John P. Barrett

A broadband low-reflection metamaterial absorber

Artificially engineered metamaterials have enabled the creation of electromagnetic materials with properties not found in nature. Recent work has demonstrated the feasibility of developing high performance, narrowband electromagnetic absorbers using such metamaterials. These metamaterials derive their absorption properties primarily through dielectric loss and impedance matching at resonance. This paper builds on that work by increasing the bandwidth through embedding resistors into the metamaterial structure in order to lower the Q factor and by using multiple elements with different resonances. This is done while maintaining an impedance-matched material at normal incidence. We thus present the design, simulation, and experimental verification of a broadband gigahertz region metamaterial absorber, with a maximum absorption of 99.9% at 2.4 GHz, and a full width at half maximum bandwidth of 700 MHz, all while maintaining low reflection inside and outside of resonance.

A low‐frequency near‐field interferometric‐TOA 3‐D Lightning Mapping Array

We report on the development of an easily deployable LF near-field interferometric-time of arrival (TOA) 3-D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with submicrosecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.

RF Limiter Metamaterial Using p-i-n Diodes

We present the design and experimental implementation of an RF limiter metamaterial using a sheet of nonlinear metamaterials. We demonstrate that complementary electric inductive-capacitive resonators loaded with nonlinear p-i-n diodes can act as RF limiter unit cells. We design and fabricate limiter metamaterials and compare them to traditional circuit limiters. Our limiter metamaterial exhibits a minimum insertion loss under 3 dB, a maximum decrease in transmission of 6.95 dB and broadband performance, with a minimum decrease in transmission of 3 dB over 18% bandwidth. The limiter metamaterial is suitable for a wide variety of practical applications requiring protection of sensitive devices from high power.

Time-varying transistor-based metamaterial for tunability, mixing, and efficient phase conjugation

We present a transistor-based microwave metamaterial exhibiting tunability over a wide range of time scales. By loading a metamaterial with a transistor, we show through theory and simulation that both the resonant frequency and quality factor of the metamaterial can be dynamically tuned with a voltage bias. We demonstrate through experiment that such a time-varying transistor-based metamaterial exhibits this tunability. The tunability is applicable to a wide range of time scales, from quasi-static effective parameter tuning to parametric pumping for mixing and phase conjugation. We then apply the metamaterial to a particular application of phase conjugation and demonstrate through simulation and experiment that a very strong phase conjugated signal is produced. We experimentally show that the mixing efficiency for a transistor metamaterial is over 30 dB stronger than that of a varactor-based phase conjugate metamaterial.

Powered and nonlinear RF metamaterials

The electromagnetic properties of metamaterials can be engineered to achieve substantially more flexibility and variety than those available from conventional materials. Adding some degree of external control or power to metamaterials enables another level of functionality. We describe our recent efforts to develop an approach for realizing powered and nonlinear metamaterials in which each unit cell contains embedded active or nonlinear elements. We demonstrate experimentally how such active and nonlinear metamaterials enable properties such as gain, nonreciprocity, and phase conjugation.

Bandwidth Tuning in Transistor Embedded Metamaterials Using Variable Resistance

Metamaterials have been previously loaded with diodes and other types of passive circuit elements. Transistors offer an alternative to these established loading elements to expand the possible capabilities of metamaterials. With embedded transistors, additional degrees of freedom are achieved and lay out the architecture for more complex electromagnetic metamaterial design. A mathematical analysis of transistor loaded SRR unit cells is described in which the transistor acts as a variable resistor. From the mathematical analysis, we calculate transmission coefficients for a single unit cell. We then experimentally measure two SRRs with tunable quality factors and thus tunable bandwidth based upon modulating the effective loading circuit resistance to confirm the calculations. From the agreement between the calculated and measured transmission coefficients, we expand the analysis to show that a slab of more densely packed unit cells can achieve negative permeability with varying degrees of dispersion.

Transistor-based metamaterials with dynamically tunable nonlinear susceptibility

We present the design, analysis, and experimental demonstration of an electromagnetic metamaterial with a dynamically tunable effective nonlinear susceptibility. Split-ring resonators loaded with transistors are shown theoretically and experimentally to act as metamaterials with a second-order nonlinear susceptibility that can be adjusted through the use of a bias voltage. Measurements confirm that this allows for the design of a nonlinear metamaterial with adjustable mixing efficiency.

Design and Full Characterization of Planar Active Magnetic RF Metamaterials

Applications of metamaterials have been limited by parasitic resistive losses. We present the design and measurement of split ring resonators with embedded active field effect transistor circuits comprising a magnetic metamaterial. We show that the imaginary part of the effective permeability changes sign within the bandwidth of the negative differential resistance region, creating a stable gain medium. We present experimental reflection and transmission measurements showing a sign change in the permeability loss term and two applications of such a metamaterial. use as a mu-negative near field parasitic element for small magnetic antennas and as an amplifying material in free space. We show the active metamaterial can provide property enhancement compared with related passive structures in both applications, providing a path to overcoming parasitic loss in previously demonstrated metamaterial structures.

Design and realization of transistor-embedded active RF metamaterials

Summary form only given. We present several results that use transistors within the context of metamaterials. Transistors are a fundamental device in the construction of several RF systems and applications. The basic device physics of the transistor allow for the convenient construction of variable resistors, oscillators, switches, and amplifiers. Embedding transistors within the confines of a split ring resonator (SRR), metamaterials can be designed to have a tunable resonant frequency, a tunable bandwidth, controlled loss cancellation and gain, and controlled oscillation. We will demonstrate how to design and realize all of the aforementioned properties.

Roadmap to electrically self-tuning metamaterials: Design and experimental validation

We present the design and experimental validation of a self-tuning metamaterial. We begin with an active two-state split ring resonator and build up to a self-contained system that includes feedback. We show the ability of the active split ring resonator to track and lock to an incident RF signal around 1 GHz. The SRR has a tuning capability of 4% bandwidth and takes a maximum of 3 seconds to track and lock to an incident signal.