Ovidio Peña-Rodríguez

Enhanced plasmonic behavior of bimetallic (Ag-Au) multilayered spheres

In this article we study the plasmonic behavior of some stable, highly biocompatible bimetallic metal-dielectric-metal (MDM) and double concentric nanoshell (DCN) structures. By simply switching the material of the inner structure from Au to Ag, the intensity of their surface plasmon resonance could be increased in the optical transparency region of the human tissues up to 20 and 60 percent for the MDM and DCN, respectively, while the biocompatibility is retained. The obtained results indicate that these novel structures could be highly suitable for surface enhanced Raman scattering and photothermal cancer therapy.

Femtosecond Laser-Controlled Tip-to-Tip Assembly and Welding of Gold Nanorods.

Directed assembly of gold nanorods through the use of dithiolated molecular linkers is one of the most efficient methodologies for the morphologically controlled tip-to-tip assembly of this type of anisotropic nanocrystals. However, in a direct analogy to molecular polymerization synthesis, this process is characterized by difficulties in chain-growth control over nanoparticle oligomers. In particular, it is nearly impossible to favor the formation of one type of oligomer, making the methodology hard to use for actual applications in nanoplasmonics. We propose here a light-controlled synthetic procedure that allows obtaining selected plasmonic oligomers in high yield and with reaction times in the scale of minutes by irradiation with low fluence near-infrared (NIR) femtosecond laser pulses. Selective inhibition of the formation of gold nanorod n-mers (trimers) with a longitudinal localized surface plasmon in resonance with a 800 nm Ti:sapphire laser, allowed efficient trapping of the (n – 1)-mers (dimers) by hot spot mediated photothermal decomposition of the interparticle molecular linkers. Laser irradiation at higher energies produced near-field enhancement at the interparticle gaps, which is large enough to melt gold nanorod tips, offering a new pathway toward tip-to-tip welding of gold nanorod oligomers with a plasmonic response at the NIR. Thorough optical and electron microscopy characterization indicates that plasmonic oligomers can be selectively trapped and welded, which has been analyzed in terms of a model that predicts with reasonable accuracy the relative concentrations of the main plasmonic species.

MieLab: A Software Tool to Perform Calculations on the Scattering of Electromagnetic Waves by Multilayered Spheres

In this paper, we present MieLab, a free computational package for simulating the scattering of electromagnetic radiation by multilayered spheres or an ensemble of particles with normal size distribution. It has been designed as a virtual laboratory, including a friendly graphical user interface (GUI), an optimization algorithm (to fit the simulations to experimental results) and scripting capabilities. The paper is structured in five different sections: the introduction is a perspective on the importance of the software for the study of scattering of light scattering. In the second section, various approaches used for modeling the scattering of electromagnetic radiation by small particles are discussed. The third and fourth sections are devoted to provide an overview of MieLab and to describe the main features of its architectural model and functional behavior, respectively. Finally, several examples are provided to illustrate the main characteristics of the software.

Polyrotaxane-Mediated Self-Assembly of Gold Nanospheres into Fully Reversible Supercrystals†

The use of a thiol-functionalized nonionic surfactant to stabilize spherical gold nanoparticles in water induces the spontaneous formation of polyrotaxanes at the nanoparticle surface in the presence of the macrocycle α-cyclodextrin. Whereas using an excess of surfactant an amorphous gold nanocomposite is obtained, under controlled drying conditions the self-assembly between the surface supramolecules provides large and homogenous supercrystals with hexagonal close packing of nanoparticles. Once formed, the self-assembled supercrystals can be fully redispersed in water. The reversibility of the crystallization process may offer an excellent reusable material to prepare gold nanoparticle inks and optical sensors with the potential to be recovered after use.

Femtosecond laser reshaping yields gold nanorods with ultranarrow surface plasmon resonances

