Efficient techniques for scattering from planar and cylindrical structures with edges

In this work, we present rigorous and efficient methods for analyzing scattering from the following structures • Tandem Slit loaded with homogeneous material • Eccentrically loaded cylinder with multiple slits • Semicircular cylinder and slit • Dielectric loaded Wedge shaped cylinder • Circular cylinder with resonant cavities and resonant cavities on circular arc. For analyzing the material loaded tandem slit configuration, the boundary value problem is formulated into a pair of simultaneous Wiener-Hopf equations via Fourier transformation. After decoupling these equations by elementary transformation, each modified Wiener-Hopf equation is reduced to a Fredholm integral equation of the second kind. The integral equations are then solved approximately to yield the Fourier transform of the diffracted fields. The inverse transform is evaluated asymptotically to obtain the far field expressions. Measurements and numerical simulations are also performed for several different geometric and material configurations. The analytic solutions compare well with measured and simulated results. The possibility of reducing beamwidth and increasing power coupled through the loaded tandem slit is explored. The analysis of the eccentrically loaded cylindrical cavity with multiple slits under plane wave illumination is formulated using two distinct approaches: (1) an integral equation/combined boundary condition (IE/CBC) formulation and (2) an integral equation/Neumann series expansion (IE/NS) formulation. The IE/NS formulation is shown to converge faster than the IE/CBC formulation based on the proper edge behavior exhibited by the Neumann series current expansion functions. Results for the backscattered radar cross section (RCS) of several geometries are presented, and the relationships between the RCS and the scatterer characteristics are explored. The applicability of the Neumann series method to find a fast method for evaluating scattering from a metallic strip and semicircular cylinder is presented. The Neumann series of different periodicity is used for studying scattering from wedge shaped cylinder. The Neumann series is also applied to study scattering from a circular cylinder with resonant cavities and resonant cavities on a circular arc. These resonant cavities on a circular arc have superdirective properties, which are useful for high gain antenna design

Extraordinary transmission through arrays of slits: A circuit theory model

Extraordinary transmission and other interesting related phenomena for 1-D periodic arrays of slits (compound diffraction gratings) have recently been the object of intense research in the optics and solid state physics communities. This case should be differentiated from the extraordinary transmission through arrays of small apertures on metal screens since small holes only support below-cutoff modes, whereas slits can also support transverse electromagnetic modes without cutoff frequency. In this paper, an equivalent-circuit approach is proposed to account for the most relevant details of the behavior of slit-based periodic structures: extraordinary transmission peaks, FabryProt resonances, and transmission dips observed in compound structures. The proposed equivalent-circuit model, based on well-established concepts of waveguide and circuit theory, provides a simple and accurate description of the phenomenon that is appropriate for educational purposes, as well as for the design of potential devices based on the behavior of the structures under study.Ministerio de Educación y Ciencia TEC2007-65376, CSD2008-00066Junta de Andalucía TIC-25

Analytical model for the transmission of electromagnetic waves through arrays of slits in perfect conductors and lossy metal screens

This paper presents a very simple analytical model for the analysis of the resonant transmission of microwaves or millimeter waves through periodically distributed slits in a thick metal screen. The model is based on equivalent circuits consisting of transmission line elements of known characteristic admittances and propagation constants loaded by capacitors. Closed-form analytical expressions are provided for all the circuit parameters. Alternatively, the circuit parameters can be quickly computed from numerical simulations carried out at a few frequency points. The proposed analytical model accounts for all the details of the observed transmission spectrum, including conventional Fabry-Pérot (FP) resonances, which are controlled by the thickness of the screen, as well as extraordinary transmission peaks, which are related to the periodicity. The range of validity of the model as a function of dimensional parameters is discussed. The experimentally observed and numerically predicted redshift of the Fabry-Pérot transmission peaks with respect to the ideal Fabry-Pérot resonance condition is accurately accounted for by the capacitors of the model. For narrow slits, the extraordinary transmission peak is linked to the singular behavior of the capacitances at the Rayleigh-Wood anomaly frequency point. Finally, the effect of the lossy nature of the metal screens is included in the model, providing accurate predictions of the transmission losses. Additionally, for lossy screens the model adequately predicts the anomalous behavior of the above mentioned redshift when the slit width becomes comparable to the skin depth in the metal, which is in good agreement with experimental and theoretical data previously reported for a single slit.Ministerio de Ciencia e Innovación P2009-0405, TEC2007-65376, TEC2010-16948, CSD2008-00066Junta de Andalucía TIC-459

