Boundary Effects on the Determination of Metamaterial Parameters from Normal Incidence Reflection and Transmission Measurements

A method is described for the determination of the effective electromagnetic parameters of a metamaterial based only on external measurements or simulations, taking boundary effects at the interfaces between a conventional material and metamaterial into account. Plane-wave reflection and transmission coefficients at the interfaces are regarded as additional unknowns to be determined, rather than explicitly dependent on the material parameters. Our technique is thus analogous to the line-reflect-line (LRL) calibration method in microwave measurements. The refractive index can be determined from S-parameters for two samples of different thickness. The effective wave impedance requires the additional assumption that generalized sheet transition conditions (GSTCs) account for the boundary effects. Expressions for the bulk permittivity and permeability then follow easily. Our method is validated by comparison with the results using the Nicolson-Ross-Weir (NRW) for determining properties of an ordinary material measured in a coaxial line. Utilizing S-parameters obtained from 3-D full wave simulations, we test the method on magnetodielectric metamaterials. We compare the results from our method and the conventional one that does not consider boundary effects. Moreover, it is shown that results from our method are consistent under changes in reference plane location, whereas the results from other methods are not.Comment: 16 pages, 16 figures. Submitted to IEEE Transactions on Antennas and Propagatio

Saturated hydraulic conductivity determined by on ground mono-offset Ground-Penetrating Radar inside a single ring infiltrometer

In this study we show how to use GPR data acquired along the infiltration of water inside a single ring infiltrometer to inverse the saturated hydraulic conductivity. We used Hydrus-1D to simulate the water infiltration. We generated water content profiles at each time step of infiltration, based on a particular value of the saturated hydraulic conductivity, knowing the other van Genuchten parameters. Water content profiles were converted to dielectric permittivity profiles using the Complex Refractive Index Method relation. We then used the GprMax suite of programs to generate radargrams and to follow the wetting front using arrival time of electromagnetic waves recorded by a Ground-Penetrating Radar (GPR). Theoretically, the 1D time convolution between reflectivity and GPR signal at any infiltration time step is related to the peak of the reflected amplitude recorded in the corresponding trace in the radargram. We used this relation ship to invert the saturated hydraulic conductivity for constant and falling head infiltrations. We present our method on synthetic examples and on two experiments carried out on sand soil. We further discuss on the uncertainties on the retrieved saturated hydraulic conductivity computed by our algorithm from the van Genuchten parameters

Low-damping epsilon-near-zero slabs: nonlinear and nonlocal optical properties

We investigate second harmonic generation, low-threshold multistability, all-optical switching, and inherently nonlocal effects due to the free-electron gas pressure in an epsilon-near-zero (ENZ) metamaterial slab made of cylindrical, plasmonic nanoshells illuminated by TM-polarized light. Damping compensation in the ENZ frequency region, achieved by using gain medium inside the shells' dielectric cores, enhances the nonlinear properties. Reflection is inhibited and the electric field component normal to the slab interface is enhanced near the effective pseudo-Brewster angle, where the effective \epsilon-near-zero condition triggers a non-resonant, impedance-matching phenomenon. We show that the slab displays a strong effective, spatial nonlocality associated with leaky modes that are mediated by the compensation of damping. The presence of these leaky modes then induces further spectral and angular conditions where the local fields are enhanced, thus opening new windows of opportunity for the enhancement of nonlinear optical processes

Extraction of Dispersive Material Parameters using Vector Network Analyzers and Genetic Algorithms

A novel method to extract dispersive properties for dielectrics over a wide frequency range is proposed. This method is based on measuring scattering parameters for planar transmission lines and applying genetic algorithms. The scattering parameters are converted into ABCD matrix parameters. The complex propagation constant of the TEM wave inside the line is obtained from A-parameters of the ABCD matrix. For planar transmission lines, analytical or empirical formulas for dielectric loss,conductor loss, anaphase constant are known. The genetic algorithm is then used to extract the Debye parameters for the dielectric substrates. FDTD modeling is used to verify the dispersive parameter extraction by comparing with the measurement

Assessment of air void content of asphalt using dielectric constant measurements by GPR and with VNA

