Rodrigo Alcaraz de la Osa

Rhodium Nanoparticles for Ultraviolet Plasmonics

The nonoxidizing catalytic noble metal rhodium is introduced for ultraviolet plasmonics. Planar tripods of 8 nm Rh nanoparticles, synthesized by a modified polyol reduction method, have a calculated local surface plasmon resonance near 330 nm. By attaching p-aminothiophenol, local field-enhanced Raman spectra and accelerated photodamage were observed under near-resonant ultraviolet illumination, while charge transfer simultaneously increased fluorescence for up to 13 min. The combined local field enhancement and charge transfer demonstrate essential steps toward plasmonically enhanced ultraviolet photocatalysis.

Quantum optical response of metallic nanoparticles and dimers.

The optical properties of metallic nanoparticles (NPs) can be described with analytical models based on fundamental quantum mechanical principles, of which the Drude model constitutes the classical limit. Here, we examine the plasmonic properties of silver and gold nanospheres and dimers, with radii ranging from 10 to 1 nm, extending from the classically described regime to the quantum size regime. We have studied the spectral extinction cross section by using the T-matrix method. The results indicate an increasingly substantial change in NP permittivity as the radius is reduced below 5 nm, showing a clear blueshift and weakening of the plasmon resonances for both silver and gold. As a consequence, we observe a dramatic change in the interaction of dimers, especially in the case of gold, where the introduction of quantum mechanically corrected optical properties quenches the plasmonic resonance and predicts an absence of the expected associated redshift.

Frequency shift between near- and far-field scattering resonances in dielectric particles.

The near-field electromagnetic scattering intensity resonances are redshifted in frequency with respect to their far-field counterparts. We derive simple, approximate, analytical formulas for this shift in the case of a plane wave interacting with a dielectric sphere. Numerical results comparing the approximate formulas to the numerically exact solutions show that the two are in good agreement. We also consider the Rayleigh limit of the formulas to gain more insight into the phenomenon.

Recent advances in metals for plasmonics applications in the UV range

Plasmonics in the UV-range constitutes a new focus of research due to new challenges arising in fields such as biology, chemistry or spectroscopy. Very recent studies point out gallium and rhodium as good candidates for these purposes because of their low oxidation tendency and at the same time, having a good plasmonic response in the UV and excellent photocatalytic properties. Here we present an overview of the current state of UV-plasmonics with our latest findings in the plasmonic activity of materials like gallium and rhodium.

The extended Kubelka-Munk theory and its application to colloidal systems

The use of nanoparticles is spreading in many fields and a frequent way of preparing them is in the form of colloids, whose characterization becomes increasingly important. The spectral reflectance and transmittance curves of such colloids exhibit a strong dependence with the main parameters of the system. By means of a two-flux model we have performed a colorimetric study of gold colloids varying several parameters of the system, including the radius of the particles, the particle number density, the thickness of the system and the refractive index of the surrounding medium. In all cases, trajectories in the L*a*b* color space have been obtained, as well as the evolution of the luminosity, chroma and hue, either for reflectance or transmittance. The observed colors agree well with typical colors found in the literature for colloidal gold, and could allow for a fast assessment of the parameters involved, e.g., the radius of the nanoparticle during the fabrication process.

Modelling metal-dielectric core-shell nanoparticles with effective medium theories

A new method for calculating the effective dielectric function of a metal-oxide core-shell nanoparticle is presented and compared with existing theories. This new approach can be helpful for predicting the reflectance, transmittance and absorbance spectra of core-shell colloids and nanocomposites which are widely used in photocatalysis or solar energy harvesting.

Using linear polarization for sensing and monitoring nanoparticle purity

We analyze the effect of contaminants on the quadrupolar magnetic, dipolar electric and dipolar magnetic resonances of silicon nanoparticles (NPs) by considering the spectral evolution of the linear polarization degree at right angle scattering configuration, PL(90°). From an optical point of view, a decrease in the purity of silicon nanoparticles due to the presence of contaminants impacts the NP effective refractive index. We study this effect for a silicon nanosphere of radius 200 nm embedded in different media. The weakness of the resonances induced on the PL(90°) spectrum because of the lack of purity can be used to quantify the contamination of the material. In addition, it is shown that Kerker conditions also suffer from a spectral shift, which is quantified as a function of material purity.

Polarimetric techniques for determining morphology and optical features of high refractive index dielectric nanoparticle size

The spectral evolution of the degree of linear polarization (PL) at a scattering angle of 90° is studied numerically for high refractive index (HRI) dielectric spherical nanoparticles. The behaviour of PL(90°) is analyzed as a function of the refractive index of the surrounding medium and the particle radius. We focus on the spectral region where both electric and magnetic resonances of order not higher than two are located for various semiconductor materials with low absorption. The spectral behavior of PL(90°) has only a small, linear dependence on nanoparticle size R. This weak dependence makes it experimentally feasible to perform real-time retrievals of both the refractive index of the external medium and the NP size R. From an industrial point of view, pure materials are nonrealistic, since they can only be provided under certain conditions. For this reason, we also study the effect of contaminants on the resonances of silicon NPs by considering the spectral evolution of PL(90°).

Near- and far-field scattering resonance frequency shift in dielectric and perfect electric conducting cylinders

The ability to infer near-field scattering properties from far-field measurements is of paramount importance in nano-optics. Recently we derived an approximate formula for predicting the frequency shift between near- and far-field intensity peaks in the case of a dielectric sphere. In this work we demonstrate that almost an identical formula can be used to predict the resonance shift of a dielectric cylinder and a perfectly conducting cylinder. We find the redshift of the resonance peak of the perfect electric conducting cylinder to be approximately 2 orders of magnitude greater than for the dielectric cylinder. The errors in our approximate analytic formula for predicting the redshift are approximately only twice as great. Furthermore, we apply the redshift formula to a silicon cylinder and discuss its magneto-dielectric properties, which may be of interest in design of metamaterials.

Metals for UV Plasmonics

Electromagnetic enhancement of metallic nanoparticles on substrates is assessed by an exhaustive numerical analysis. The potential of each technologically promising metal for UV-plasmonic applications may be ascertained.