A polarization converting metasurface that operates in the X-band is designed and fabricated. The metasurface converts a linearly polarized incident electromagnetic wave to an orthogonal polarization upon reflection. The design has a subwavelength thickness of 1.27 mm (0.04 λ), a half-power bandwidth of 22%, and a polarization conversion ratio of 99.4%. The metasurface was fabricated with a low-cost printed-circuitboard process. It is realized by cascading three patterned metallic sheets in order to achieve the desired polarization conversion and high reflection. The polarization conversion is accomplished by guiding and coupling electromagnetic energy between pairs of orthogonal slots in a ground plane, placed at the edges of each unit cell. The performance of an experimental prototype showed close agreement with simulation.

Almost ten years have passed since the first experimental attempts of enhancing functionality of radiofrequency metamaterials by embedding active circuits that mimic behavior of hypothetical negative capacitors, negative inductors and negative resistors. While negative capacitors and negative inductors can compensate for dispersive behavior of ordinary passive metamaterials and provide wide operational bandwidth, negative resistors can compensate for inherent losses. Here, the evolution of aforementioned research field, together with the most important theoretical and experimental results is reviewed. In addition, some very recent efforts that go beyond idealistic impedance negation and make use of inherent non-ideality, instability, and non-linearity of realistic devices, are highlighted. Finally, a very fundamental, but still unsolved issue of common theoretical framework that connects causality, stability, and non-linearity of networks with negative elements is stressed.

To date, research on metasurfaces has predominantly focused on those with polarizabilities that are tangent to the metasurface. Only a few theoretical works have characterized metasurfaces with normal polarizabilities. Here, we provide full analytical, numerical, and experimental characterizations of a metasurface that can be described with polarizabilities in all three spatial directions. First, a set of equations is derived that allows a surface distribution of scatterers to be replaced by a sheet boundary condition. It is shown that the extraction of unknown polarizabilities in the normal direction necessarily requires scattering parameters obtained from oblique incidence. Closed-form expressions that relate scattering parameters to surface susceptibilities in all three spatial directions are given. It is shown that the reflection and transmission properties of the metasurface can be predicted, for an arbitrary angle of incidence, from the sheet parameters. In addition, we report the experimental characterization of a metasurface with polarizabilities in the normal direction. The free-space measurements were performed on a recently proposed DB metasurface at 10 GHz. Experiments confirmed that the sheet parameters do not change with angle of incidence. Consequently, it was possible to extract surface susceptibilities in the normal direction from measured transmission parameters alone.

The concept of a self-oscillating non-Foster unit cell, intended for use in future active metasurfaces, is introduced. It is based on two orthogonally polarized small antennas, connected via negative impedance converter (NIC). A NIC converts the admittance of the first antenna into a negative admittance, cancelling the admittance of the second antenna within a theoretically infinite bandwidth. This system behaves as a special kind of a nearly perfectly matched small active antenna that supports oscillations at every frequency within NIC’s operating bandwidth. Frequency tuning is achieved either by inclusion of a resonant circuit or by injection locking from an external source. The influence of NIC dispersion, non-linearity and antenna type on the system performances, is discussed, and appropriate design guidelines are given. Several scaled experimental demonstrators of non-Foster self-oscillating unit cell, operating in lower RF range, were designed, manufactured, and tested. Experimental results proved the correctness of the basic idea and showed stable self-oscillations with tuning range from 1:2 to 1:3.

In this article, we present a physically oriented design of broadband RF negative non-Foster capacitors. The design procedure is based on the matrix decomposition of Linvill’s floating negative impedance converter (NIC). The decomposition allows optimization of the NIC circuit regarding the accuracy of generated capacitance. Furthermore, it provides a clear physical picture of the influence of particular circuit elements on the system properties. The methodology is verified by the design and measurements of a negative capacitor in a 0.5–2 GHz range, suitable for applications in bandpass filters with the NIC-based impedance inverters. Experimental results were found to be in good agreement with the theoretical predictions, verifying the correctness of the presented approach.

Temporal modulation unlocks possibilities to dynamically control and modify the response of electromagnetic systems. Employing explicit dependencies of circuit or surface parameters on time enables the engineering of systems with conventionally unachievable functionalities. Here, we propose an alternative approach that enables the emulation of electromagnetic systems that can have arbitrary frequency dispersion and nonlinear properties, including the non-Foster response. In particular, we show that a proper modulation of a time-varying capacitor allows one to mimic a static inductance, capacitance, or resistance having arbitrary values, both positive and negative. We discuss necessary modifications of determined ideal modulation functions that ensure the stability of the system. To demonstrate the applicability of the proposed method, we introduce and simulate an invisible sensor, i.e., a device that does not produce any scattering and is capable of sensing. Three different geometries are proposed and validated using full-wave simulations. In addition to that, we discuss the stability of the systems that are modulated externally. We believe that this study introduces an alternative paradigm of using time modulations to engineer system responses that can be applied not only to electromagnetic systems (in electronics, microwaves, and optics) but also in other branches of physics.