For several years, Ground Penetrating Radar (GPR) has been used in Finland to evaluate the air void content of asphalt pavements. Air void content is an important quality measure of pavement condition for both old and new asphalt pavements. The objective is to investigate if the existing GPR technique and application employed in Finland is sufficiently accurate to be used as a quality control tool in assessing the compaction of newly laid asphalt pavements. The work comprised field and laboratory experiments and a review of the existing PANK calibration method for the GPR measurements. Field experiments were conducted in the summer of 2013 on highways Vt3 and Vt12, near the City of Tampere. The test roads were paved with SMA16 using an approx. 40 mm thick layer of new asphalt. Roads were measured with GPR several times during the fall of 2013. A total of 36 cores and 2 slabs were obtained from the roads and tested in the laboratory with a Vector Network Analyzer. Measurements were done with a 7 to 17 GHz transmission configuration to measure the reference dielectric constant of the asphalt mixture. A major finding is that the PANK calibration method for the GPR inadvertently reduces observed density variations and may introduce a systematic bias. This makes pavements appear to be more homogenous and dense than they actually are according to conventional measurements.Maatutkaa (Ground Penetrating Radar, GPR) on käytetty Suomessa pitkään asfalttipäällysteiden tyhjätilan määrittämiseen. Tyhjätila on tärkeä kriteeri sekä uusien että vanhojen asfalttipäällysteiden laadun selvittämisessä. Tavoitteena on tutkia, onko Suomessa tällä hetkellä käytössä oleva GPR-tekniikka ja sen soveltaminen tarpeeksi tarkkaa uusien asfalttipäällysteiden tiiveyden mittaamiseen. Työ koostui kenttä- ja laboratoriotutkimuksista sekä GPR-mittausten kalibrointiin käytetyn PANK-kalibraatiomallin arvioinnista. Kenttäkokeet suoritettiin kesällä 2013 Tampereen lähellä valtateillä 3 ja 12. Teiden päällyste oli tyyppiä SMA16, ja uuden asfalttikerroksen paksuus oli 40 mm. Tiet mitattiin 1 GHz maatutkalla useita kertoja syksyn 2013 aikana. Teiltä otettiin 36 poranäytettä ja 2 laattanäytettä, jotka testattiin laboratoriossa vektoripiirianalysaattorilla. Asfalttiseoksen dielektrisyysvakio mitattiin 7-17 GHz läpimittauskonfiguraatiolla vertailuarvojen saamiseksi. Tärkein havainto oli se, että PANK-kalibraatiomallin käyttö maatutkamittauksissa vähentää havaittuja tiheyden vaihteluita ja saattaa lisätä systemaattisen virheen mittauksiin. Tämä saa päällysteet näyttämään tasalaatuisemmilta ja tiiviimmiltä kuin mitä ne oikeasti ovat

Development of Forward and Inversion Schemes for Cross-Borehole Ground Penetrating Radar

Tomography is an imaging technique to develop a representation of the internal features of material using a penetrating wave, such as an electromagnetic wave. The calculation method used is an example of an inverse problem, which is a system where the input and the output are known but the internal parameters are not. These parameters can be estimated by understanding the responses of a penetrating wave as it passes through the unknown media. A forward problem is just the opposite; the internal structure and input penetrating wave is known and the output is determined. For both forward and inverse problems, raytracing is needed to define the raypath through the medium and inversion techniques are used to minimize the error for a discretized matrix of material properties. To assess various inversion techniques for use in shallow karst conditions, three synthetic karst geology models, each with increasing complexity, were generated. Each model was analyzed using forward modeling techniques to compare the calculated tomograms from known geometry and material properties. Gaussian Raytracing with LSQR inversion technique performed the best. This technique, Gaussian Raytracing with LSQR, was then applied to an inversion problem; cross-borehole ground penetrating radar data was collected at a karst geology field site and tomograms were produced. The resulting tomography confirmed information detailed in the driller\u27s logs and features between boreholes were identified. This confirmed that cross-borehole ground penetrating radar is an applicable technique for use in geotechnical site characterization activities in karst areas

Tunable Infrared Metamaterials

Metamaterials are engineered periodic composites that have unique refractive-index characteristics not available in natural materials. They have been demonstrated over a large portion of the electromagnetic spectrum, from visible to radiofrequency. For applications in the infrared, the structure of metamaterials is generally defined using electron-beam lithography. At these frequencies, the loss and dispersion of any metal included in the composite are of particular significance. In this regard, we investigate deviations from the Drude model due to the anomalous skin effect. For comparison with theoretical predictions, the optical properties of several different metals are measured, both at room temperature and at 4 K. We extend this analysis to the coupling between plasmon and phonon modes in a metamaterial, demonstrating that very thin oxide layers residing at the metal-substrate interface will significantly affect the spectral location of the overall resonance. Oxide-thickness-dependent trends are then explored in some detail. Potential applications of this general area of study include surface-enhanced infrared spectroscopy for chemical sensing, and development of narrowband notch filters in the very long wavelength infrared. We then consider various possibilities for development of tunable infrared metamaterials. These would have wide applicability in dynamically variable reflectance surfaces and in beam steering. We consider several methods that have been previously shown to produce tunable metamaterials in the radio frequency band, and explore the challenges that occur when such techniques are attempted at infrared frequencies. A significant advance in tunable-infrared-metamaterial technology is then demonstrated with the use of thermochromic vanadium dioxide thin films. Highlights include the first demonstration of a tunable reflectarray in the infrared for active modulation of reflected phase, the first demonstration of a tunable resonance frequency in the thermal infrared band, and the largest resonance-frequency shift recorded to date in any part of the infrared. Finally, future work is proposed that holds the promise of wideband frequency tuning and electronically-controllable metamaterials

