In this letter, we propose all-dielectric Huygens' metasurface structures to construct high numerical aperture flat lenses and beam deflecting devices. The designed metasurface consists of two-dimensional array of all dielectric nanodisk resonators with spatially varying radii, thereby introducing judiciously designed phase shift to the propagating light. Owing to the overlap of Mie-type magnetic and electric resonances, high transmission was achieved with rigorous design analysis. The designed flat lenses have numerical aperture value of 0.85 and transmission values around 80%. It also offers easy fabrication and compatibility with available semiconductor technology. This spectrally and physically scalable, versatile design could implement efficient wavefront manipulation or beam shaping for high power laser beams, as well as various optical microscopy applications without requiring plasmonic structures that are susceptible to ohmic loss of metals and sensitive to the polarization of light.

In many scientific and engineering researches, Bayesian surrogate models are utilized to handle nonlinear data for regression and classification tasks. In this thesis, we consider a real-life problem, functional response modeling of metamaterial and its rapid design, to which we establish and test such models. To familiarize with this subject, some fundamental electromagnetic physics are provided.. Noticing that the dispersive data are usually in rational form, a two-stage modeling approach is proposed, where in the first stage, a universal link function is formulated to rationally approximate the data with a few discrete parameters, namely poles and residues. Then they are used to synthesize equivalent circuits, and surrogate models are applied to circuit elements in the second stage.. To start with a regression scheme, the classical Gaussian process (GP) is introduced, which proceeds by parameterizing a covariance function of any continuous inputs, and infers hyperparameters given the training data. Two metamaterial prototypes are illustrated to demonstrate the methodology of model building, whose results are shown to prove the efficiency and precision of probabilistic pre- dictions. One well-known problem with metamaterial functionality is its great variability in resonance identities, which shows discrepancy in approximation orders required to fit the data with rational functions. In order to give accurate prediction, both approximation order and the presenting circuit elements should be inferred, by classification and regression, respectively. An augmented Bayesian surrogate model, which integrates GP multiclass classification, Bayesian treed GP regression, is formulated to provide a systematic dealing to such unique physical phenomenon. Meanwhile, the nonstationarity and computational complexity are well scaled with such model.. Finally, as one of the most advantageous property of Bayesian perspective, probabilistic assessment to underlying uncertainties is also discussed and demonstrated with detailed formulation and examples.

Le travail de cette thèse concerne la conception de nouvelles architectures d'antennes miniatures ou multifréquences en utilisant la technique inspirée des métamatériaux. Les antennes proposées sont munies de nouvelles formes d'éléments parasites qui permettent d'obtenir des structures compactes et donc facilement intégrables dans les terminaux mobiles. Par la suite, l'étude et la conception d'antennes directives et reconfigurables en diagramme de rayonnement par la technique inspirée des métamatériaux a été menée. En première partie, une étude paramétrique sur les paramètres pertinents de la structure a été effectuée afin de bien comprendre le fonctionnement de celle-ci. En deuxième partie, des antennes miniatures et directives sont proposées. Finalement, de nouvelles structures directives et reconfigurables en diagramme de rayonnement sont présentées. / The work of this thesis concerns the design of new architectures of miniature antennas by using the concept of metamaterials-inspired.The proposed antennas integrate new shapes of parasitic elements that allow to obtain compact structures and therefore easily integrated into mobile devices.

This paper presents and evaluates a new terahertz metamaterial absorber (MMA) for sensing applications. Because of its unique properties, metamaterial-based sensors are widely employed in a variety of applications. The reported structure comprises of two identical metallic patches, a dielectric spacer and a ground metal plane. The finite element approach has been utilized to simulate and analyse the design. It is found that the MMA offered a prominent resonant peak with near 100% absorbance at frequency 4.5 THz due to the resultant effect of coupling between the two identical patches. In addition, surface current distribution, absorption mechanism and structural parametric analysis has also been investigated. The peak is designated as 'A', with a line width of 0.02 THz and a quality factor (Q-factor) of 225, which is sensitive to the refractive index of the environment (RI). As a result of its highly sensitive sensing capabilities, the proposed design can be employed as a sensor for refractive index, having 1.6 THz per refractive index unit (RIU) sensitivity and figure of merit (FoM) of 80 in terms of change in RI of the environment. The majority of biomedical samples have RI of 1.3 to 1.36, which is worth highlighting. Thus, biomedical applications may be possible with the suggested sensor.

