Alexander R. Katko

A microwave metamaterial with integrated power harvesting functionality

We present the design and experimental implementation of a power harvesting metamaterial. A maximum of 36.8% of the incident power from a 900 MHz signal is experimentally rectified by an array of metamaterial unit cells. We demonstrate that the maximum harvested power occurs for a resistive load close to 70 Ω in both simulation and experiment. The power harvesting metamaterial is an example of a functional metamaterial that may be suitable for a wide variety of applications that require power delivery to any active components integrated into the metamaterial.

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.

Nonlinear and active RF metamaterial applications using embedded devices

Nonlinear metamaterials have received considerable attention in recent years. The inclusion of nonlinear and active effects in metamaterials expands the possibilities for engineering media with designer properties. We detail our recent efforts to create nonlinear and active metamaterials at RF with useful properties through the inclusion of embedded nonlinear or active elements. We demonstrate some of the possible applications of such nonlinear and active metamaterials experimentally, with properties including saturable absorption, phase conjugation, and power harvesting.

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.

Transistor-embedded acousto-optic and nonlinear metamaterials

Summary form only given. We present experimental results showing the application of transistor-embedded metamaterials to realize nonlinear and acousto-optic metamaterials. Due to their nonlinear properties, transistors can be used to realize a large variety of useful radio frequency (RF) functions and applications. These include switches, oscillators, amplifiers, tunable resistors and capacitors, and other devices. Metamaterials provide a natural platform for embedding circuit elements at RF to implement useful functionality. The useful properties of transistors can be exploited by embedding transistors within metamaterial unit cells. We will demonstrate how transistors allow the realization of acousto-optic and nonlinear metamaterials. Transistors can be biased to operate in a number of regimes. For this work we focus on the linear regime of the transistor. In this bias range, we can consider the transistor to act as a controllable resistor in parallel with a capacitor, with tuning provided by a bias on the gate of the transistor. Embedding this within a metamaterial allows us to dynamically tune the resonant frequency and quality factor (Q) of the metamaterial by altering the gate bias of the transistor. By using an AC gate bias to tune the transistor, we can construct a nonlinear metamaterial. We demonstrate its nonlinearity by using the metamaterial as a mixer. An acoustic signal incident on a transducer such as a piezoelectric membrane or microphone produces an AC voltage. We apply this voltage to the transistor bias. An incident acoustic signal will modulate the resonant frequency or Q of the metamaterial. We construct a wireless acousto-optic modulator using the nonlinear acousto-optic metamaterial and experimentally demonstrate its efficacy.

Functional metamaterials for wireless phase conjugation

Functional metamaterials provide useful properties for the design of electromagnetic devices. In this work we demonstrate that functions including high nonlinearity and amplification can be included in metamaterials to realize time reversal imaging.