Abstract
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.
Highlights
The advent of metamaterials and metasurfaces, or artificial structures composed of subwavelength periodic units, allowed us to design a wide range of electromagnetic properties including even ones not available from nature[1,2,3]
All the past studies were evaluated with only surface waves or free-space waves at a normal angle[23,24,27], in reality electromagnetic waves scatter from various structures or boundaries and illuminate such metasurfaces at oblique angles
It turns out from these figures that as seen in normal incidence[27], there appears a clear difference between absorptance of 50 ns short pulses and that of continuous waves (CWs), when the input power and incident angle were respectively set to 0 dBm and 20 degrees, which is a relatively small angle
Summary
Waves at the Same Frequency received: 09 June 2016 accepted: 18 July 2016 Published: 12 August 2016. 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 expected to give us an additional degree of freedom to control electromagnetic waves. Metasurfaces were recently designed with several circuit elements including schottky diodes so that they enabled us to sense difference in the waveforms of incoming waves or pulse widths[23,24,25] (Fig. 1) This new capability to distinguish different waves even at the same frequency was expected to give us another degree of freedom to control electromagnetic waves, thereby leading to development of new kinds of microwave devices and applications such as waveform-selective wireless communications[26]. More precise measurement can be performed by taking account of the frequency dependence of the gain and S11 of the antennas used, we adopted the values at 4.0 GHz for all the frequencies for the sake of simplicity
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