Αντικείμενο της παρούσας διδακτορικής διατριβής είναι η ανάλυση, μοντελοποίηση και βελτιστοποιημένη σχεδίαση περιοδικών μεταεπιφανειών με τη βοήθεια μοντέλων ενεργών επιφανειακών επιδεκτικοτήτων. Το βασικό κίνητρο για την παρούσα εργασία αποτέλεσε η σημαντική ανάπτυξη του συγκεκριμένου επιστημονικού κλάδου και η πληθώρα πρακτικών εφαρμογών που έχουν προταθεί τα τελευταία λίγα χρόνια. Προς τον σκοπό αυτό προτάθηκε ένα αναλυτικό μοντέλο το οποίο βασίζεται στην διπολική προσέγγιση. Η ανάλυση παρουσιάστηκε, αρχικά, για την αναπαράσταση ενεργού μέσου μιας διδιάστατης περιοδική δομής ηλεκτρικά-μικρών, με εφαπτόμενων σκεδαστών (μεταφίλμ). Αξιοποιώντας έναν συμπαγή μητρωικό συμβολισμό, ο ζητούμενος πίνακας ενεργών επιφανειακών επιδεκτικοτήτων της διάταξης υπολογίζεται ως το άθροισμα τριών όρων με διακριτή φυσική σημασία: του πίνακα πολωσιμοτήτων του μετα-ατόμου που απαρτίζει το μεταφίλμ, [α], του πίνακα των συντελεστών ενδοεπιπεδικών αλληλεπιδράσεων, [C], και του κυματικού πίνακα, [D]. Οι εξαγ ...
Όλα τα τεκμήρια στο ΕΑΔΔ προστατεύονται από πνευματικά δικαιώματα.
Περίληψη σε άλλη γλώσσα
The scope of the present doctoral thesis is the analysis, modeling, and optimal design of periodic metasurfaces via the employment of effective surface susceptibility models. The main motivation for this work has been the significant growth of this particular research field and its various practical applications which have been developed during the past few years. To this end, an analytical model based on the dipole approximation is, initially, proposed for the effective-medium representation of 2-D periodic arrays of electrically-small, non-intersecting scatterers (metafilm). By employing a compact mathematical formulation, the desired surface susceptibility matrix of the structure is computed as the sum of three terms with distinct physical meaning: the polarizability matrix of the meta-atom forming the metafilm, [α], the intraplanar interaction coefficients’ matrix, [C], and the wave matrix, [D]. The effective parameters are non-local in nature, since their values depend on the wave ...
The scope of the present doctoral thesis is the analysis, modeling, and optimal design of periodic metasurfaces via the employment of effective surface susceptibility models. The main motivation for this work has been the significant growth of this particular research field and its various practical applications which have been developed during the past few years. To this end, an analytical model based on the dipole approximation is, initially, proposed for the effective-medium representation of 2-D periodic arrays of electrically-small, non-intersecting scatterers (metafilm). By employing a compact mathematical formulation, the desired surface susceptibility matrix of the structure is computed as the sum of three terms with distinct physical meaning: the polarizability matrix of the meta-atom forming the metafilm, [α], the intraplanar interaction coefficients’ matrix, [C], and the wave matrix, [D]. The effective parameters are non-local in nature, since their values depend on the wavevector of the incident radiation. Moreover, by exploiting the Babinet duality principle, the problem of a metallic screen perforated with periodic arrays of holes (metascreen) is properly mapped to the solution of its complementary metafilm. This approach, which is carefully extended up to the limit of the first Wood-Rayleigh anomaly for the case of normal incidence, allows the prediction of the intriguing extraordinary transmission phenomenon, for holes of canonical geometrical shapes.On the other hand, analytical expressions for the reflection and transmission coefficients of arbitrary metasurfaces are analytically derived, by taking into account the most general case of surface susceptibility matrix. These formulas incorporate the co-polarized and cross-polarized Fresnel coefficients and depend on both φ and θ Euler angles. This result constitutes an adequate generalization of various existing formulas and forms an invaluable tool for the validation of any surface susceptibility model. Furthermore, energy balance conditions are extracted for the case of lossless metasurfaces.Also, as a realistic application of metasurfaces, the design and performance analysis of microwave absorbers is thoroughly investigated. The proposed device is polarization-insensitive and its performance remains practically unchanged for incident angles up to 60ο. The necessary matching of the structure to the surrounding space occurs due the destructive interference of the reflected waves, while the electromagnetic energy is converted into heat, mainly due to the losses of the dielectric substrate. Finally, by utilizing the metamaterial scalability property, the frequency bandwidth of the initial design is significantly improved, without increasing the overall size of the absorber. The absorption spectra of the fabricated prototypes, measured via a properly designed experimental setup inside an anechoic chamber, appear to be in very good agreement with the numerical simulation outcomes, thus verifying the accuracy of the featured design procedure.
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