Carlos E. Roman-Velazquez

Controlled anisotropic deformation of Ag nanoparticles by Si ion irradiation

The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance unambiguously show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.

Optical circular dichroism of single-wall carbon nanotubes

The circular dichroism (CD) spectra of single-wall carbon nanotubes are calculated using a dipole approximation. The calculated CD spectra show features that allow us to distinguish between nanotubes with different angles of chirality, and diameters. These results provide theoretical support for the quantification of chirality and its measurement, using the CD line shapes of chiral nanotubes. It is expected that this information would be useful to motivate further experimental studies.

Designing the plasmonic response of shell nanoparticles: Spectral representation

A spectral representation formalism in the quasistatic limit is developed to study the optical response of nanoparticles, such as nanospheres, nanospheroids, and concentric nanoshells. A transfer matrix theory is formulated for systems with an arbitrary number of shells. The spectral representation formalism allows us to analyze the optical response in terms of the interacting surface plasmons excited at the interfaces by separating the contributions of the geometry from those of the dielectric properties of each shell and surroundings. Neither numerical nor analytical methods can do this separation. These insights into the physical origin of the optical response of multishelled nanoparticles are very useful for engineering systems with desired properties for applications in different fields ranging from materials science and electronics to medicine and biochemistry.

Theoretical Study of Surface Plasmon Resonances in Hollow Gold-Silver Double-Shell Nanostructures †

A theoretical model has been developed to study the optical properties of metallic multishell structures on the nanometer scale. The Mie theory was generalized for multiconcentric spherical shell nanostructures and employed to determine the effects and importance of the different parameters of the system such as thickness, size, and other material properties, for instance, the medium index of refraction. A unique hollow gold-silver double-shell structure is used as an example to test the model developed with recent experiments. The surface plasmon resonance (SPR) absorption spectrum of this structure has been calculated as a function of various parameters, including shell thickness and diameter. Using parameters similar to those previously reported experimentally, very good agreement has been found between calculated and experimentally measured SPR spectra, which validates the model. The results provide new insights into the fundamental properties of complex metal nanostructures that give us the ability to control the optical response, which has important implications in the synthesis of new metal nanostructures as well as their application in emerging technologies.

High-multipolar effects on the Casimir force: The non-retarded limit

We show that the dispersive force between a spherical nanoparticle (with a radius

Dispersive force between dissimilar materials: Geometrical effects

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Spectral representation of the nonretarded dispersive force between a sphere and a substrate

100 nm) and a substrate is enhanced by several orders of magnitude when the sphere is near to the substrate. We calculate exactly the dispersive force in the non-retarded limit by incorporating the contributions to the interaction from of all the multipolar electromagnetic modes. We show that as the sphere approaches the substrate, the fluctuations of the electromagnetic field, induced by the vacuum and the presence of the substrate, the dispersive force is enhanced by orders of magnitude. We discuss this effect as a function of the size of the sphere.We implement a spectral representation formalism to calculate exactly the non-retarded Casimir force or dispersive van der Waals force, between a spherical nanoparticle and a planar substrate beyond the dipolar approximation. For a sphere of radius R separated a distance z from the substrate, we find that high-order multipole interactions induce extra factors R/z in the force, as compared to the dipole power law of ~ R3/z3. As a consequence, at small separations the non-retarded Casimir force increases by several orders of magnitude with respect to that estimated in the usual dipole approximation.

Multipolar plasma resonances in supported alkali-metal nanoparticles

We calculate the Casimir force or dispersive van der Waals force between a spherical nanoparticle and a planar substrate, both with arbitrary dielectric properties. We show that the force between the sphere and half-space can be calculated through the interacting surface plasmons of the bodies. Using a Spectral Representation formalism, we show that the force of a sphere made of a material A and a half-space made of a material B differs from the case when the sphere is made of B, and the half-space is made of A. We find that the difference depends on the plasma frequency of the materials, the geometry, and the distance of separation between the sphere and half-space. The differences show the importance of the geometry, and make evident the necessity of realistic descriptions of the system beyond the Derjaguin Approximation or Proximity Theorem Approximation.We calculate exactly the Casimir force between a spherical particle and a plane, both with arbitrary dielectric properties, in the non-retarded limit. Using a Spectral Representation formalism, we show that the Casimir force of a sphere made of a material A and a plane made of a material B, differ from the case when the sphere is made of B, and the plane is made of A. The differences in energy and force show the importance of the geometry, and make evident the necessity of realistic descriptions of the sphere-plane system beyond the Proximity Theorem approximation.

The role of geometry on dispersive forces

We develop a spectral representation formalism to calculate the nonretarded Casimir force between a spherical particle and a substrate, both with arbitrary local dielectric properties. The spectral formalism allows us to study systematically such force as a function of its geometrical properties separately from its dielectric properties. The calculated force is attractive, and at a small separations it is orders of magnitude larger for nanometric-size spheres than for micrometer particles. We also found that the force depends more on the dielectric properties of the sphere than of the substrate.

Circular dichroism simulated spectra of chiral gold nanoclusters: A dipole approximation

Abstract Multipolar effects in the polarizability of metallic potassium particles on a silicon substrate were studied using differential reflectance spectroscopy. The experimental spectra were compared with calculations of the effective polarizability of particles of different shapes leading to the conclusion that the resonances in the spectra correspond to excitations of substrate-induced multipolar modes in the particle–substrate system.