Vasundara V. Varadan

Unique Retrieval of Complex Permittivity and Permeability of Dispersive Materials From Reflection and Transmitted Fields by Enforcing Causality

The electromagnetic properties of general classes of materials can be obtained by inverting the measured or numerically simulated reflection ( S 11) and transmission ( S 21) coefficients through a known thickness of a planar slab of the material. The major difficulty is the uncertainty in the change of phase of the transmitted field, if the phase change exceeds 2mpi, m=0,1,2,.... This happens for thick samples as well as for highly dispersive materials. An infinite number of solutions are generated for the imaginary part of the complex wavenumber gamma = alpha + jbeta for different choices of m. The choice of m can be made unique by noting that for a causal medium, gamma is an analytic function of frequency and this results in a Kramers-Kronig-type relation between alpha and beta. Since alpha is independent of the choice of m and, hence, uniquely determined, a Kramers-Kronig reconstruction of beta using alpha will provide a guideline for the correct choice of m. This is a foolproof approach even for totally unknown materials since no initial guess is required. Indeed, this approach will work in optics and ultrasonics, and should also be effective for determining the effective refractive index of photonic-bandgap structures that are also highly dispersive.

Tuning the effective properties of metamaterials by changing the substrate properties

Tunable properties of materials are highly desirable in many applications. Metamaterials with negative properties (permittivity, permeability, or both) can have many applications if such properties can be made tunable or can be switched alternatively between positive and negative property behavior. This paper is a numerical study of the effect of substrate properties on the effective properties of a metamaterial slab. We present both simulation results and measurement data for a specific split-ring resonator structure for two different substrate thicknesses and demonstrate very good agreement. Then, using finite element simulation, varying the permittivity of the substrate from 1 to 14 while keeping its physical thickness fixed, we show that the resonance frequency drops from ∼16to∼6GHz. Alternately, when the physical thickness of the substrate is varied from 0.05to2mm, keeping its permittivity fixed, the resonance frequency decreases from ∼13.2to∼9.2GHz. In each case, the effective refractive index is re...

Effective properties of split-ring resonator metamaterials using measured scattering parameters: Effect of gap orientation

This paper presents experimental S-parameter data (amplitude and phase) in the 8–26GHz range for split-ring resonator (SRR) metamaterial samples exhibiting electric (E) and/or magnetic (H) resonances at 10.4GHz depending on the orientation of the gaps of the SRRs with respect to the E and H fields. Use of a thru, reflect, line (TRL) calibrated focused plane wave, free space microwave system permits inversion of the complex permittivity, permeability, and refractive index of planar metamaterial samples from measurement of the reflected and transmitted S parameters of the system. Samples were prepared by patterning concentric SRRs on a dielectric substrate. Three different orientations of the SRR have been used in the experimental study; (i) SRRparallel—gaps in the split rings parallel to the incident E field to realize a dielectric resonance; (ii) SRRrandom—randomly oriented gaps to realize simultaneous dielectric and magnetic resonances; and (iii) SRRperpendicular—gap oriented perpendicular to the E-field to realize a strong magnetic resonance. The experimentally extracted material properties show very good agreement with numerical simulations of effective properties giving rise to a negative magnetic permeability. A second resonance at 23GHz clearly establishes a passband between 10.4 and 23GHz for all samples. The extraction of effective properties from measured S parameters is a useful development in metamaterials research and the study of random orientations of gaps with respect to the E field is an interesting design for metamaterials that may lead to negative index behavior with further optimization.This paper presents experimental S-parameter data (amplitude and phase) in the 8–26GHz range for split-ring resonator (SRR) metamaterial samples exhibiting electric (E) and/or magnetic (H) resonances at 10.4GHz depending on the orientation of the gaps of the SRRs with respect to the E and H fields. Use of a thru, reflect, line (TRL) calibrated focused plane wave, free space microwave system permits inversion of the complex permittivity, permeability, and refractive index of planar metamaterial samples from measurement of the reflected and transmitted S parameters of the system. Samples were prepared by patterning concentric SRRs on a dielectric substrate. Three different orientations of the SRR have been used in the experimental study; (i) SRRparallel—gaps in the split rings parallel to the incident E field to realize a dielectric resonance; (ii) SRRrandom—randomly oriented gaps to realize simultaneous dielectric and magnetic resonances; and (iii) SRRperpendicular—gap oriented perpendicular to the E-field...

Electrically Small, Millimeter Wave Dual Band Meta-Resonator Antennas

A meta-resonator antenna is one in which a metamaterial resonator is the radiating element of the antenna. In this paper, meta-resonator antennas are developed using multilayer low temperature co-fired ceramics techniques. A pair of split-ring resonators (SRR) is used as the radiating element of the antenna. The two SRRs have different resonance frequencies due to the opposite placement of the gaps and the antenna can operate at both frequencies. Other multiband antennas can be designed by adding different metamaterial resonators. No matching network is required since feeding is by inductive/capacitive coupling. The size of the meta-resonator antenna is 10% of a conventional microstrip antenna. The electrical size (ka) of the antenna is 0.386, the bandwidth is 2%, gain is 3.76 dB and efficiency is 71%. An omnidirectional meta-resonator antenna is designed by removing the ground plane and by using a microstrip line as the feed line. The feed line can also serve as a monopole antenna if desired. The radiation pattern, efficiency and gain of the omnidirectional meta-resonator antenna are similar to those of a monopole antenna.

Evaluation of a new self-contained, ambulatory, objective cough monitor.

