Yifeng Fan

Direct Radiations of Surface Plasmon Polariton Waves by Gradient Groove Depth and Flaring Metal Structure

An efficient method to radiate surface plasmon polaritons (SPPs) directly using metallic corrugated strip with gradient grooves and flaring structure has been proposed, in which the gradient grooves are used to overcome the big mismatch of wavenumbers between SPP modes and radiated spatial waves while the flaring structure provides impedance matching in wide frequency band. Both numerical simulations and measurement results suggest good direct radiation performance of the proposed structure in a wide frequency band from 5 to 20 GHz, in which the measured radiation patterns show a high gain level of 9.9 dBi on average, and the average efficiency can achieve 92%. The proposed direct radiation structure is of great application values in communication systems and integrated circuits.

Characterization of active metamaterials based on negative impedance converters

Negative impedance converters (NICs) are used to create impedance loads that can effectively cancel the inductive properties of magnetic dipoles, resulting in active metamaterials with increased bandwidth and reduced loss for μ-near-zero (MNZ) and negative-Re(μ) (MNG) media. We demonstrate techniques for analyzing the stability and characterizing the magnetic properties of effective media loaded with NICs. Specifically, we apply the Nyquist criterion to validate the stability of sample active metamaterials. It is shown that the practical NIC-loaded metamaterial may maintain stability and reduce dispersion, albeit with reduced performance as compared to the ideal NIC load.

An Active Wideband and Wide-Angle Electromagnetic Absorber at Microwave Frequencies

Two-dimensional (2-D) metamaterials (MTMs) can be used to create perfect electromagnetic absorbers. In this letter, a novel active MTM absorber with non-Foster loads is proposed. For obliquely incident plane waves with both transverse-electric (TE) and transverse-magnetic (TM) polarizations, its effective circuit model is analytically demonstrated, accounting the material losses. Based on the circuit model, a stability characterization is introduced to give the design principles for the non-Foster elements to achieve a wideband and wide-angle metamaterial absorber (MA). These active elements are achieved by a two-port non-Foster circuit based on the resonant tunneling diodes. For the purpose of verification, a sample active MA design is presented, exhibiting a high electromagnetic absorption rate for wideband and wide-angle incidence for both TE and TM polarizations at microwave frequencies, as compared to its passive counterpart.

Noise analysis of broadband active metamaterials with non-Foster loads

Active metamaterials loaded with non-Foster inclusions may exhibit broadband and low-loss material properties, as compared to their passive counterparts. However, the study of their noise performance still remains to be explored. In addition to the Johnson-Nyquist noise from lossy conductors, nonlinear materials used for the design of negative impedance converters (NICs) will give rise to noise as well. In this paper, a general analytical model is developed for the prediction of the noise performance of broadband, stable active metamaterials based on NICs. The noise performance is analyzed in relation to stability, and the effective material properties. The techniques developed in this paper can be applied generally to metamaterials for which equivalent circuit models exist, and results are shown demonstrating the power spectral density and the noise figure of the specific case of an active metamaterial consisting of loaded loop arrays. Results are compared to and verified with circuit simulations.

Wave propagation in reconfigurable broadband gain metamaterials at microwave frequencies

The wave dispersion characteristics for loop array-based metamaterials were analyzed, based on the general transmission line model of a one-dimensional host medium interacting with a chain of coupled loops. By relating the wave propagation constant and the effective parameters of the coupled host medium, we showed that an active medium embedded with non-Foster loaded loop array can be designed to exhibit broadband negative material parameters with positive gain. Accounting for all interactions, the stability of the active medium was investigated, further yielding necessary design specifications for the non-Foster loads. Subsequently, an experimental demonstration was provided to verify the theoretical analysis, showing that stable reconfigurable broadband gain metamaterials at microwave frequencies can be obtained with proper negative impedance converter design.

Noise power in active broadband metamaterials

Active metamaterials loaded with non-Fosters inclusions may exhibit broadband and low-loss material properties, as compared to their passive counterparts. However, the study of their noise performance still remains to be explored. In addition to the Johnson-Nyquist noise from lossy conductors, nonlinear materials used for the design of negative impedance converters (NICs) will give rise to noise as well. In this paper, the noise power in active metamaterials is analyzed in relation to its stability.

Recent Advances in Practical Metamaterial Engineering

Metamaterials have been used to implement materials with near-zero and negative values of the relative electric permittivity (e), as well as magnetic permeability (μ). These properties enable potentially significant applications, including superlenses, invisibility cloaks, and wideband artificial magnetic conductors. However, it has been proven that passive materials with these properties are necessarily narrow-banded and lossy. Active inclusions can be used to overcome these limitations, albeit at the expense of possible instability and noise issues, and added complexity. We examine a promising possible active implementation: the non-Foster circuit (NFC). Non-Foster circuits are used to create negative impedance loads, and have experienced widespread use for amplifying analog signals in trans-Atlantic telephony cables. There is also a long body of research into their applications for increasing antenna bandwidth. In this presentation, we demonstrate their application to multiple unit-cell metamaterials, ...

Active circuits for improved metamaterial performance

The performance of passive antennas and metamaterials are restricted by fundamental physical limits. For example, due to inherent limitations imposed by causality, metamaterials are both lossy and narrowbanded. We will demonstrate active loads that can be used to improve the performance of both metamaterials and antennas.