Zero-Group-Velocity Propagation Of Electromagnetic Wave Through Nanomaterial

This research will investigate the problem on the propagation of electromagnetic wave through a specific nanomaterial. The nanomaterial analyzed is a material consisting of a field of Pt nanorods. This field of Pt nanorods are deposited on a substrate which consists of a RuO2 nano structure. When the nanorod is exposed to an electron beam emitted by a TEM (Transmission electron microscopy). A wave disturbance has been observed. A video taken within the chamber shows a wave with a speed in the scale of um/s (Á?10Á?^(-6) m/s), which is 14 orders of magnitude lower than speed of light in free space (approximate 3ÁÁ?10Á?^8 m/s ). A physical and mathematical model is developed to explain this phenomenon. Due to the process of fabrication, the geometry of the decorated Pt nanorod field is assumed to be approximately periodic. The nanomaterials possess properties similar to a photonic crystal. Pt, as a noble metal, shows dispersive behaviours that is different from those ones of a perfect or good conductors. A FDTD algorithm is implemented to calculate the band diagram of the nanomaterials. To explore the dispersive properties of the Pt nanorod field, the FDTD algorithm is corrected with a Drude Model. The analysis of the corrected band diagram illustrates that the group velocity of the wave packet propagating through the nanomaterial can be positive, negative or zero. The possible zero-group velocity is therefore used to explain the extremely low velocity of wave (wave envelope) detected in the TEM

Numerical tools for computational design of acoustic metamaterials

The notion of metamaterials as artificially engineered structures designed to obtain specific material properties, typically unachievable in naturally occurring materials, has captured the attention of the scientific and industrial communities. Among the broad range of applications for such kind of materials, in the field of acoustics, the possibility of creating materials capable of efficiently attenuating noise in target frequency ranges is of utmost importance for a lot of industrial areas. In this context, the so-called locally resonant acoustic metamaterials (LRAMs) can play an important role, as their internal topology can be designed to exhibit huge levels of attenuation in specific frequency regions by taking advantage of internal resonance modes. With a proper, optimized topological design, LRAMs can be used, for instance, to build lightweight and thin noise insulation panels that operate in a low-frequency regime, where standard solutions for effectively attenuating the noise sources require dense and thick materials. Given the importance of the topological structure in obtaining the desired properties in acoustic metamaterials, the use of novel numerical techniques can be exploited to cre-ate a set of computational tools aimed at the analysis and design of optimized solutions. These are based on three fundamental pillars: (1) the multiscale homogenization of complex material structures in the microscale to get a set of effective properties capa-ble of describing the material behavior in the macroscale, (2) the model-order reduc-tion techniques, which are used to decrease the computational cost of heavy computa-tions while still maintaining a sufficient degree of accuracy, and (3) the topology optimi-zation methods that can be employed to obtain optimal configurations with a given set of constraints and a target material behavior. This set of computational tools can be applied to design acoustic metamaterials that are both efficient and practical, i.e. they behave according to their design specifications and can be produced easily, for in-stance, making use of novel additive manufacturing techniques.La concepció dels metamaterials com a estructures dissenyades artificialment amb l’objectiu d’obtenir un conjunt de propietats que no són assolibles en materials de manera natural, ha captat l’atenció de les comunitats científiques i industrials. Dins de l’ampli ventall d’aplicacions que se’ls pot donar als metamaterials, si ens centrem en el camp de l’acústica, la possibilitat de crear un material capaç d’atenuar de manera efectiva sorolls en rangs de freqüència concrets és de gran interès en multitud d’indústries. En aquest context, els anomenats “locally resonant acoustic metamaterials” (LRAMs) destaquen per la possibilitat de dissenyar la seva topologia interna per tal que produeixin elevats nivells d’atenuació en regions concretes de l’espectre de freqüències. Amb un disseny topològic òptim, els LRAMs poden servir, per exemple, per a la construcció de panells lleugers aïllants de soroll, que operin en rangs de freqüències baixos, en els quals la solució clàssica requereix de materials d’elevada densitat i espessor. Donada la importància de l’estructura topològica dels metamaterials acústics en l’obtenció de les propietats desitjades, resulta convenient l’ús de mètodes numèrics punters per al desenvolupament d’un conjunt d’eines computacionals que tinguin per objectiu l’anàlisi i el disseny de solucions òptimes. Tals eines es fonamenten en tres pilars: (1) la homogeneïtzació multiescala d’estructures de material complexes a una escala micro que derivi en l’obtenció de propietats efectives que permetin descriure el comportament del material a una escala macro, (2) tècniques de reducció per minimitzar l’esforç computacional mantenint nivells de precisió suficients i (3) mètodes d’optimització topològica emprats per a l’obtenció de configuracions òptimes donat un conjunt de restriccions i unes propietats de material objectiu. Aquestes eines computacionals es poden aplicar al disseny de metamaterials acústics que resultin eficients i pràctics a la vegada, és a dir, que es comportin segons les especificacions de disseny i siguin fàcilment fabricables, per exemple, mitjançant tècniques punteres d’impressió 3D