Objective and backgroundObjective monitoring of cough may be preferred to subjective reporting of the symptom in clinical and research settings. Therefore, a self-contained, ambulatory cough monitoring system is needed that is non-invasive, usable for children and adults of all ages, inexpensive, and highly accurate with easy to use analysis software.MethodologyAfter development of a new device, 15 subjects with frequent coughing were recorded with the novel cough monitor and a simultaneous video recording in order to validate the monitor compared with a gold standard. Two investigators independently analyzed the recordings and counted the number of coughs during the study period from both the cough monitor and the video recording.ResultsWhen measuring agreement between the two investigators, the sample concordance correlation coefficient for audio counts was 0.998 (p < 0.001). In the comparison of video counts, the sample concordance correlation coefficient was 0.997 (p < 0.001). For the comparison of investigator 1s video counts to the corresponding audio counts, the sample concordance correlation coefficient was 0.968 (p = 0.026). For the comparison of investigator 2s video counts to the corresponding counts, the sample concordance correlation coefficient was 0.973 (p = 0.015).ConclusionWe have developed and piloted a new, valid, and reproducible method of objectively recording and analyzing cough. This device appears to be useful for subjects of any age and in clinical and research settings.

Fishnet metastructure for efficiency enhancement of a thin film solar cell

We propose embedment of a fishnet metastructure in the back passivation layer for enhancing the efficiency of hydrogenated amorphous silicon (a-Si:H) thin film solar cells. Incident light excites a plasmon resonance that results in frequency dependent effective impedance for the embedding layer so that the input impedance satisfies impedance matching condition. Reflection is very low under this condition. A planar 20 nm-thick fishnet structure is embedded in the back passivation layer of the solar cell to enhance light absorption near the bandgap of a-Si:H. This enhancement remains over a wide range of incident angles. Detailed electromagnetic modeling of the absorption in different layers of the solar cell is performed. Only absorption in the a-Si:H is included in computing the photocurrent generation. 64% of the total absorbed energy at resonance is in the silicon layer and this absorption is uniformly distributed inside the silicon. Based on the enhancement of photocurrent density near the bandgap of a...

Incorporation of a light and carrier collection management nano-element array into superstrate a-Si:H solar cells

Superstrate a-Si:H solar cells incorporating a nano-column array for light and photocarrier collection have been fabricated and evaluated. It is found that the short circuit current density (JSC) is significantly increased while the open circuit voltage and fill factor are not detrimentally affected by this architecture. Numerical analysis of JSC matches experiment and shows that the enhanced JSC observed is due to both effective absorber thickness and photonic-plasmonic effects. Further analysis shows that this nano-column architecture can lead to a 42% increase in conversion efficiency over that of the planar control for a 200 nm absorber thickness cell.

Electric and magnetic resonances in symmetric pairs of split ring resonators

Orientation of the gap of a split ring resonator determines whether the resonance is an electric or magnetic response. When the gap of a split ring resonator is parallel to the incident E-field, an electric resonance is excited, and when the gap is perpendicular to the E-field, a magnetic resonance is excited. In this paper, we show that strong coupling between adjacent symmetric split ring resonators can give rise to dual electric and magnetic resonances if the intercell spacing is small enough. By varying the interparticle spacing within a unit cell, we can position the dual resonances as desired. Inverting the simulated reflection and transmission coefficients of a periodic slab of symmetric pairs of split ring resonators, the permittivity and permeability can be extracted and are shown to result in negative properties at resonance. Through a careful analysis of the extracted and Lorentz model fits of the permittivity and permeability, together with the simulated S-parameters, we have established a cle...

Temperature Dependence of Resonances in Metamaterials

Metamaterials are increasingly being proposed for many different microwave applications. Metamaterials are highly resonant structures and metamaterial device design is based on the resonance frequency for both civilian and military applications. These applications are often in a high-temperature environment. In this paper, we have performed an experimental study of the shift in resonance frequency of selected metamaterials (split-ring resonators and thin wires) in the 25°C -400°C range using FR4 and low-temperature co-fired ceramic substrates. We have provided a theoretical explanation for the observed shift in resonance frequency by calculating the shift in frequency resulting from the temperature dependence of the substrate permittivity and electrical conductivity of the metal, as well as thermal expansion of the metallo-dielectric structures comprising the metamaterial. The measured downward shift of the resonance frequency with increasing temperature agrees very well with full-wave finite-element simulation. It is found that the change of permittivity of the substrate is the primary effect, while the thermal expansion of the metallic structure is a secondary effect. The reducing resonance strength is due to the decrease in electrical conductivity with rising temperature. This study can be applied to other metamaterial metallo-dielectric structures or other planar microwave resonators printed on dielectric substrates.

Embedded Wideband Metaresonator Antenna on a High-Impedance Ground Plane for Vehicular Applications

A conformal embedded wideband metaresonator antenna is proposed for military vehicular applications. Metamaterials are artificial materials that exhibit plasmonic resonances with subwavelength sizes of metallic structures. Metaresonator antennas use metamaterial structures as radiators to reduce the size of antennas and design multiband antennas. A small-dipole antenna is placed on a high-impedance ground plane with a conjoined split-ring resonator (SRR). The total volume of the antenna, including the effectively high impedance ground plane, is only 0.51λ0 × 0.41λ0 × 0.05 λ0. The embedded multilayer ceramic antenna was fabricated using a low-temperature co-fired ceramic (LTCC) technique and is well suited for embedment in the armor. Very good agreement was obtained between full-wave simulation results and measurements of the reflection coefficient and radiation pattern.