Light manipulation through periodic plasmonic corrugations

textCollective oscillations of free electrons localized in a small volume have drawn a lot of attention for the past decades. These so-called plasmons have special optical properties that can be used in many applications ranging from optical modulators to sensing of small quantities of molecules. Large numbers of extensive plasmonic applications are being based on the capability of light manipulation proposed by the periodic nanostructure and its optical response. By controlling over the way in which plasmonic modes interact with incident radiation, periodic corrugation opens up the possibility of developing new and exciting photonic devices. The goal of doctoral research presented herein is to investigate at a fundamental level of several corrugated metallic structures which may offer effective control of the optical response by coupling radiation to plasmonic modes. By controlling morphologies and material compositions, sophisticatedly engineered nanostructure may allow the coupling of electromagnetic waves into desired spectral/spatial modes in a way that an effective tuning of macroscopic optical properties in desired domain can be achieved. This dissertation is dedicated to answer the following question, if and how one can manipulate the optical responses by use of different nanostructures and various materials. Based on devised analytical models proposed for various corrugated nanostructures, we show that I. spatial and II. spectral manipulation of light can be realized. Specifically, we investigate how the grating array interacts with light. To understand those periodic nanostructures showing inherently dispersive nature, firstly the diffraction of light and accompanying effects are studied with the analytical models and numerical simulation. On this basis, we show the optical response is readily tunable, and efficiently controlled by the morphology and dielectric property of the corrugations. The outline of doctoral research is broadly categorized into (1) theoretical considerations on the topic of plasmonics, (2) specific insight in the analytical model of the various nanostructures, and (3) investigation of the plasmonic properties of the fabricated structures. Lastly, the discussion of outlook to possibilities and future experiments will close the dissertation.Electrical and Computer Engineerin

Comparison of Mutual Coupling Phenomena in Subwavelength Ridged Circular Apertures and Half-Wavelength Dipole Antenna Arrays

The resonant microwave transmission characteristics of several coupled subwavelength ridged circular aperture arrangements in a thin metallic film are investigated using the three-dimensional finite-difference time-domain (3D FDTD) method. Simple equivalent circuits represented by the self and mutual conductances that have been quantitatively extracted for each resonance condition, including the mutual coupling effects, are also devised with the help of virtual magnetic current elements. Furthermore, a duality is identified between the ridged circular apertures and conventional half-wavelength dipole arrays based on comparing the respective resonance conditions

Comparison of Mutual Coupling Phenomena in Subwavelength Ridged Circular Apertures and Half-Wavelength Dipole Antenna Arrays

The resonant microwave transmission characteristics of several coupled subwavelength ridged circular aperture arrangements in a thin metallic film are investigated using the three-dimensional finite-difference time-domain (3D FDTD) method. Simple equivalent circuits represented by the self and mutual conductances that have been quantitatively extracted for each resonance condition, including the mutual coupling effects, are also devised with the help of virtual magnetic current elements. Furthermore, a duality is identified between the ridged circular apertures and conventional half-wavelength dipole arrays based on comparing the respective resonance conditions

Resonant Transmission Through A Metagrating With Dielectric-Loaded Slots

In this thesis we will focus our attention on the transmission through a single dielectric-loaded slot and an array of slots in an infinite thick metallic screen. We use a mode-matching technique to obtain approximate analytical formulas for the transmission through the perforated metallic screen. We analyze an infinite single slot or array of slots in an infinite metallic screen of finite thickness, where the slot is much smaller than or comparable to the wavelength of the incident field. Strong transmission can be accomplished by loading the slot with a high dielectric constant material. We include the effect of higher-order modes in the slot, which will be seen to result in some unusual resonance phenomena. Including a higher-order mode in our calculation reveals the presence of new narrowband (Fano) resonances besides the broad resonances (Fabry-Perot resonances) that are due to the fundamental parallel-plate TEM mode. The approximate analytical formula predicts new narrowband (Fano) resonances with very good agreement in magnitude and frequency location with a numerical method. We include the effect of introducing a gap region of different dielectric permittivity within the slot on resonances due to both the fundamental parallel-plate TEM mode and to a higher-order mode in the slot. Our study reveals how the thickness of the gap within the slot, as well as the dielectric constant of the substance that fills the gap, can control the location and magnitude of resonances. This analytical model allows us to study two different loading geometries (a gap and grooves) and analyze the influence on both kinds of resonances (Fabry-Perot and Fano). By utilizing image theory, we were able to investigate a structure closely related to the infinite array of slots: that of a rectangular waveguide loaded by a dielectric slot in a thick metallic screen. Our results are validated by comparison with a full-wave numerical finite-element simulation as well as by experimental measurements. The experimental results revealed another kind of resonance phenomenon due to small transverse air gaps within the dielectric loaded slot.</p

Analytical circuit model for 1-D periodic T-shaped corrugated surfaces

An analytical circuit model is obtained to study the reflection of TM polarized electromagnetic waves that impinge obliquely on a 1-D periodic corrugated surface consisting of dielectric-loaded T-shaped planar corrugations backed by an infinite ground plane. The model is based on transmission line theory and equivalent lumped-element circuits. For the case of perfect conductors, the topology of the circuit is directly inferred from a rigorous full-wave formulation of the periodic problem without using any heuristic argument. This procedure leads to fully analytical expressions for all the circuit parameters. Ohmic losses are further incorporated in the model under the assumption of strong skin effect. The results thus obtained are compared with those given by an accurate Method of Moments numerical code and HFSS software showing a very good agreement. The strong numerical efficiency as well as the good physical insight provided by the present equivalent circuit model can be advantageously employed for the analysis and/or design of a variety of devices. As examples of the latter, the circuit model is used for the first-stage design of an electrically thin hard impedance surface, a corrugated surface that prevents specular reflection, and an absorber.Ministerio de Ciencia e Innovación TEC2010-16948, CSD2008-0006