Optical Nanostructures For Controllable And Tunable Optical Properties

Optical nanostructures are heterogeneous media containing subwavelength inclusions in periodic or aperiodic fashion. The optical properties of optical nanostructure can be controlled and tuned using their constituent material properties and spatial arrangement of the inclusions. While optical nanostructures have been widely studied, controllable and tunable nanostructures using low loss transparent materials has not been studied in detail in the literature. The objective of this research is to perform efficient design and analyses of controllable and tunable optical nanostructures using low loss transparent materials. To that end, versatile and highly accurate numerical methods like finite different tie domain and plane wave expansion methods are reviewed first. These methods and compared in terms of their speed, accuracy, and memory requirement. Different kind of optical nanostructures, consisting of low index transparent materials, are analyzed to study their controllability. For example, single scatterers are optimized to obtain highly direction forward scattering using low index materials. Then, the minimum refractive index required for establishing optical bandgap in a planar periodic nanostructure was established. Using the bandgap, highly sensitive transparent sensors are designed using low index materials. It is found that the numerical methods can analyze small or periodic nanostructure, while requiring significant computational resources. As an alternative to numerical modelling, analytical effective medium approximations are considered. The available approximations are reviewed, and their limitations are pointed out. Using the Mie scattering theory, the Maxwell-Garnett approximation is extended so that it can account for arbitrary size, as well as different physical structures, of the inclusions. The derived effective medium approximation is tested on a wide variety of optical nanostructure, both periodic and aperiodic. Good agreement between analytical and experimental results are established. The utility of the approximation in designing controllable and tunable optical nanostructure is demonstrated by modelling the dynamic optical properties of magnetic colloids and verifying them experimentally. The effective medium approximation can be a very fast, and efficient method of modelling the controllable and tunable properties of optical nanostructure, when applied judiciously. The applicability, limits of validity, and limitation of the approximation is also discussed. Using the analytical framework, controllable optical nanostructure that can mimic optical elements, e.g., focusing lenses, are designed. The relationship between physical structure of the inclusions and the imparted phase by the nanostructure is studied using effective medium approximation and numerical methods. The effective medium approximation can predict the imparted phase with high accuracy, while requiring a fraction of the computation resources compared to numerical methods. Based on the relationship between imparted phase and physical structure of the inclusions, it is possible to design optical nanostructure with controllable spatial phase profile. Using this property, nanostructured optical elements are designed. Their far-field properties are calculated using analytical scalar theory. The analytical results matched well with numerical and experimental results. In conclusion, an analytical method for designing and analyzing tunable and controllable optical nanostructure is derived and verified with experimental results. The analytical method is significantly more efficient compared to numerical methods, while being similarly accurate compared to experimental results. The research in this work can lead to efficient design of optical nanostructure for many different fields

Design of an All-dielectric Sublayer for Enhanced Transmittance In Stacked Antenna Array Applications

In spatially constrained applications, the overlapping of antenna arrays can be unavoidable and its presence can lead to a blockage in the line-of-sight for the underlying antennas. Although previous investigations focused predominantly on the contribution of the ground plane and feed network-which were resolved through the use of frequency selective surfaces and proper feed network design, respectively-it is believed that the ground plane, substrate, and patch regions can emplace a substantial combined impedance. To rectify the transmission through these layers, an all-dielectric implementation is suggested based on the properties of complementary media and Fabry-Perot resonance shifting phenomena. Consequently, both spherical inclusion based dielectric metamaterials and regular dielectrics are suggested, such that additional conductive losses are avoided and surface wave coupling becomes less plausible; while making possible both negative and positive refractive indices. The transfer matrix method and effective medium theory are jointly implemented to examine the required properties of the dielectric or metamaterial sublayer on the basis of providing a transmittance on the order of a known high transmittance analog. Mie theory provides the basis behind the effective properties of the spherical inclusion based metamaterial whereby the required dimensions and permittivity can be determined by a sequential quadratic programming optimization in MATLAB. It is found that the metamaterial emplaces constraints on fabrication which are not currently feasible. Therefore, the equally practicable positive refractive index solutions in a regular dielectric are proposed as the most viable alternative and utilized to determine a functional bandwidth