or incoherent microscope illumination. Comparison of quantitative experimental and numerical results is used to estimate parameters describing the geometry of a nano-object, such as the diameter or the aspect ratio. In conjunction with the high-throughput capabilities of widefield image analysis, quantitative cross-section measurements and optical characterization of the geometry can provide a thorough statistical appraisal of the dispersity of the structural and optical properties of a sample. Therefore, this thesis represents a significant step towards an ‘all-optical’ characterization of nano-objects, complementing costly and time-consuming electron microscopy techniques

Tailoring core size, shell thickness, and aluminium doping of Au@ZnO core@shell nanoparticles

Plasmonic materials, such as gold nanoparticles (AuNPs), exhibit significant extinction and near-field enhancement across the visible and near-infrared spectrum, attributable to localized surface plasmon resonances (LSPRs). Epsilon-near-zero (ENZ) materials, such as aluminium doped zinc oxide (AZO) are known in non-linear optics for their ability to generate and manipulate light-matter interactions through processes like higher harmonic generation.Combining doped ZnO with plasmonic materials therefore holds promise for enhancing non-linear efficiencies and tuning their operational wavelengths. To date, however, only top-down structures based on plasmonically decorated thin ENZ films have been realized, and no colloidal and scalable route to obtain these hybrid materials has been reported yet. Here, we introduce a novel colloidal synthesis approach for fabricating Au@AZO core@shell nanoparticles with tunable core size, shell thickness, and dopant concentration, allowing for the spectral alignment of the LSPRs of the AuNPs with the non-linear optical properties of the AZO shells. Our method involves the colloidal syntheis of gold cores followed by an ascorbic acid-assisted process to deposit polycristalline ZnO and AZO shells, resulting in core diameters ranging from 25 to 69 nm, shell thicknesses from 16 to 47 nm, and aluminium doping levels between 0 and 4 at%. Our procedure widens the range of hybrid plasmonic nanostructures that can be colloidally synthesised, opening new possibilities for the large scale fabrication of high-performance nanomaterials for integration in photonic, photocatalytic, and sensing applications

Quantitative optical microspectroscopy, electron microscopy, and modelling of individual silver nanocubes reveals surface compositional changes at the nanoscale

The optical response of metal nanoparticles is governed by plasmonic resonances, which depend often intricately on the geometry and composition of the particle and its environment. In this work we describe a method and analysis pipeline unravelling these relations at the single nanoparticle level through a quantitative characterization of the optical and structural properties. It is based on correlating electron microscopy with micro-spectroscopy measurements of the same particle immersed in media of different refractive index. The optical measurements quantify the magnitude of both the scattering and the absorption cross sections, while the geometry measured in electron microscopy is used for numerical simulations of the cross section spectra accounting for the experimental conditions. We showcase the method on silver nanocubes of nominal 75nm edge size. The large amount of information afforded by the quantitative cross section spectra, and measuring the same particle in two environments, allows us to identify a specific degradation of the cube surface. We find a layer of tarnish, only a few nanometers thick, a fine surface compositional change of the studied system which would be hardly quantifiable otherwise

Quantitative measurement of the optical cross-sections of single nano-objects by correlative transmission and scattering micro-spectroscopy

The scattering and absorption of light by nano-objects is a key physical property exploited in many applications, including biosensing and photovoltaics. Yet, its quantification at the single object level is challenging, and often requires expensive and complicated techniques. We report a method based on a commercial transmission microscope to measure the optical scattering and absorption cross-sections of individual nano-objects. The method applies to micro-spectroscopy and wide-field image analysis, offering fine spectral information and high throughput sample characterization. Accurate cross-section determination requires a detailed modeling of the measurement, which we develop, accounting for the geometry of the illumination and detection, as well as for the presence of a sample substrate. We demonstrate the method on three model systems (gold spheres, gold rods, and polystyrene spheres), which include metallic and dielectric particles, spherical and elongated, placed in a homogeneous medium or on a dielectric substrate. Furthermore, by comparing the measured cross-sections with numerical simulations, we are able to determine structural parameters of the studied system, such as the particle diameter and aspect ratio. Our method therefore holds the potential to complement electron microscopy as a simpler and cost-effective tool for structural characterization of single nano-objects

Quantitative high-throughput optical sizing of individual colloidal nanoparticles by wide-field imaging extinction microscopy

We present a wide-field imaging technique recently developed by us to measure quantitatively the optical extinction cross section σext of individual nanoparticles. The technique is simple, high speed, and enables the simultaneous acquisition of hundreds of nanoparticles in the wide-field image for statistical analysis, with a sensitivity corresponding to the detection of a single gold nanoparticle down to 2nm diameter. Notably, the method is applicable to any nanoparticle (dielectric, semiconducting, metallic), and can be easily and cost-effectively implemented on a conventional wide-field microscope. Of specific significance for accurate quantification, we show that σext depends on the numerical aperture of the microscope illumination due to the oblique incidence, even for spherical particles in an isotropic environment. This "long shadow" effect needs to be taken into account when comparing σext to theoretical values calculated under plane wave illumination at normal incidence. Owing to the accurate experimental quantification of σext, one can then use it to determine the nanoparticle size, as demonstrated here on gold nanoparticles of 30nm nominal diameter. This technique thus has the potential to become a simple and cost-effective new tool for accurate size characterization of single small nanoparticles, complementing time consuming and expensive methods such as electron microscopy

Quantitative morphometric analysis of single gold nanoparticles by optical extinction microscopy: material permittivity and surface damping effects

Quantifying the optical extinction cross section of a plasmonic nanoparticle has recently emerged as a powerful means to characterize the nanoparticle morphologically, i.e., to determine its size and shape with a precision comparable to electron microscopy while using a simple optical microscope. In this context, a critical piece of information to solve the inverse problem, namely, calculating the particle geometry from the measured cross section, is the material permittivity. For bulk gold, many datasets have been reported in the literature, raising the question of which one is more adequate to describe specific systems at the nanoscale. Another question is how the nanoparticle interface, not present in the bulk material, affects its permittivity. In this work, we have investigated the role of the material permittivities on the morphometric characterization of defect-free ultra-uniform gold nanospheres with diameters of 10 nm and 30 nm, following a quantitative analysis of the polarization- and spectrally-resolved extinction cross section on hundreds of individual nanoparticles. The measured cross sections were fitted using an ellipsoid model. By minimizing the fit error or the variation of the fitted dimensions with color channel selection, the material permittivity dataset and the surface damping parameter g best describing the nanoparticles are found to be the single crystal dataset by Olmon et al. [Phys. Rev. B 86, 235147 (2012)] and g ≈ 1, respectively. The resulting nanoparticle geometries are in good agreement with transmission electron microscopy of the same sample batches, including both 2D projection and tomography

Manipulating Light with Tunable Nanoantennas and Metasurfaces

The extensive progress in nanofabrication techniques enabled innovative methods for molding light at the nanoscale. Subwavelength structured optical elements and, in general, metasurfaces and metamaterials achieved promising results in several research areas, such as holography, microscopy, sensing and nonlinear optics. Still, a demanding challenge is represented by the development of innovative devices with reconfigurable optical properties. Here, we review recent achievements in the field of tunable metasurface. After a brief general introduction about metasurfaces, we will discuss two different mechanisms to implement tunable properties of optical elements at the nanoscale. In particular, we will first focus on phase-transition materials, such as vanadium dioxide, to tune and control the resonances of dipole nanoantennas in the near-infrared region. Finally, we will present a platform based on an AlGaAs metasurface embedded in a liquid crystal matrix that allows the modulation of the generated second harmonic signal