Numerical tools for computational design of acoustic metamaterials

Tesi en modalitat de compendi de publicacionsThe notion of metamaterials as artificially engineered structures designed to obtain specific material properties, typically unachievable in naturally occurring materials, has captured the attention of the scientific and industrial communities. Among the broad range of applications for such kind of materials, in the field of acoustics, the possibility of creating materials capable of efficiently attenuating noise in target frequency ranges is of utmost importance for a lot of industrial areas. In this context, the so-called locally resonant acoustic metamaterials (LRAMs) can play an important role, as their internal topology can be designed to exhibit huge levels of attenuation in specific frequency regions by taking advantage of internal resonance modes. With a proper, optimized topological design, LRAMs can be used, for instance, to build lightweight and thin noise insulation panels that operate in a low-frequency regime, where standard solutions for effectively attenuating the noise sources require dense and thick materials. Given the importance of the topological structure in obtaining the desired properties in acoustic metamaterials, the use of novel numerical techniques can be exploited to cre-ate a set of computational tools aimed at the analysis and design of optimized solutions. These are based on three fundamental pillars: (1) the multiscale homogenization of complex material structures in the microscale to get a set of effective properties capa-ble of describing the material behavior in the macroscale, (2) the model-order reduc-tion techniques, which are used to decrease the computational cost of heavy computa-tions while still maintaining a sufficient degree of accuracy, and (3) the topology optimi-zation methods that can be employed to obtain optimal configurations with a given set of constraints and a target material behavior. This set of computational tools can be applied to design acoustic metamaterials that are both efficient and practical, i.e. they behave according to their design specifications and can be produced easily, for in-stance, making use of novel additive manufacturing techniques.La concepció dels metamaterials com a estructures dissenyades artificialment amb l’objectiu d’obtenir un conjunt de propietats que no són assolibles en materials de manera natural, ha captat l’atenció de les comunitats científiques i industrials. Dins de l’ampli ventall d’aplicacions que se’ls pot donar als metamaterials, si ens centrem en el camp de l’acústica, la possibilitat de crear un material capaç d’atenuar de manera efectiva sorolls en rangs de freqüència concrets és de gran interès en multitud d’indústries. En aquest context, els anomenats “locally resonant acoustic metamaterials” (LRAMs) destaquen per la possibilitat de dissenyar la seva topologia interna per tal que produeixin elevats nivells d’atenuació en regions concretes de l’espectre de freqüències. Amb un disseny topològic òptim, els LRAMs poden servir, per exemple, per a la construcció de panells lleugers aïllants de soroll, que operin en rangs de freqüències baixos, en els quals la solució clàssica requereix de materials d’elevada densitat i espessor. Donada la importància de l’estructura topològica dels metamaterials acústics en l’obtenció de les propietats desitjades, resulta convenient l’ús de mètodes numèrics punters per al desenvolupament d’un conjunt d’eines computacionals que tinguin per objectiu l’anàlisi i el disseny de solucions òptimes. Tals eines es fonamenten en tres pilars: (1) la homogeneïtzació multiescala d’estructures de material complexes a una escala micro que derivi en l’obtenció de propietats efectives que permetin descriure el comportament del material a una escala macro, (2) tècniques de reducció per minimitzar l’esforç computacional mantenint nivells de precisió suficients i (3) mètodes d’optimització topològica emprats per a l’obtenció de configuracions òptimes donat un conjunt de restriccions i unes propietats de material objectiu. Aquestes eines computacionals es poden aplicar al disseny de metamaterials acústics que resultin eficients i pràctics a la vegada, és a dir, que es comportin segons les especificacions de disseny i siguin fàcilment fabricables, per exemple, mitjançant tècniques punteres d’impressió 3D.Postprint (published version

Architecturing materials at mesoscale: some current trends

This article overviews several areas of research into architectured materials which, in the opinion of the authors, are most topical and promising. The classes of materials considered are based on meso scale designs inspired by animate and inanimate Nature, but also on those born in the minds of scientists and engineers, without any inspiration from Nature. We present the principles governing the design of the emerging materials architectures, discuss their explored and anticipated properties, and provide an outlook on their future developments and applications

