Active metamaterials with gain compensation

Metamaterials are artificial composite materials that can provide unique and useful properties that do not normally exist in natural materials and may lead to many new and exciting capabilities for electromagnetic applications. However, many of the practical applications are limited by two fundamental issues that have not yet been solved, namely, loss and narrow bandwidth inherently associated with a typical passive metamaterial. In this talk, active metamaterial incorporating gain medium / device will be briefly reviewed first. Experimentally demonstrated 1-D composite right/ lefthanded transmission lines with simultaneous negative phase constant and negative attenuation constant will be discussed. Measurement results on the linear and nonlinear properties of the 1-D transmission line will be presented. Balanced and un-balanced designs as well as a zero-th order antenna consist of a single unit-cell will be described. In addition, volumetric negative index structure based on wire and split-ring resonator arrays with net gain will be presented. This active metamaterial with embedded microwave tunnel diodes at the unit cell level exhibits a band-limited Lorentzian dispersion with an over-compensated loss (gain) and a negative refractive index is demonstrated [6]. It shows examples of subwavelength wire and SRR cells with embedded resonant tunneling diode with negative differential resistance (NDR).