Laser-shaping nanoparticles For many applications of the plasmon resonances of metal nanoparticles, it is necessary to have narrow resonance lines. However, most methods for synthesizing nanoparticles create a distribution of sizes and shapes that broaden the resonance lines. González-Rubio et al. annealed gold nanorods dispersed in an aqueous solution of a surfactant with carefully tuned ultrafast (femtosecond) laser pulses. This approach reshaped the nanoparticles to create a near-uniform distribution with resonance lines nearly as sharp as for a single nanorod. Science, this issue p. 640 Heating nanoparticles with femtosecond laser pulses can reshape them and sharpen their plasmon resonances. The irradiation of gold nanorod colloids with a femtosecond laser can be tuned to induce controlled nanorod reshaping, yielding colloids with exceptionally narrow localized surface plasmon resonance bands. The process relies on a regime characterized by a gentle multishot reduction of the aspect ratio, whereas the rod shape and volume are barely affected. Successful reshaping can only occur within a narrow window of the heat dissipation rate: Low cooling rates lead to drastic morphological changes, and fast cooling has nearly no effect. Hence, a delicate balance must be achieved between irradiation fluence and surface density of the surfactant on the nanorods. This perfection process is appealing because it provides a simple, fast, reproducible, and scalable route toward gold nanorods with an optical response of exceptional quality, near the theoretical limit.

Kinetics of color center formation in silica irradiated with swift heavy ions: Thresholding and formation efficiency

We have determined the cross-section σ for color center generation under single Br ion impacts on amorphous SiO2. The evolution of the cross-sections, σ(E) and σ(Se), show an initial flat stage that we associate to atomic collision mechanisms. Above a certain threshold value (Se > 2 keV/nm), roughly coinciding with that reported for the onset of macroscopic disorder (compaction), σ shows a marked increase due to electronic processes. In this regime, a energetic cost of around 7.5 keV is necessary to create a non bridging oxygen hole center-E′ (NBOHC/E′) pair, whatever the input energy. The data appear consistent with a non-radiative decay of self-trapped excitons.

Ionoluminescence as Sensor of Structural Disorder in Crystalline SiO2: Determination of Amorphization Threshold by Swift Heavy Ions

Ionoluminescence (IL) has been used in this work as a sensitive tool to probe the microscopic electronic processes and structural changes produced on quartz by the irradiation with swift heavy ions. The IL yields have been measured as a function of irradiation fluence and electronic stopping power. The results are consistent with the assignment of the 2.7 eV (460 nm) band to the recombination of self-trapped excitons at the damaged regions in the irradiated material. Moreover, it was possible to determine the threshold for amorphization by a single ion impact, as ?1.7 keV/nm, which agrees well with the results of previous studies.

Optical Waveguides Fabricated by Ion Implantation/Irradiation: A Review Optical Waveguides Fabricated by Ion Implantation/Irradiation: A Review

Ovidio Pena-Rodriguez1,2, Jose Olivares1,2, Mercedes Carrascosa3, Angel Garcia-Cabanes3, Antonio Rivera4 and Fernando Agullo-Lopez1 1Centro de Microanalisis de Materiales (CMAM), Universidad Autonoma de Madrid (UAM), Cantoblanco, Madrid 2Instituto de Optica, Consejo Superior de Investigaciones Cientificas (IO-CSIC), Madrid 3Departamento de Fisica de Materiales C-IV, Universidad Autonoma de Madrid, Madrid 4Instituto de Fusion Nuclear, Universidad Politecnica de Madrid, Madrid Spain

Reduction of scattering using thin all-dielectric shells designed by stochastic optimizer

Adaptive differential evolution method has been used for optimization of all-dielectric multilayer coatings in order to reduce total scattering from spherical targets. The optimal refractive index profiles have been found for various sizes of targets and thicknesses of coatings. Few profile types that appear to be optimal for various geometrical parameters have been identified. Scattering of the target with diameter of 0.75 λ has been reduced by 85% using 0.16 λ thick coating formed by isotropic dielectric materials. For larger targets, scattering reduction becomes smaller, but it still reaches 50% for targets with the diameter of 1.5 λ. The obtained designs provide a route to implement cloaking without the use of magnetic and anisotropic metamaterials.

Understanding the ion-induced elongation of silver nanoparticles embedded in silica

In this work we have studied the elongation of silver nanoparticles irradiated with 40 MeV Bromine ions by means of in situ optical measurements, transmission electron microscopy and molecular dynamics simulations. The localized surface plasmon resonance of silver nanoparticles has a strong dependence on the particle shape and size, which allowed us to obtain the geometrical parameters with remarkable accuracy by means of a fit of the optical spectra. Optical results have been compared with transmission electron microscopy images and molecular dynamics simulations and the agreement is excellent in both cases. An important advantage of in situ measurements is that they yield an extremely detailed information of the full elongation kinetics. Final nanoparticle elongation depends on a complex competition between single-ion deformation, Ostwald ripening and dissolution. Building and validating theoretical models with the data reported in this work should be easier than with the information previously available, due to the unprecedented level of kinetic details obtained from the in situ measurements.