Hiroki Wakatsuchi

Customised broadband metamaterial absorbers for arbitrary polarisation

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.

Circuit-based nonlinear metasurface absorbers for high power surface currents

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.

Generalized scattering control using cut-wire-based metamaterials

We numerically show how multiple band scattering parameters for arbitrary polarization can be easily designed by using cut-wire-based (CW-based) metamaterials, which enable us to control six fundamental scattering properties, i.e., reflection, transmission, absorption, and polarization independence, dependence and change. The structure shown here is composed of three-independent metamaterial designs, each of which has a different scattering profile. The use of the simple CW-based metamaterials is expected to facilitate complex scattering parameter design, which potentially contributes to the development of more complex and multiple band metamaterial applications.

Switchable nonlinear metasurfaces for absorbing high power surface waves

We demonstrate a concept of a nonlinear metamaterial that provides power dependent absorption of incident surface waves. The metasurface includes nonlinear circuits which transform it from a low loss to high loss state when illuminated with high power waves. The proposed surface allows low power signals to propagate but strongly absorbs high power signals. It can potentially be used on enclosures for electric devices to protest against damage. We experimentally verify that the nonlinear metasurface has two distinct states controlled by the incoming signal power. We also demonstrate that it inhibits the propagation of large signals and dramatically decreases the field that is leaked through an opening in a conductive enclosure.

Cut-Wire Metamaterial Design Based on Simplified Equivalent Circuit Models

Effective equivalent circuits are used for the prediction of resonant and absorbing behavior of cut-wire-based (CW-based) metamaterials. Firstly, an equivalent circuit applicable to electric resonance frequencies of single CW metamaterial arrays is considered. Secondly, the equivalent circuit is extended for prediction of magnetic resonance frequencies of symmetrically paired CW metamaterial arrays and asymmetrically paired CW metamaterial arrays. Finally, since the magnetic resonance of the symmetrically paired CW arrays is analogous to the resonance of the CW metamaterial absorbers, i.e., absorbing behavior of the absorbers, the absorptance peak frequencies of CW metamaterial absorbers are estimated. Close agreement is obtained with numerically obtained values, the difference being typically 4, 6, 4, and 2% for the single CW, symmetrically paired CW, asymmetrically paired CW metamaterials and CW metamaterial absorbers, respectively. The paper concludes with discussions pointing out differences with a previous equivalent circuit and improvements to the proposed equivalent circuits.

Waveform Selectivity at the Same Frequency

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.

Performance of Customizable Cut-Wire-Based Metamaterial Absorbers: Absorbing Mechanism and Experimental Demonstration

Performance of customizable cut-wire-based (CW-based) metamaterial absorbers is studied both numerically and experimentally. In the first part of the paper the fundamental absorbing performance of the CW-based metamaterial absorbers is numerically investigated. In this paper two simple geometrical modifications are proposed to easily enhance the absorbing performance: use of geometrical symmetries and wider CWs. The latter part of the paper shows an experimental demonstration of the CW-based metamaterial absorbers using a metal box lined with metamaterial absorber. The metal box has a resonant frequency which reduces the shielding effectiveness of the metal box but disappears when multiple CW absorbing elements are introduced. The absorbing performance is also confirmed by numerical simulations. The CW-based metamaterial absorbers have several important features, e.g., the simple geometry, both arbitrary enhancement and reduction of the absorbing performance and multiple band operation, which may be useful in satisfying a wide range of wave absorber applications.

Waveform-selective metasurfaces with free-space wave pulses at the same frequency

Waveform-selective metasurfaces were recently reported to enable us to distinguish different surface waves even at the same frequency in accordance with their waveforms or pulse widths. In this study, we demonstrate that such new characteristics are applicable to controlling not only surface waves but also free-space waves including ones normal to metasurfaces. Both simulation and measurement results show selective absorption or transmission for specific pulses at the same frequency. Thus, the waveform selectivity is expected to create a wider range of new applications, for instance, waveform-selective antennas and wireless communications.

Synthesis and Design of Programmable Subwavelength Coil Array for Near-Field Manipulation

We present the design and experimental demonstration of a programmable subwavelength coil array, which can manipulate the evanescent magnetic field leading to a precisely controlled electric-field pattern at an image plane in the near-field zone. A synthesis methodology is outlined for 2-D near-field manipulations. Simulation based on a full-wave electromagnetic solver clearly demonstrates the focused electric-field pattern, and its implementation is discussed. An example of a three-layer subwavelength coil array with 48 units is fabricated and measured. The consensus of the measured results with the simulated and synthesized results verified the presented synthesis method. Such a device, capable of producing a programmable magnetic field and electric field, respectively, in orthogonal planes, will find applications in biomedical devices such as neural stimulation, and potentially other areas such as noncontact charging.

Responses of Waveform-Selective Absorbing Metasurfaces to Oblique Waves at the Same Frequency.

Conventional materials vary their electromagnetic properties in response to the frequency of an incoming wave, but these responses generally remain unchanged at the same frequency unless nonlinearity is involved. Waveform-selective metasurfaces, recently developed by integrating several circuit elements with planar subwavelength periodic structures, allowed us to distinguish different waves even at the same frequency depending on how long the waves continued, namely, on their pulse widths. These materials were thus expected to give us an additional degree of freedom to control electromagnetic waves. However, all the past studies were demonstrated with waves at a normal angle only, although in reality electromagnetic waves scatter from various structures or boundaries and therefore illuminate the metasurfaces at oblique angles. Here we study angular dependences of waveform-selective metasurfaces both numerically and experimentally. We demonstrate that, if designed properly, capacitor-based waveform-selective metasurfaces more effectively absorb short pulses than continuous waves (CWs) for a wide range of the incident angle, while inductor-based metasurfaces absorb CWs more strongly. Our study is expected to be usefully exploited for applying the concept of waveform selectivity to a wide range of existing microwave devices to expand their functionalities or performances in response to pulse width as a new capability.