1,721,019 research outputs found
A Tri-vial Contrast Theory for Electromagnetic Invisibility: Cloaking and Non-radiating Sources
Full text linkNovel closed-form expressions and general conditions for achieving electromagnetic invisibility are exposed and detailed in terms of a familiar concept in visual and reading contexts: the contrast. Defined as a normalized difference between some property P and the same homogeneous property of the background Pb, the contrast has been used as a parameter in Microwave Imaging problems to detect the presence of buried or unknown scatterers in a given scenario. However, when the contrast is low and it approaches zero, it becomes difficult to reveal the presence of objects
in a defined background: such structures appear to be undetectable or invisible. Starting from this observation, all the mathematical conditions for electromagnetic invisibility are reported from two different perspectives: non-radiating sources, when the excitation is internal to the structure itself, and cloaking devices, when the excitation comes from the external world and it is impinging into the device itself. The interests in such invisible structures come from different research areas: optics, for the non-radiating feature of anapole modes, microwave engineering, for surface waves excitation in cloaking devices, and physics, for coordinate transformation in electromagnetics. All these aspects are treated and discussed, with the introduction of two novel concepts for achieving the zeros of the scattered field: the nulling of the average sources in the domain of interest, namely weak solution, and the zeros of the equivalent sources, namely strong solution. The first approach is validated in the quasi-static limit, where closed-form expressions
can be written for dielectric-plasmonic systems and for dielectric-metallic patterned particles. In this subwavelength approximation, it turns out that to achieve a non-radiating or cloaking effect, objects can be modeled in terms of lumped elements and the local condition in the domain of interest can be written in the form of the Kirchhoff’s Current Law. The second approach is validated beyond the quasi-static limit and it can be written in the form of a matching problem, where objects can be modeled by lumped admittances and the fields are decomposed in terms of harmonic waves. In order to achieve the non-radiating or cloaking feature, a cancellation effect has to take place for one (or more) harmonic mode(s) and this is performed for dielectric objects by inserting a lumped surface admittance at the object’s contour. Results are shown for single harmonic cancellation, where the surface admittance function Zs appears to be purely reactive and it is computed exploiting the notion of contrast: as a normalized difference between the admittance of the bare object and the admittance of the background region. In this case of single harmonic suppression, the choice is limited to the harmonic mode which is dominant in the scattered field (in the quasi-static limit, n=0) and such dominant scattering contribution changes as the frequency is increased (n ≥ 1 beyond the subwavelength limit). For multi-harmonic cancellation, results indicate that the surface admittance functions Zs appears to be of complex value, with resistive and reactive parts that start oscillating around the azimuthal coordinate, where the admittance mantle cloak is inserted. This oscillating behaviour, especially in the resistive part, is quite sensitive to the changing in the direction of incidence for the incoming wave, but it is able to suppress an arbitrary portion of the harmonic content (n = 0,1, ..,M).
Investigations on the effects of the frequency regime and on the change of the direction of incidence are performed for volumetric systems, where the transition from the subwavelength limit to any frequency regime leads to the transition from plasmonics to the use of all-positive dielectric coatings. Exploiting the formulation of an Inverse Scattering Problem, it is possible to manipulate the internal configuration of the electromagnetic fields in many dielectric structures, as a function of the radius of the coating region, in order to synthesize the zeros of the external scattered field
in a certain frequency band and for a finite number of direction of arrival for the incoming wave. The arrangement of the dielectric permittivity in the coating layer follows the characteristics of the imposed specifications, especially the number of direction of arrivals (for which the all-dielectric device is cloaked) that ensures the same number of axis of symmetry on the dielectric structure. Concluding remarks and final recap of this PhD study are indicated
Kirchhoff's current law as local cloaking condition: theory and applications
No full textStarting from the electric field integral equation (EFIE), it is shown how, for quasi-static cloaking (i.e. zero scattered fields), the EFIE can be reduced to classical Kirchhoff's current law (KCL). The KCL, if considered according to all-dielectric or metal-dielectric structures, is demonstrated to realise devices based on plasmonic or mantle cloaking, respectively. The KCL generalises scattering cancellation theory for arbitrary shape devices in homogeneous backgrounds and it can be extended as a local cloaking condition beyond quasi-static regime. A cloaking device can be seen as a node (i.e. no current source or sink) that ensures all current densities to be compensated even for more complicated low scattering devices
Invisibility and cloaking structures as weak or strong solutions of Devaney-Wolf theorem
No full textInspired by a general theorem on non-radiating sources demonstrated by Devaney and Wolf, a unified theory for invisible and cloaking structures is here proposed. By solving Devaney-Wolf theorem in the quasi-static limit, a weak solution is obtained, demonstrating the existence of Anapole modes, Mantle Cloaking and Plasmonic Cloaking. Beyond the quasi-static regime, a strong solution of Devaney-Wolf theorem can be formulated, predicting general non-scattering devices based on directional invisibility, Transformation Optics, neutral inclusions and refractive index continuity. Both weak and strong solutions are analytically demonstrated to depend on the concept of contrast, mathematically defined as a normalized difference between constitutive parameters (or wave-impedance property) of a material and its surrounding background
A Tissue Engineering product development pathway
