We present the first use of a waveform-dependent absorbing metasurface for high-power pulsed surface currents. The new type of nonlinear metasurface, composed of circuit elements including diodes, is capable of storing high power pulse energy to dissipate it between pulses, while allowing propagation of small signals. Interestingly, the absorbing performance varies for high power pulses but not for high power continuous waves (CWs), since the capacitors used are fully charged up. Thus, the waveform dependence enables us to distinguish various signal types (i.e. CW or pulse) even at the same frequency, which potentially creates new kinds of microwave technologies and applications.Conventional absorbers [1,2] are independent of incoming power levels and therefore absorb not only destructive high power signals but also small signals necessary for antenna communications. These two effects can be decoupled by introducing nonlinearity into a periodic structure or metasurface [3] such that the artificially engineered surface possesses a low power surface impedance that is different from its high power absorbing response. Furthermore, we can take advantage of the fact that high power microwave signals generally come from pulsed sources, and design a structure in which the high power microwave energy is rectified and stored as a static field in the surface, and then dissipated between pulses. This provides freedom in the choice of lossy components, allowing us to further decouple the high power absorption properties from the small signal behavior and also design the surface to respond specifically to short pulses.Many nonlinear metamaterials and metasurfaces derive their nonlinear properties from altering the resonant paths of induced electric currents with nonlinear media (e.g. through semiconductive substrates [4][5][6][7] or geometrical modifications induced by heating [8] or magnetic force [9,10]). These differ from our nonlinear metasurfaces based on diodes or nonlinear circuits. Diodes switch between nonconducting and conducting states depending on the applied voltage and therefore are capable of handling incoming signals differently depending on the magnitude. Because diodes rectify high power signals and convert much of the energy to a static field, our nonlinear metasurfaces perform differently from others that convert incoming signals into high order modes [11,12] or exploit wave mixing techniques [13].To understand how such a structure absorbs high power pulse energy consider Figs. 1 (a) and (b). Our metasurface is composed of circuit components (i.e. diodes, capacitors and resistors) as well as a conductive ground plane and periodic metal patches, separated by a dielectric substrate, Rogers 3003. During a pulse (i.e. * hirokiwaka@gmail.com † dsievenpiper@eng.ucsd.edu Fig. 1 (a)) high power surface currents propagate on the patches. Once the voltage difference across a diode reaches the turn-on voltage, electric charges flow into a capacitor and the energy is stored. Between pulses ( Fig. 1 (b)) the stored ele...
Electromagnetic properties depend on the composition of materials, i.e. either angstrom scales of molecules or, for metamaterials, subwavelength periodic structures. Each material behaves differently in accordance with the frequency of an incoming electromagnetic wave due to the frequency dispersion or the resonance of the periodic structures. This indicates that if the frequency is fixed, the material always responds in the same manner unless it has nonlinearity. However, such nonlinearity is controlled by the magnitude of the incoming wave or other bias. Therefore, it is difficult to distinguish different incoming waves at the same frequency. Here we present a new concept of circuit-based metasurfaces to selectively absorb or transmit specific types of waveforms even at the same frequency. The metasurfaces, integrated with schottky diodes as well as either capacitors or inductors, selectively absorb short or long pulses, respectively. The two types of circuit elements are then combined to absorb or transmit specific waveforms in between. This waveform selectivity gives us another degree of freedom to control electromagnetic waves in various fields including wireless communications, as our simulation reveals that the metasurfaces are capable of varying bit error rates in response to different waveforms.
The role of frequency is very important in electromagnetics since it may significantly change how a material interacts with an incident wave if the frequency spectrum varies. Here, a new kind of microwave window is demonstrated that has the unique property of controlling transmission and reflection based on not only the frequency of an incoming wave but also the waveform or pulse width. This is achieved by designing a planar periodic surface with circuit elements including diodes, which convert most of the incoming signal to zero frequency. This surface can preferentially pass or reject different kinds of signals, such as short pulses or continuous waves, even if they occur at the same frequency. Such a structure can be used, for example, to allow long communication signals to pass through, while rejecting short radar pulses in the same frequency band. It is related to the classic frequency selective surface, but adds the new dimension of waveform selectivity, which is possible only by introducing nonlinear electronics into the surface. Thus, the study is expected to provide new solutions to both fundamental and applied electromagnetic issues ranging from traditional antenna design and wireless communications to emerging areas such as cloaking, perfect lenses, and wavefront shaping.
This paper shows that customised broadband absorption of electromagnetic waves having arbitrary polarisation is possible by use of lossy cut-wire (CW) metamaterials. These useful features are confirmed by numerical simulations in which different lengths of CW pairs are combined as one periodic metamaterial unit and placed near to a perfect electric conductor (PEC). So far metamaterial absorbers have exhibited some interesting features, which are not available from conventional absorbers, e.g. straightforward adjustment of electromagnetic properties and size reduction. The paper shows how with proper design a broad range of absorber characteristics may be obtained.
We demonstrate a concept of circuit-based nonlinear metasurface absorbers that absorb high power surface currents but not small signals. The nonlinear absorbing behavior is achieved through the use of diodes integrated into the metasurface. The diodes rectify high power signals to produce a static field, whose energy is stored in capacitors and then dissipated with resistors. We present electromagnetic simulations of these structures and validate the results with the first measurements of nonlinear metasurface absorbers. The metasurfaces can potentially contribute to solving a wide range of microwave interference issues due to their power dependent electromagnetic response.
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