David E. Fernandes

Optical tractor beam with chiral light

We suggest a novel mechanism to induce the motion of a chiral material body towards an optical source. Our solution is based on the interference between a chiral light beam and its reflection on an opaque mirror. Surprisingly, it is theoretically shown that the electromagnetic response of the material may be tailored in such a way that independent of the specific body location with the respect to the mirror, it is always pushed upstream against the photon flow associated with the incoming wave. Moreover, it is proven that by controlling the handedness of the incoming light it may be possible to harness the sign of the optical force, switching from a pulling force to a pushing force.

Analytical Solution for the Stopping Power of the Cherenkov Radiation in a Uniaxial Nanowire Material

We derive closed analytical formulae for the power emitted by moving charged particles in a uniaxial wire medium by means of an eigenfunction expansion. Our analytical expressions demonstrate that, in the absence of material dispersion, the stopping power of the uniaxial wire medium is proportional to the charge velocity, and that there is no velocity threshold for the Cherenkov emission. It is shown that the eigenfunction expansion formalism can be extended to the case of dispersive lossless media. Furthermore, in the presence of material dispersion, the optimal charge velocity that maximizes the emitted Cherenkov power may be less than the speed of light in a vacuum.

Single-Beam Optical Conveyor Belt for Chiral Particles

A different paradigm is proposed to selectively manipulate and transport small engineered chiral particles and discriminate different enantiomers using unstructured chiral light. It is theoretically shown that the response of a chiral metamaterial particle may be tailored to enable an optical conveyor belt operation with no optical traps, such that for a fixed incident light helicity the nanoparticle is either steadily pushed towards the direction of the photon flow or steadily pulled against the photon flow, independent of its position. Our findings create distinct opportunities for unconventional optical manipulations of tailored nanoparticles and may have applications in sorting racemic mixtures of artificial chiral molecules and in particle delivery.

Time evolution of electron waves in graphene superlattices

The time evolution of electron waves in graphene superlattices is studied using both microscopic and “effective medium” formalisms. The numerical simulations reveal that in a wide range of physical scenarios it is possible to neglect the granularity of the superlattice and characterize the electron transport using a simple effective Hamiltonian. It is verified that as general rule the continuum approximation is rather accurate when the initial state is less localized than the characteristic spatial period of the superlattice. This property holds even when the microsocopic electric potential has a strong spatial modulation or in presence of interfaces between different superlattices. Detailed examples are given both of the time evolution of initial electronic states and of the propagation of stationary states in the context of wave scattering. The theory also confirms that electrons propagating in tailored graphene superlattices with extreme anisotropy experience virtually no diffraction.

Asymmetric Mushroom-Type Metamaterials

We study the scattering of electromagnetic waves by mushroom-type metamaterials such that the metallic wire array and the patch grids have different symmetry centers. Based on a quasi-static model, we develop an analytical formalism to compute the reflection and transmission characteristics of a metamaterial slab, and derive suitable boundary conditions for the patch grid interfaces. The theory is successfully compared with full-wave simulations.

Simulating electron wave dynamics in graphene superlattices exploiting parallel processing advantages

Abstract This work introduces a parallel computing framework to characterize the propagation of electron waves in graphene-based nanostructures. The electron wave dynamics is modeled using both “microscopic” and effective medium formalisms and the numerical solution of the two-dimensional massless Dirac equation is determined using a Finite-Difference Time-Domain scheme. The propagation of electron waves in graphene superlattices with localized scattering centers is studied, and the role of the symmetry of the microscopic potential in the electron velocity is discussed. The computational methodologies target the parallel capabilities of heterogeneous multi-core CPU and multi-GPU environments and are built with the OpenCL parallel programming framework which provides a portable, vendor agnostic and high throughput-performance solution. The proposed heterogeneous multi-GPU implementation achieves speedup ratios up to 75x when compared to multi-thread and multi-core CPU execution, reducing simulation times from several hours to a couple of minutes. Program summary Program title: GslSim. Program Files doi: http://dx.doi.org/10.17632/prmfv63nj6.1 Licensing provisions: GPLv3. Programming language: C, OpenCL and Matlab for results analysis. Nature of problem: Computing the time evolution of electron waves in graphene superlattices is a time consuming process due to the high number of necessary nodes to discretize the spatial and time domains. Solution method: We develop a simulator based on the C/OpenCL standards to study the time evolution of electron waves in graphene superlattices by exploiting hardware architectures such as graphics processing units (GPUs) to speedup the computation of the pseudospinor.

Bistability in mushroom-type metamaterials

Here, we study the electromagnetic response of asymmetric mushroom-type metamaterials loaded with nonlinear elements. It is shown that near a Fano resonance, these structures may have a strong tunable, bistable, and switchable response and enable giant nonlinear effects. By using an effective medium theory and full wave simulations, it is proven that the nonlinear elements may allow the reflection and transmission coefficients to follow hysteresis loops, and to switch the metamaterial between “go” and “no-go” states similar to an ideal electromagnetic switch.

Measurement of the Specific Absorption Rate using a single electric field sensor

The Specific Absorption Rate (SAR) is usually measured using electric field probes based on dipole antennas, loaded with radio frequency (RF) detector diodes and connected to differential amplifiers through high-impedance transmission lines. The most common configuration consists of three mutually orthogonal electric short dipole antennas, loaded with a fast switching detector, providing broadband characteristics. In this work, we describe the measurement of the SAR with a sensor based on a single dipole antenna. It is shown that this reduces the complexity of the system without compromising the accuracy and frequency response. The probe is tested by measuring the SAR for a dual-band Planar Inverted-F antenna.