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

Abstract

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 Fast-Wave Propagation in a Non-Foster Circuit Loaded Waveguide

“…The retrieved effective material parameters from the measured -parameters identify an effectively uniform medium with a constant (less than 10% variation) phase velocity of over 60-120 MHz, or 2:1 bandwidth. Compared to [11] and [12], the central operating frequency has been increased from 20 to 90 MHz, and the bandwidth is still significantly more broadband than the conventional metallic waveguides. Furthermore, the measurement results have been verified by Kramers-Kronig relations and the measured near-field distribution along the microstrip line, which demonstrate that the accomplished waveguide can be regarded as a stable, causal, and homogeneous material rather than a lumped element.…”

“…Therefore, the velocity of the start and end of the information cannot exceed . To put it another way, as discussed in [11], [12], and [28], the speed of the information is determined by the "precursor velocity" or the "front velocity" rather than the group velocity, and they are never faster than . However, the "precursor velocity" or the "front velocity" argument can not clearly and comprehensively answer the question of whether a band-limited "superluminal" propagation exists or not, and thus it is still an open question calling for further scientific debate.…”

“…Both papers reported measurements on single unit cell and the multiple-unit-cell simulations for investigating the effects of the large-scale EM structures. The only available measurement of a multi-section FW structure was presented over the bandwidth of 2-40 MHz in [11] and [12], where the negative capacitors were realized with op-amp circuits. As is well known, an op-amp circuit is an integrated circuit (IC) whose available operating frequency is usually below 50 MHz owing to the fixed gain-bandwidth (GBW) product and its parasitics.…”

Stability of embedded non-Foster metamaterials with potentially unstable circuit parameters

“…A number of investigators have recently presented promising results in the development of non-Foster metamaterials such as wideband artificial magnetic conductors [1], wideband composite metamaterial and metasurfaces [2], wideband metamaterial structures [3,4], and measurements of wideband epsilon-near-zero metamaterials with gain [5]. However, the active devices inherent in non-Foster metamaterials present the potential for instability [6,7].…”

Metamaterials: The early years in the USA

“…These principles have been used by them to realize lossless magnetic [123] and non-reciprocal [124] microwave metamaterials. Non-Foster elements to achieve dispersionless, wide bandwidth and superluminal effects in metamaterials have been studied by Hrabar's team at the University of Zagreb [125][126][127]. Superluminal waveguides based on non-Foster circuits have been reported by Sievenpiper's UCSD group [128].…”