306 research outputs found
A Radial-Dependent Dispersive Finite-Difference Time-Domain Method for the Evaluation of Electromagnetic Cloaks
A radial-dependent dispersive finite-difference time-domain (FDTD) method is
proposed to simulate electromagnetic cloaking devices. The Drude dispersion
model is applied to model the electromagnetic characteristics of the cloaking
medium. Both lossless and lossy cloaking materials are examined and their
operating bandwidth is also investigated. It is demonstrated that the perfect
"invisibility" from electromagnetic cloaks is only available for lossless
metamaterials and within an extremely narrow frequency band.Comment: 18 pages, 10 figure
Multi-Layered Plasmonic Covers for Comb-Like Scattering Response and Optical Tagging
We discuss the potential of multilayered plasmonic particles to tailor the
optical scattering response. The interplay of plasmons localized in thin
stacked shells realizes peculiar degenerate cloaking and resonant states
occurring at arbitrarily close frequencies. These concepts are applied to
realize ultrasharp comb-like scattering responses and synthesize staggered,
ideally strong super-scattering states closely coupled to invisible states. We
demonstrate robustness to material losses and to variations in the background
medium, properties that make these structures ideal for optical tagging.Comment: 15 pages, 4 figure
Asymmetric control of light at the nanoscale
Breaking reciprocity at the nanoscale can produce directional formation of images due to the asymmetric nonlinear optical response of subwavelength anisotropic resonators. The self-induced passive non-reciprocity has advantages compared to magnet or time modulation approaches and may impact both classical and quantum photonics
Ground-Plane Quasi-Cloaking for Free Space
Ground-plane cloak designs are presented, which minimize scattering of
electromagnetic radiation from metallic objects in the visible spectrum. It is
showed that simplified ground-plane cloaks made from only a few blocks of
all-dielectric isotropic materials, either embedded in a background medium or
in free space, can provide considerable cloaking performance while maintaining
their broadband nature. A design which operates isolated in free space that
cloaks radiation originating from a specified direction is also analyzed. These
schemes should be much easier to be demonstrated experimentally compared to
full designs.Comment: 10 pages, 4 figure
FDTD modelling of electromagnetic transformation based devices
PhDDuring this PhD study, several finite-difference time-domain (FDTD) methods were
developed to numerically investigate coordinate transformation based metamaterial
devices. A novel radially-dependent dispersive FDTD algorithm was proposed and
applied to simulate electromagnetic cloaking structures. The proposed method can ac-
curately model both lossless and lossy cloaks with ideal or reduced parameters. It was
demonstrated that perfect “invisibility” from electromagnetic cloaks is only available
for lossless metamaterials and within an extremely narrow frequency band. With a
few modifications the method is able to simulate general media, such as concentrators
and rotation coatings, which are produced by means of coordinate transformations
techniques. The limitations of all these devices were thoroughly studied and explo-
red. Finally, more useful cloaking structures were proposed, which can operate over a
broad frequency spectrum.
Several ways to control and manipulate the loss in the electromagnetic cloak ba-
sed on transformation electromagnetics were examined. It was found that, by utili-
sing inherent electric and magnetic losses of metamaterials, as well as additional lossy
materials, perfect wave absorption can be achieved. These new devices demonstrate
super-absorptivity over a moderate wideband range, suitable both for microwave and
optical applications.
Furthermore, a parallel three-dimensional dispersive FDTD method was introdu-
ced to model a plasmonic nanolens. The device has its potential in subwavelength
imaging at optical frequencies. The finiteness of such a nano-device and its impact
on the system dynamic behaviour was numerically exploited. Lastly, a parallel FDTD
method was also used to model another interesting coordinate transformation based
device, an optical black hole, which can be characterised as an omnidirectional broad-
band absorber
A combined immersed boundary/phase-field method for simulating two-phase pipe flows
The investigation of the flow in a pipe is a major issue for the pipeline capacity but
also plays an important role for the control and prevention of phenomena that could
damage the pipe, such as corrosion, erosion, and the potential formation of wax or their
deposits. Therefore, the characterization of the flow patterns is also a major issue for the
prediction of the distribution over the cross-section of the pipe, in order to understand
any problems that may interrupt or shut down the operation of the production line.
The main purpose of the present effort is to develop an appropriate numerical method
for simulating two-phase pipe flows. Advanced Computational Fluid Dynamics (CFD)
methods are employed as Navier-Stokes solver, while a Phase-Field method is used to
simulate the interfacial region between the two fluids. A Ghost-Cell Immersed Boundary
Method (GCIBM) was developed and implemented for the reconstruction of smooth rigid
boundaries (pipe wall) based on the work of Tseng and Ferziger (2003). The method was
also modified in order to incorporate appropriate boundary conditions for coupling the
Phase-Field and Navier-Stokes solvers for two-phase pipe flows. Tseng and Ferziger (2003)
used the GCIBM for turbulent single-phase flows; the present modified version comprises
a continuation of the method for handling two-phase pipe flows. The computational
model is capable of handling large density and viscosity ratios with good accuracy.
The developed GCIBM algorithm was validated against analytical solutions for single and
two-phase pipe flow, presenting very good agreement. The computational model was
compared to available experimental data from the literature for single rising bubbles and
bubble coalescence in vertical pipe also with good agreement. The numerical method was
used to investigate the lateral wall effects of a 3-D single bubble in a viscous liquid for
different pipe diameters and bubble flow regimes. The dynamics of 3-D Taylor bubbles
was also examined in vertical pipes for different properties of fluids (e.g. air-water system)
and dimensionless parameters relevant to the problem (e.g. ReB, Eo, Mo). The numerical
results were compared with available experimental and numerical data from the literature,
presenting good agreement.Open Acces
Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides
The intriguing physics of exceptional points (EPs) and spectral singularities in open (non-Hermitian) active photonic systems has recently sparked increased interest among the research community. These spectral degeneracies have been obtained in asymmetric active and passive photonic configurations but their demonstration with symmetric active plasmonic structures still remains elusive. In this paper, we present a nanoscale active plasmonic waveguide system consisting of an array of periodic slits that can exhibit exceptional points and spectral singularities leading to several functionalities. The proposed symmetric active system operates near its cutoff wavelength and behaves as an effective epsilon-near-zero (ENZ) medium. We demonstrate the formation of an EP that is accessed with very low gain coefficient values, a unique feature of the proposed nanoscale symmetric plasmonic configuration. Reflectionless ENZ transmission and perfect loss compensation are realized at the EP which coincides with the effective ENZ resonance wavelength of the proposed array of active plasmonic waveguides. When we further increase the gain coefficient of the dielectric material loaded in the slits, a spectral singularity occurs at the ENZ resonance leading to superscattering (lasing) response at both forward and backward directions. These interesting effects are achieved by materials characterized by very small gain coefficients with practical values and at subwavelength scales due to the strong and homogeneous field enhancement inside the active slits at the ENZ resonance leading to enhanced light-matter interaction. We theoretically analyze the obtained EP, as well as the divergent spectral singularity, using a transmission line model, and investigate the addition of a second incident wave and nonlinearities in the response of the proposed active ENZ plasmonic system. Our findings provide a route towards interesting nanophotonic applications, such as reflectionless active ENZ media, unidirectional coherent perfect absorbers, nanolasers, and strong optical bistability and all-optical switching nanodevices
- …