Experimental and Novel Analytic Results for Couplings in Ordered Submicroscopic Systems: from Optomechanics to Thermomechanics

Theoretical modelling of challenging multiscale problems arising in complex (and sometimes bioinspired) solids are presented. Such activities are supported by analytical, numerical and experimental studies. For instance, this is the case for studying the response of hierarchical and nano-composites, nanostructured solid/semi-fluid membranes, polymeric nanocomposites, to electromagnetic, mechanical, thermal, and sometimes biological, electrical, and chemical agents. Such actions are notoriously important for sensors, polymeric films, artificial muscles, cell membranes, metamaterials, hierarchical composite interfaces and other novel class of materials. The main purpose of this project is to make significant advancements in the study of such composites, with a focus on the electromagnetic and mechanical performances of the mentioned structures, with particular regards to novel concept devices for sensing. These latter ones have been studied with different configuration, from 3D colloidal to 2D quasi-hemispherical micro voids elastomeric grating as strain sensors. Exhibited time-rate dependent behavior and structural phenomena induced by the nano/micro-structure and their adaptation to the applied actions, have been explored. Such, and similar, ordered submicroscopic systems undergoing thermal and mechanical stimuli often exhibit an anomalous response. Indeed, they neither follow Fourier’s law for heat transport nor their mechanical time-dependent behavior exhibiting classical hereditariness. Such features are known both for natural and artificial materials, such as bone, lipid membranes, metallic and polymeric “spongy” composites (like foams) and many others. Strong efforts have been made in the last years to scale-up the thermal, mechanical and micro-fluidic properties of such solids, to the extent of understanding their effective bulk and interface features. The analysis of the physical grounds highlighted above has led to findings that allow the describing of those materials’ effective characteristics through their fractional-order response. Fractional-order frameworks have also been employed in analyzing heat transfer to the extent of generalizing the classical Fourier and Cattaneo transport equations and also for studying consolidation phenomenon. Overall, the research outcomes have fulfilled all the research objectives of this thesis thanks to the strong interconnection between several disciplines, ranging from mechanics to physics, from structural health monitoring to chemistry, both from an analytical and numerical point of view to the experimental one

Engineering aperiodic spiral order for photonic-plasmonic device applications

Thesis (Ph.D.)--Boston UniversityDeterministic arrays of metal (i.e., Au) nanoparticles and dielectric nanopillars (i.e., Si and SiN) arranged in aperiodic spiral geometries (Vogel's spirals) are proposed as a novel platform for engineering enhanced photonic-plasmonic coupling and increased light-matter interaction over broad frequency and angular spectra for planar optical devices. Vogel's spirals lack both translational and orientational symmetry in real space, while displaying continuous circular symmetry (i.e., rotational symmetry of infinite order) in reciprocal Fourier space. The novel regime of "circular multiple light scattering" in finite-size deterministic structures will be investigated. The distinctive geometrical structure of Vogel spirals will be studied by a multifractal analysis, Fourier-Bessel decomposition, and Delaunay tessellation methods, leading to spiral structure optimization for novel localized optical states with broadband fluctuations in their photonic mode density. Experimentally, a number of designed passive and active spiral structures will be fabricated and characterized using dark-field optical spectroscopy, ellipsometry, and Fourier space imaging. Polarization-insensitive planar omnidirectional diffraction will be demonstrated and engineered over a large and controllable range of frequencies. Device applications to enhanced LEDs, novel lasers, and thin-film solar cells with enhanced absorption will be specifically targeted. Additionally, using Vogel spirals we investigate the direct (i.e. free space) generation of optical vortices, with well-defined and controllable values of orbital angular momentum, paving the way to the engineering and control of novel types of phase discontinuities (i.e., phase dislocation loops) in compact, chip-scale optical devices. Finally, we report on the design, modeling, and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing, nanoantennas for resonant near-field optical concentration of radiation to individual nanowires, and aperiodic double resonance surface enhanced Raman scattering substrates

