Frederico Nunes

Equivalent Circuits and Nanoplasmonics

We show how a circuit analysis, used widely in electrical engineering, finds application to problems of light wave injection and transport in subwavelength structures in the optical frequency range. Lumped circuit and transmission-line analysis may prove helpful in the design of plasmonic devices with standard, functional properties.

High-frequency response of subwavelength-structured metals in the petahertz domain

Electromagnetic plane waves, incident on and reflecting from a dielectric-conductor interface, set up a standing wave in the dielectric with the B-field adjacent to the conductor. It is shown here how the harmonic time variation of this B-field induces an E-field and a conduction current J (c) within the skin depth of a real metal; and that at frequencies in the visible and near-infrared range, the imaginary term sigmai of the complex conductivity sigma = sigma(r) + isigma(i) dominates the optical response. Continuity conditions of the E-field through the surface together with the in-quadrature response of the conductivity determine the phase relation between the incident E-M field and J(c). If slits or grooves are milled into the metal surface, a displacement current in the dielectric gap and oscillating charge dipoles at the structure edges are established in quadrature phase with incident field. These dipoles radiate into the aperture and launch surface waves from the edges. They are the principle source of light transmission through the apertures.

A long-range surface plasmon-polariton waveguide ring resonator as a platform for (bio)sensor applications

A ring plasmon-polariton resonator has been investigated and proposed as a promising platform for (bio)sensor devices. Many applications related to sensitive, miniaturized, and lab-on-a-chip devices can be realized with the structure proposed in this work. This structure consists of a rib-type ring configuration employing a metallic film sandwiched by dielectric layers. The adopted operating wavelength is ? = 1550?nm. Our theoretical investigation shows that the proposed structure not only has low loss, and consequently a long propagation distance (2.17?mm), but also a good lateral field confinement and a quality factor Qtot?1146. This high quality factor is essential for sensing applications where the conductivity of analyte mixtures is allowed to vary.

Analysis of dispersive and dissipative media with optical resonances

In this paper we analyze the problem of light-matter interaction when absorptive resonances are imbedded in the material dispersion. We apply an improved approach to aluminum (Al) in the optical frequency range to investigate the impact of these resonances on the operating characteristics of Al-based nanoscale devices. Quantities such as group velocity, stored energy density, and energy velocity, normally obtained using a single resonance model [Wave Propagation and Group Velocity (Academic Press, 1960), Nat. Mater. 11, 208 (2012)], are now accurately calculated regardless of the medium adopted. We adapt the Loudon approach [Nat. Mater. 11, 208 (2012)] to media with several optical resonances and present the details of the extended model. We also show pertinent results for Al-based metal-dielectric-metal (MDM) waveguides, around spectral resonances. The model delineated here can be applied readily to any metal accurately characterized by Drude-Lorentz spectral resonance features.

Metal–Insulator–Metal Surface Plasmon Polariton Waveguide Filters With Cascaded Transverse Cavities

In this work we propose a new approach for the design of resonant structures aiming at wavelength filtering applications. The structure consists of a subwavelength metal-insulator-metal (MIM) waveguide presenting cascaded cavities transversely arranged in the midpoint between the input and output ports. An extra degree of freedom added to this design consists in tilting the cavities around their midpoints which, besides effectively increasing the quality factor of the cavity, helps extending its range of applications by tuning multiple wavelengths.

DESIGN CONSIDERATIONS ON A DISPERSION COMPENSATING COAXIAL FIBER

This work reports on the performance of a dispersion compensating coaxial fiber as a function of its geometric parameters. Our analysis is carried out by solving the wave equation under the linearly polarized approximation, which leads to transcendental equations that provide the effective index of refraction. The highest efficiency at a fixed wavelength is achieved for a suitably chosen geometry and this choice constitutes an important factor to determine the general shape of the wavelength-dependent dispersion curve.

A Feasible Novel Technique for Breast Cancer Imaging Using UWB-Microwave Antennas

Microwave Imaging (MI) of breast cancer is an emerging non-invasive and non-ionizing technique for breast cancer diagnosis based on microwave radiation backscattered by breast tissues. Usually, MBI explore dielectric properties of breast tissues to increase the imaging contrast between tumor and healthy tissue through an Inverse Image Reconstruction Algorithm. Furthermore, MBI systems could be cost-effectively, compact and recent developments drive them to wearable soon. Even X-ray Mammography is the gold-standard for imaging tumors inside breast; it still is low contrast for early diagnostic, painful and age- and dose-restricted because ionizing radiation. Then, in long-term, our group is motivated to develop MBI technique and cost-effectively wearable systems for in vivo early diagnosis of breast cancer as a complimentary to X-ray Mammography. In short-term, we had been designing our systems by simulations, manufacturing and initial in vitro tests in lab. This work presents initial results of design and simulation of a system based on hard and/or flexible antennas ranging between 0.001 GHz to 3 GHz for in vitro experimentation. In detail, two different shapes (rectangular patch and circular slot) of Micro strip Patch antennas were designed and simulated in FR4, Cotton, Polyester and Pyralux Polyimide (for Flexible antennas) materials. The resonance frequency of antennas depends on permittivity of substrate material and its geometry, so the High Frequency Simulation Software (HFSS) simulated scattering parameters of designed antennas, which were also tested on mimic-phantoms of breast. The simulation of rectangular patch and bow-tie antennas resulted on a like-Gaussian curve of microwave emission / detection peaked at 1.9-2.7 GHz with ~50-90MHz band width.

Using confocal microscopy to characterize nanoplasmonic structures responsible for light transmission

The optical properties of nanostructured metallic nanofilms have been extensively studied in last few years. It was observed, for a wide variety of structures an enhancement in the transmission that can be explained as resulting from surface plasmon polaritons (SPP) waves propagating at the interface between the metallic film and the surrounding dielectric and/or substrate. In this work we utilize confocal microscope images as a useful tool to characterize the optical response of a set of concentric nanorings in the presence of SPP waves. We show for the first time the influence of the metal thickness on the light intensity profile. Reflected and transmitted light for concentric nanorings were observed under excitation of different laser wavelengths (405-633nm) as well as white light. Microscopy imaging with polarized light showed not only the spatial pattern of the radiation transmitted through these apertures but also a significant dependence of these patterns on the film thickness. The behavior was theoretically analyzed via basic principles as well as numerical simulation with standard software. A possible explanation is describing each ring as a source of radiation formed by two dipole systems, one electric dipole aligned to the applied electric field and a second one, a magnetic dipole, associated to a loop-antenna having an azimuthally non-homogeneous current dependence. This preliminary model is an ongoing study which may be useful to explain the behavior of the transmitted light. Analysis also showed the potential of confocal microscope for imaging nanostructures as well as for quantitative information on SPP excitation.

A confocal microscopy study on the transmission of light through a single sub-wavelength slit

We measured a single sub-wavelength slit using confocal microscopy. The transmitted light was measured and the dependence with the input laser polarization was characterized. Results may be related to the coupled SPP throughout the slit.

A Proposed Quantum Mechanics Mechanism for (e-, h+) Charges Separation Applied to Photosynthesis and Energy Production Efficiency Improvement

Based on concepts in semiconductor band gap engineering (the staggered one), a qualitative model is proposed for the first step mechanism in artificial catalysis and natural systems such as photosynthesis in green leaves.