Nonlinear optics is the study of interactions of materials with intense light beams made possible by the invention of laser. Arguably the most trivial but technologically most important nonlinear optical effect is the intensity-dependent nonlinear refraction: an intense light beam can temporarily and reversibly change the refractive index of a material. However, the changes to the refractive index of a material due to the presence of a strong laser beam are very weak---maximum on the order of $10^{-3}$---and tend to be a small fraction of the linear refractive index. It must be noted that at optical frequencies vacuum has a refractive index of 1 and glass has a refractive index of 1.5. Thus, one of the foundational assumptions of nonlinear optics is that the nonlinear optical changes to material properties are always a small perturbation to the linear response. In the 58-year history of nonlinear optics, one of the overarching themes of research has been to find ways to increase the efficiency of nonlinear interactions. This thesis is a collection of six manuscripts motivated by our experimental finding that at least in a certain class of materials the above long-standing view of nonlinear optics does not necessarily hold true. We have found that in a material with low refractive index, known as an epsilon-near-zero material or ENZ material, the nonlinear changes to the refractive index can be a few times larger than the linear refractive index, i.e. the nonlinear response becomes the dominant response of the material in the presence of an intense optical beam. We believe that the results presented in this thesis collectively make a convincing case that ENZ materials are a promising platform for nonlinear nano-optics.

The light manipulation performance of plasmonic structures has been widely studied at wavelengths spanning from microwave and terahertz radiation to infrared and visible light. Plasmonic nanostructures with designed sizes and geometries displays strong enhancement and confinement of electromagnetic fields that known for tailoring spectra. By exploiting these properties, color printing at the diffraction limit in the visible light regime has recently been demonstrated. Diatoms, one kind of unicellular microalgae, widely exist in aquatic environments and are well-known for their light-manipulating properties. The abundant biologically evolved micro- and nano-pores enable diatom frustules to be remarkably studied achieving superior optical performances in sensing and solar cell applications. In this work, the diatom-inspired metamaterials for subwavelength plasmonic color printing have been investigated. The nanoporous structures on diatom frustules are examined to develop both Hierarchical Diatom-inspired Nanopattern (HDN) and Misaligned Hierarchical Diatom-inspired Nanopattern (MHDN) with different structural parameters on the metamaterial design. Using finite-element simulations, the practical metal-insulator-metal (MIM) configurations are screened, and electric field distribution is evaluated to uncover the physical mechanisms responsible for color printing. Nanofabrication and optical measurement are conducted as complementary validation for simulation analysis and present the practicality for application. / 2024-01-15T00:00:00Z

Thesis advisor: Krzysztof Kempa / “Simulations are like an experiment but on a computer.” – K. Kempa. Powerful ideas can be explored in immense detail and unmatched flexibility through computational resources. Combined with the beauty of electromagnetics, worlds of situations and problems can be uncovered. Of the many interesting phenomena available to study, a relatively recent explosion of engineered plasmonic materials has benefitted greatly from numerical breakthroughs in simulating Maxwell’s equations. Using these tools on novel metamaterial systems, composite materials with precisely designed structural features, the analysis and optimization probes the unique capabilities they have interacting with light. Example phenomena from this work includes fundamental principle breaking, extraordinary optical transmission, negative refraction, and superconductivity enhancement. The systems that harbor such outstanding feats fall into the umbrella term of metamaterials, each with distinct geometry and contrasting electrical properties that allow for an engineered control of the effective structural dielectric function. As the response to electromagnetic radiation, manipulating the dielectric function is key to creating and discovering the effects that control light, without changing any chemistry. This work scales pedagogically through the different types of metamaterials, beginning first with 2D planar checkerboard structures with highly non-linear percolation. In combination with spoofed plasmonics, the longstanding symmetry of the Babinet principle is challenged. Layers of checkerboards are then stacked and translated to create subwavelength gaps for which plasmonic coupling between layers aids in optical transmission. In fact, there is similar physics controlling other layered quasi-complementary structures shown by comparison to experimental transmittance data. A further stage introduces photonic crystals constructed out of 3D periodic lattice of nanoparticles. Photonic band structure calculations for properly designed systems suggest the possibility of bandwidths of the IR spectrum where the crystal has a negative refractive index. Such a material property allows for the invention of lenses that beat the diffraction limit, applicable to subwavelength imaging. Lastly, non-local extensions to plasmonics are theoretically worked into expressions for superconductivity, creating a resonant anti-shielding effect, in composite topological crystal/superconductor layered arrangements. Applying this to known topics, like Bi2Se3 and MgB2, shows significant boost to electron pairing and thus rises in superconducting critical temperature. Central to all the systems and effects explored are the modifications made to the dielectric function of each effective medium. Supported by electromagnetic simulations and theoretical efforts, the listed engineered materials transform the dielectric environment purposefully to originate the mentioned exotic optical phenomena. / Thesis (PhD) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Yes