Nonreciprocal Electromagnetics of Layered Media

In plasmonic systems, interaction of light and surface plasmons leads to excitation of surface plasmon polaritons (SPPs) carrying energy on the surface. In an isotropic plasmonic system, the SPPs optical response is reciprocal, which means that the forward and backward surface waves have identical propagation behaviors and SPPs refract when they encounter a discontinuity on the surface. In order to excite SPPs resilient to the surface disorders, the system reciprocity needs to be broken by different techniques such as applying an external magnetic bias. In this case, the plasmonic system becomes a gyrotropic medium. Recently, it has been shown that magnetized continuous plasmonic systems such as semiconductors and graphene support unidirectional SPPs, the surface waves that are propagating only in one direction and are robust to the surface impurities. This topic has attracted the attention of many researchers, including our group. In this work, we study the properties of unidirectional SPPs in different plasmonic configurations. Our findings set a solid foundation for future active nonreciprocal plasmonic devices based on unidirectional SPPs. First, we study SPPs in the well-known topological Voigt configuration. Since indium antimonide (InSb) crystal is often cited as a suitable magneto-optics platform that supports unidirectional SPPs, we evaluate the functionality of this crystal as a topological platform by considering realistic conditions. So, using the far-field time-domain THz spectroscopy measurement, our group, along with colleagues at the University of West Virginia, examine the magneto-optical effects of the undoped InSb crystal at different temperatures varied from 5K to 300K. We apply a multi-carrier material model to consider the effect of both electrons and holes charge carriers. Then, using the measured data we examine the unidirectional SPPs and discuss the constrains that limit applications. We design a grating metallic coupler on the surface of the magnetized InSb to launch unidirectional SPPs. The measured reflection data reveals strongly nonreciprocal SPPs that are tunable by temperature and magnetic field intensity. The patterned InSb sample is tilted to examine topological behavior. The measured data are consistent with the theoretical predictions. Next, via simulation we study unidirectional SPPs on the surfaces of a magnetized plasma slab coated by a dielectric material below the plasma frequency. The equi-frequency contours are extracted from dispersion surface which follows by obtaining the group velocity vectors to estimate the SPP propagation behaviors at different operation frequencies. We mainly focus on a frequency window wherein there exists narrow-beam unidirectional SPPs. We present a Green\u27s function model for a gyrotropic slab to examine the effect of thickness on the narrow SPP beams. We observe that when the slab is thin, in addition of two excited narrow beams at the top interface, two other narrow beams form at the bottom interface due to energy coupling. We characterize them by an asymptotic dispersion relation derived from a quasi-static approach. Then, we study the nonlocality effects and Chern numbers in a continuous plasma medium. Topological SPPs are characterized by integer Chern invariants. When a continuous plasma systems is model by the (overly simplistic, but often used) local Drude model, there is a dispersion band that is ill-behaved at large wavenumbers and assigned by a non-integer Chern number. In this case, the number of unidirectional edge modes cannot be determined using the bulk-edge correspondence principle. This problem has been previously solved by introducing an ad hoc material model which includes a spatial cutoff wavenumber in the model. However, the proposed nonlocal model leads to some difficulties such as non-realistic material response at large wavenumbers and the need to interpolate the interfaced materials so that the Chern numbers sum to zero as they must. To overcome this issue, we instead suggest applying the hydrodynamic material model which is a more realistic, physical, nonlocal model. In this case, we evaluate the Chern numbers and dispersion bands. We show that this model form a complete, self consistent model that clarifies the topological physics of plasma continua. In the next work, we propose a new plasmonic configuration to excite nonreciprocal curved SPPs. We demonstrate that by applying radial bias in a plasma system, one-way SPPs travel on a circular path, unlike in an axially-biased system which supports SPPs with linear trajectory. We derive a Green\u27s function model for a radially-biased plasma system to examine curved SPPs. A nonreciprocal circular junction is proposed to effectively guide SPPs on the curvature. Finally, we examine the unidirectional SPPs in two-dimensional plasmonic platform. It has been previously shown that graphene monolayer biased by external magnets supports unidirectional edge modes. Here, we evaluate the magneto-optical effects of graphene/chromium triiodide (CrI3) heterostructure. The exchange field between layers provides an effective out of magnetic field. The optical conductivity is a tensor with non-zero off-diagonal elements which manifest the nonreciprocal response. We obtain one-way edge modes and Faraday rotation in this multi-layer structure. However, we argue that the nonreciprocal response of this heterostructure is weaker than the isolated graphene biased by external magnets. Therefore, CrI3 magnetic monolayer does not work as an alternative magnetic source that causes strong non-reciprocity