Full text linkTissue engineering is a field of inquiry and research that uses engineering techniques and principles of biological sciences to develop functional substitutes for reconstruction of damaged organs. Commercial translation of tissue engineering products is currently in progress all over the world. Many companies are moving their interest towards this market segment that grows by 6% per year. Aim of this thesis is to probe the possibility of developing tissue engineering products in the most cost-effective way, minimizing the industrial risk and developing a specific fund raising model. Tissue engineering is based on three main features: cells, scaffolds and bioreactors. Cells are seeded on a scaffold and cultured in a bioreactor in order to obtain a tissue engineering product. Nevertheless, developing cell carrying products is hampered by certification claims ("advanced therapies" certification rules) that unbearably increase R&D and certification costs and can be faced by either big companies or start-ups of big companies and spin-offs of complex aggregates of research centers involved in advanced cell research. On the other hand, scaffolds (certification class IIb) and bioreactors for tissue engineering (certification class I) can be developed with a lower economic effort, being the competition based on innovation, since their market is in the "growth phase" for scaffolds and in the "introduction phase" for bioreactors in the Levitt's product life cycle theory. Purpose of this thesis is to basically study scaffold and bioreactor features, then to preliminarily design some models of bioreactors and, eventually, to set a business model, based on private and public fund raising, aimed to the development of scaffolds for dental implantology and of bioreactors for cardiovascular and bone tissue engineering. Finally, a business plan of a company being spin-off of Politecnico di Torino and industrial start-up has been elaborate
Controlling surface waves with metasurfaces: From planar propagation to conformal cloaking
No full textSurface electromagnetic modes, occasionally formed by natural-existing 2D boundary conditions, can be enhanced and controlled transforming contact surfaces into artificially manipulated metasurfaces: propagation and radiation patterns can be properly shaped according to the desired specifications. Focusing the attention on microstrip technology, deep investigations based on dispersion engineering show how controlling surface waves lead to exotic applications, spanning from tunable planar propagation (creation of pass/stop bands) to conformal radiation control for cloaking purpose
The volume electric field integral equation for dielectric cloaking at any frequency regime
No full textAnalytical manipulations on the Volume Electric Field Integral Equation (VEFIE) are performed for cloaking dielectric volumes (with maximum dimension D) from incoming fields (with wavelength λ). Three main mathematical conditions are derived for any frequency regime (D/λ). The first confirms plasmonic cloaking in quasi-static regime; the second demonstrates plasmonic cloaking beyond the subwavelength limit; the third ensures cloaking with all-natural dielectric materials beyond the quasi-static frequency band
Multipolar passive cloaking by nonradiating anapole excitation
Full text linkIn this paper, we demonstrate the relation between cloaking effect and its nonradiating state by considering the destructive multipolar interaction between near-field scattering by bare object and surrounding coating located in its proximity. This cloaking effect is underpinned by anapole mode excitation and it occurs as destructive interference between the electric dipole moment, generated by a bare object (here a central metallic scatterer) and the toroidal moment, formed inside the cloak (a surrounding cluster of dielectric cylinders). Numerical results show how a cloaking effect based on the formation of the anapole mode can lead to an overall nonradiating metamolecule with all-dielectric materials in the coating region
Invisible Structures as Particular Solutions of Devaney-Wolf Theorem on Non-Radiating Sources
No full textStarting from Devaney-Wolf theorem on non-radiating sources, a general theory for invisible structures is presented, compactly deriving non-scattering devices based on Directional Invisibility, Plasmonic Cloaking, Mantle Cloaking and Anapole modes
Inverse Scattering Homogenization method for conformal metamaterial structures
No full textThe composite nature of metamaterial structures, made up of metallic and dielectric parts, repeated and modulated in a certain order in the space, increases the numerical complexity of electromagnetic analysis: homogenizing such structures with a continuous dielectric model that achieves the same global electromagnetic response could be challenging, but it could give more information about the physical mechanism of wave propagation through it. The idea developed in this paper is to associate an equivalent permittivity to a novel metamaterial architecture (i.e. the microstrip-line-based metamaterial), solving an Inverse Scattering Problem (ISP). The expected global electromagnetic response involves the cloaking effect, artificially created by the radiation of surface waves propagating around the periodic structure. In the proposed Inverse Scattering Homogenization method (ISHM), the equivalent permittivity results exhibit values between zero and one, further tested in a Forward Scattering Problem (FSP), in order to establish a one-to-one correspondance between heterogeneous (physical) and homogeneous (mathematical) mode
Population-based algorithms applied to Inverse Scattering Problem for dielectric coatings
No full textCharacterization of cloaking, realized by conformal dielectric coatings on a fixed core cylinder and carried out through an Inverse Scattering Problem (ISP), is presented as the minimization process of a proper cost functional. In order to overcome the overall non-linearity issue of the inverse problem related to the data equation, the idea presented in this paper is to exploit the bilinear form of the ISP in two successive steps. The increase in the computational cost is compensated by an hybrid scheme with local search (i.e., CG-FFT) for finding the auxiliary variable (e.g., total field) and with global optimization algorithms (i.e., GA, SGA, BBO and PSO) for the main unknown of the inverse scattering problem (i.e., complex permittivity
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