Metamaterials are artificial materials and named by those who work in the microwave material area. According to existing documentation, the metamaterials have relative permittivity and/or relative permeability of values less than 1, including negative values. If the material has negative permittivity and permeability at the same time, the material is also referred to as simultaneously negative medium (DNG medium). Such medium has several features that any natural medium is not equipped with: negative refraction, backward phase, and evanescent wave amplification [5]. Though the medium does not exist in nature, it seems that it can be artificially made through synthesizing metallic insertions inside the natural dielectrics [2]. Due to its unique feature of negative refraction and this feature is not equipped with any reported natural medium, the concept of making perfect lenses with metamaterials has attracted attentions in recent years. However a number of questions need to be answered: How can we quantize the refractive index of the metamaterial given that the permittivity and permeability are known or vice versa; can the metamaterial be made isotropic medium under effects of different incident angles? The answer to the first question will help us to define the dimension of the lenses more efficiently; and the answer to the latter question will help determine if such medium is capable of being used to make lenses. Previous publications from others demonstrated the negative refraction phenomenon of metamaterials though this phenomenon is restricted to a very narrow band [4] [11]. The derivation of the negative refractive index through full-wave simulation and comparison with its value through calculating the simulated negative permittivity and permeability obtained from the simulated scattering matrix have not been reported. The work carried in this thesis fully explored the ways to address this and answer those questions mentioned in previous paragraph. To fully understand the negative refraction effect of metamaterial, the author built a mathematical geometric model to calculate refractive index for rectangular metamaterial slab. With this approach, the refractive index can be obtained provided that incident and peak-receive angle are known. In order to achieve a metamaterial with isotropy property, the author also presented three different types of metamaterial slabs: parallel-arranged, vertical-arranged and cross-arranged slab of capacitive-loaded-loops (CLL) in front of standing probes or posts, which are also called CLL-P slabs. The three arrangements are differentiated by the way unit cell is oriented. With the geometric model, the author obtained refractive indexes for three metamaterial slabs at different incident angles through numerical simulation. The refractive indexes have negative values at all circumstance, which shows the negative refraction phenomena unique to the metamaterial. Unlike the other two CLL-P slabs, the cross-arranged CLL-P slab has near constant refractive index and constant received amplitude regardless of incident angles. This result can be attributed to the symmetrical topology of unit cell in x-y plane. To better explain refractive effects occurred for those three CLL-P slabs, the author also employed a way to calculate the effective permittivity and permeability using scattering matrix. Based on effective permittivity and permeability obtained, the analytical values of refractive indexes have been calculated at resonance point. To check the refractive indexes calculated from two different methods: using Snell's Law based geometric approach and using permittivity/permeability obtained from scattering matrix, two results are compared against each other and agree well. Knowing effective permittivity and permeability is very useful for calculating other parameters of the CLL-P slab such as wave impedance and mismatch loss etc. With all the simulation for parallel-arranged, vertical-arranged, and cross-arranged CLL-P slabs, from simulation results, it is found that the cross-arranged slab has the property of isotropy at different incident angles since the coupling between incident magnetic field and CLL loop will maintain constant. As a validation process, the CLL-P simulation result in parallel waveguide is compared with prior simulation (HFSS) and measurements of refractive focusing of the same structure, and both simulation results agree with measurements. The full-wave simulation tools FEKO that employs the Method of Moments (MoM) is used in the two ways of estimating the negative refractive index of the medium. / Master of Science

Additive Manufacturing (AM, also known as 3D printing) can produce novel three-dimensional structures using low-loss dielectric materials. This enables the construction of dielectrics with complex shapes that enable innovative microwave applications such as resonators, filters, and metamaterial lenses. This thesis addresses the production and characterization of cellular structures of various designed densities created with a low loss ceramic material, alumina (aluminum oxide), via vat photopolymerization. The permittivity of these printed structures is variable over roughly an octave, with a range of relative permittivites from 1.78 to 3.60, controlled via part geometry. Two additional materials, ferrite and nickel, have been explored for inclusion within these dielectric structures to enable the production of multi-material electromagnetic structures with conductive, magnetic, and dielectric elements. / Master of Science