Acoustic and Ultrasonic Beam Focusing Through Aberrative and Attenuative Layers

Ultrasonic nondestructive evaluation (NDE) is a well-established technique to assess material properties and material state in noninvasive way. However, conventional NDE technologies are limited by the thick top coatings over the structure and, therefore, require time consuming removal and replacement of the coatings to perform the inspection. In biological application, although ultrasonic NDE is a safer method in compared to other radioactive non-invasive techniques, aberration of acoustic beams is more common as it encounters multiple tissue layers of complex geometry with nonhomogeneous properties. These limit the use of ultrasonic NDE in engineering and biological applications. To alleviate this problem, recently developed multifunctional metamaterials are studied and proposed as an ad-hoc metastructure to focus acoustic ultrasonic wave beam. One of the intriguing features of these metastructures is that it can be utilized along with conventional NDE transducers. In general, exotic acoustical features such as acoustic transparency, ultrasonic beam focusing, acoustic band gap and super lensing capabilities are extracted using metamaterial structures. While metamaterials can focus an ultrasonic beam at specific frequency, unwanted distortion of the output wave fields at neighboring sonic frequencies are obvious in the host medium. However, ultrasonic wave focusing by virtue of negative refraction and simultaneous transparency of the metamaterial at sonic frequencies are uncommon due to their frequency disparity. In this research, two metamaterial structures are proposed: 1) to achieve acoustic beam focusing at ultrasonic frequency and keep the structure transparent to the sonic frequencies (\u3c20 \u3ekHz)an array of butterfly-shaped thin ring resonators are proposed and 2) to achieve wave focusing and generating Bessel Beam propagation through a thick composite plate a novel high symmetry interlocking micro-structure is studied and proposed as an ad-hoc metastructure infront of the ultrasonic NDE transducers, . 1) The butterfly metamaterial with local ring resonators or butterfly crystals (BC) were previously proposed to create wide band gaps (~7 kHz) at ultrasonic frequencies above 20 kHz. However, in this research a unique sub-wavelength scale wave focusing capability of the butterfly metamaterial utilizing the negative refraction phenomenon is demonstrated, while keeping the metamaterial block transparent to the propagating wave at lower sonic frequencies below the previously reported bandgaps. 2) A novel high symmetry interlocking micro-structure is recently being investigated with optimized geometry for extracting improved mechanical properties such as high stiffness-high damping and high strength-high toughness. However, the study of elastic wave propagation through these high symmetry micro-structures is still in trivial stage. In this dissertation, the band structures, mode shapes and equifrequency contours at multiple frequencies are studied for this interlocking architecture and it was discovered that at specific ultrasonic frequency wave focusing and generating Bessel Beams are possible. Through modal analysis such phenomena are physically explained. The finite element simulations are performed for long distance wave propagation and the results are post-processed to show the actual existence of Bessel Beam phenomenon at ultrasonic frequency ~271 kHz. A concluding simulation is performed using ad-hoc interlocking metastructure to propagate wave through a combination of attenuating epoxy and composite plate. Full penetration of wave inside thick composite plate is clearly observed. To visualize the wave propagation in engineered materials, like composites and metastructures, a reliable but fast wave simulation tool is required. Wave propagation in Metastructure in conjunction with attenuative composite structure or aberrative biological surfaces, is difficult to accomplish. Traditional approach uses Finite Element Method (FEM) which is consistently known to be difficult at higher ultrasonic frequencies due to spurious reflection at element boundaries. Hence, to reduce the number of elements in the structure a new simulation tool using spectral information is necessary. In this dissertation, a computational tool based on higher order Spectral Element Method is developed from scratch to solve temporal wave propagation problem in threedimensional composite structures. This tool will facilitate to understand the wave damage interaction and optimize the geometric dimensions to construct the metastructure in later times. There are multiple computational tools available now-a-days to simulate wave propagation problems. Among others, Distributed Point Source Method (DPSM), Finite Element Method (FEM), Semi Analytical Finite Element (SAFE), Local Interaction Simulation Approach (LISA), Peri-Elastodynamic (PED) are few of them. DPSM is frequency domain based computational tool which is unable to solve temporal problem proposed in this research. PED is suitable to solve wave propagation in metallic structure; however, it has not yet been implemented in composite structures. Although FEM is a flexible method to implement for complex geometries, spurious reflection, lower accuracy and higher computational time make it less effective. To overcome the disadvantages encountered by FEM, Spectral Element Method (SEM) is recently proposed for its higher accuracy and fast convergence. Therefore, in this dissertation, SEM has been proposed to visualize high frequency ultrasonic wave in a range of 1 MHz to 7.5 MHz, which is not available in current literature. Various modules of the computer code using MATLAB are developed and simulation was performed for wave propagation through a 24-ply laminated composite plate. The simulation results were compared with experimental observations, and a good agreement of simulation and experiment was observed