Leonardo A. Ambrosio

Integral localized approximation description of ordinary Bessel beams and application to optical trapping forces

Ordinary Bessel beams are described in terms of the generalized Lorenz-Mie theory (GLMT) by adopting, for what is to our knowledge the first time in the literature, the integral localized approximation for computing their beam shape coefficients (BSCs) in the expansion of the electromagnetic fields. Numerical results reveal that the beam shape coefficients calculated in this way can adequately describe a zero-order Bessel beam with insignificant difference when compared to other relative time-consuming methods involving numerical integration over the spherical coordinates of the GLMT coordinate system, or quadratures. We show that this fast and efficient new numerical description of zero-order Bessel beams can be used with advantage, for example, in the analysis of optical forces in optical trapping systems for arbitrary optical regimes.

Gradient forces on double-negative particles in optical tweezers using Bessel beams in the ray optics regime.

Gradient forces on double negative (DNG) spherical dielectric particles are theoretically evaluated for v-th Bessel beams supposing geometrical optics approximations based on momentum transfer. For the first time in the literature, comparisons between these forces for double positive (DPS) and DNG particles are reported. We conclude that, contrary to the conventional case of positive refractive index, the gradient forces acting on a DNG particle may not reverse sign when the relative refractive index n goes from |n|>1 to |n|<1, thus revealing new and interesting trapping properties.

Radiation pressure cross sections and optical forces over negative refractive index spherical particles by ordinary Bessel beams

When impinged by an arbitrary laser beam, lossless and homogeneous negative refractive index (NRI) spherical particles refract and reflect light in an unusual way, giving rise to different scattered and internal fields when compared to their equivalent positive refractive index particles. In the generalized Lorenz-Mie theory, the scattered fields are dependent upon the Mie scattering coefficients, whose values must reflect the metamaterial behavior of an NRI scatterer, thus leading to new optical properties such as force and torque. In this way, this work is devoted to the analysis of both radial and longitudinal optical forces exerted on lossless and simple NRI particles by zero-order Bessel beams, revealing how the force profiles are changed whenever the refractive index becomes negative.

Fundamentals of negative refractive index optical trapping: forces and radiation pressures exerted by focused Gaussian beams using the generalized Lorenz-Mie theory

Based on the generalized Lorenz-Mie theory (GLMT), this paper reveals, for the first time in the literature, the principal characteristics of the optical forces and radiation pressure cross-sections exerted on homogeneous, linear, isotropic and spherical hypothetical negative refractive index (NRI) particles under the influence of focused Gaussian beams in the Mie regime. Starting with ray optics considerations, the analysis is then extended through calculating the Mie coefficients and the beam-shape coefficients for incident focused Gaussian beams. Results reveal new and interesting trapping properties which are not observed for commonly positive refractive index particles and, in this way, new potential applications in biomedical optics can be devised.

Diffraction–attenuation resistant beams: their higher-order versions and finite-aperture generations

Recently, a method for obtaining diffraction-attenuation resistant beams in absorbing media has been developed in terms of suitable superposition of ideal zero-order Bessel beams. In this work, we show that such beams keep their resistance to diffraction and absorption even when generated by finite apertures. Moreover, we shall extend the original method to allow a higher control over the transverse intensity profile of the beams. Although the method is developed for scalar fields, it can be applied to paraxial vector wave fields, as well. These new beams have many potential applications, such as in free-space optics, medical apparatus, remote sensing, and optical tweezers.

Trapping double negative particles in the ray optics regime using optical tweezers with focused beams

The capabilities of optical tweezers to trap DNG (double negative) spherical particles, with both negative permittivity and permeability, are explored in detail by analyzing some interesting theoretical features not seeing in conventional DPS (double positive) particles possessing positive refractive index. The ray optics regime is adopted and, although this regime is quite simple and limited, its validity is already known and tested for DPS particles such as biological cells and molecules trapped by highly focused beams. Simulation results confirm that even for ray optics, DNG particles present unusual and interesting trapping characteristics.

Inversion of gradient forces for high refractive index particles in optical trapping

The unexpected fact that a spherical dielectric particle with refractive index higher than the surrounding medium will not always be attracted towards high intensity regions of the trapping beam is fully demonstrated here using a simple ray optics approach. This unusual situation may happen due to the inversion of gradient forces, as shown here. Therefore, conventional schemes, such the one based on the use of two counter-propagating beams to cancel the scattering forces, will fail to trap the particle. However, effective trapping still can be obtained by adopting suitable incident laser beams.

Spin angular momentum transfer from TEM 00 focused Gaussian beams to negative refractive index spherical particles

We investigate optical torques over absorbent negative refractive index spherical scatterers under the influence of linear and circularly polarized TEM00 focused Gaussian beams, in the framework of the generalized Lorenz-Mie theory with the integral localized approximation. The fundamental differences between optical torques due to spin angular momentum transfer in positive and negative refractive index optical trapping are outlined, revealing the effect of the Mie scattering coefficients in one of the most fundamental properties in optical trapping systems.

Nanocircuits and Nanoimpedances of Nonmagnetic Plasmonic Nanoparticles From the Mie Theory Point of View

In this study, we show how the Mie theory fits into the recently proposed nanocircuitry for a spherical nanoscatterer. Because this theory must give the same results for the scattering problem as those provided by the dipole approximation, regardless of any particular electromagnetic response functions, a comparison is made between these two approaches for the internal and fringing nanoimpedances associated with the scattering of dielectric and plasmonic spheres. It is shown that, because of plasmonic resonances observed when the real part of the permittivity assumes a negative value, the simple dipole model, usually reliably adopted in the conventional positive refractive index case, must be used with care. This extends some previous works and provides a more complete picture of the physical nanoimpedance associated with the fringing fields, specially at the plasmonic resonances.

RLC Circuit Model for the Scattering of Light by Small Negative Refractive Index Spheres

We study the scattering properties of very small spheres made of a negative refractive index (NRI) using circuit network principles. The Lorenz-Mie theory is used to represent the transverse magnetic and transverse electric scattering coefficients based on resistor, inductor, and capacitor (RLC) ladder circuit models derived from continued fractions. The lumped elements are then analyzed and a comparison is made between these new elements and those expected for conventional positive refractive index spheres. This provides an alternative interpretation for the fact that the scattering efficiency Qsca for NRI spheres with size parameters x <;<; 1 does not always obey Rayleighs formula Qsca ~ x4. The RLC circuits obtained here, besides being very simple, provide accurate physical insights into the problem of the scattering of light by NRI spheres, therefore serving as a new theoretical background for studying the scattering properties of negative refractive index metamaterials.