Ivan Bonic

Negative capacitor paves the way to ultra-broadband metamaterials

Experimental demonstration of the overcoming of basic dispersion-energy constraints in metamaterials with the help of active non-Foster negative capacitors is reported. The experimental metamaterial operates in RF regime, and it is based on air transmission line loaded with negative capacitors. Measurement results clearly show almost dispersionless Epsilon-Near-Zero behavior, accompanied with superluminal both phase and group velocities, over a bandwidth of more than four octaves (2 MHz-40 MHz). The principle of periodic loading of transmission line with negative capacitors may find applications in ultra-broadband active metamaterials for antennas and cloaking technology.

Ultra-broadband simultaneous superluminal phase and group velocities in non-Foster epsilon-near-zero metamaterial

Experimental investigation of ultra-broadband simultaneous superluminal phase and group velocity in recently introduced epsilon-near-zero (ENZ) non-Foster metamaterial is reported. The phase and group velocities were extracted both from the measurements of scattering parameters in the frequency domain and the measurements of propagation time of broadband Gaussian pulse in time domain. Reported superluminal effects are extremely broadband (relative bandwidth of 180% (1:20)) and they are fundamentally different from widely reported narrowband experiments based on the anomalous dispersion. The phenomenon of simultaneous superluminal phase and group velocities may find applications in transformation electromagnetic devices and in communication systems.

Broadband superluminal effects in ENZ active metamaterial

Recently, it has been shown possible to construct an (almost) dispersionless active ENZ metamaterials and transmission lines based on non-Foster elements. These structures are inherently extremely broadband (several octaves) and they may support counter-intuitive superluminal phase and group velocities. However, causality is always preserved due to finite bandwidth of any realistic non-Foster element. Here, the measurements of broadband superluminal phase and group velocity of an active transmission line operating in lower RF regime (10 MHz – 40 MHz) are reported.

Dispersionless ENZ and MNZ metamaterials based on non-Foster elements

Recently introduced concept of active non-Foster metamaterial is reviewed with an emphasis on the counterintuitive background physics. The main fundamental and technological limitations such as stability and selection criteria for active elements are highlighted. It is shown that non-Foster-based active metamaterials are stable if equivalent permittivity or permeability is positive number. Since this number can be (almost) arbitrarily small, the construction of practical broadband ENZ or MNZ metamaterials is feasible. Several examples of extremely broadband (bandwidth of more than two octaves) 1D and 2D active ENZ metamaterials developed at University of Zagreb are presented.

Homogenization of active transmission-line-based ENZ metamaterials

An active dispersionless ENZ metamaterial, based on periodic loading of a transmission line with non-Foster negative capacitors, has been introduced recently. Direct experimental determination of the equivalent permittivity of homogenized active transmission line might be difficult due to inherent instability of negative capacitors. It is also difficult to develop a realistic model of active negative capacitor, which is a prerequisite for numerical homogenization. As a first step, one can model the active metamaterial as a transmission line periodically loaded with one-port ‘black boxes’, properties of which are derived from measured scattering parameters of a single negative capacitor. This method is demonstrated in homogenization of the experimental active ENZ metamaterial operating in the lower RF band (20MHz-120 MHz).

On bandwidth of transmission-line-based and inclusion-based non-foster ENZ metamaterials

The bandwidth of recently introduced transmission-line-based and inclusion-based non-Foster active ENZ metamaterials is studied numerically and experimentally. It is shown that the inclusion-based non-Foster metamaterials exhibit narrower operating bandwidth (up to two octaves) comparing to the transmission-line-based non-Foster metamaterials (more than four octaves). This difference occurs due to frequency behavior of equivalent coupling network that connects neighboring unit cells.