| The manipulation of polarizations as well as achieving selective absorption andreflection of electromagnetic (EM) waves have important applications in various civiland military technologies such as imaging, antenna, communication system,electromagnetic compatibility (EMC), stealth and camouflage The emerging EMmetamaterials provide a totally new strategy to achieve and optimize the above twopurposes. In this thesis, we carry out the following research works focusing on thepolarization and reflection/absorption control:Firstly, we designed and fabricated a broadband and high performance90metamaterial linear polarization converter (MLPC) via giant and achromaticamplitude modulation based on Fabry-Perot cavity resonance between an asymmetricsingle split ring resonator (as-SRR) and the ground plane. The simulated cross-polarized reflectance is above0.8and the co-polarized one is below0.09in the widefrequency range of10-17.6GHz. Meanwhile, the corresponding polarizationconversion ratio (PCR) is above0.9in this band. The ultrahigh PCR above0.98isalso observed in the relatively wide frequency range of9.8-12.5GHz and16-17.3GHz, comparing to the reported literatures. The influences of asymmetric degree and spacer loss tan eon co-polarized reflectance, cross-polarized reflectance, andPCR have been studied. Results show that the co-polarized electric field componentdecreases with the increasing of, and converts to cross polarized waves with nearunity efficiency. Furthermore, the increasing of tan ealmost has no effects on the co-polarized electric field component (<0.09), but decreases the cross-polarized electricfield component and PCR. Fabry-Perot cavity resonance model indicates that thebroadband and high performance of linear polarization conversion originates from thebroadband polarization responding of the asymmetric structure, multiple polarizationcoupling processes and low spacer loss. At last, the amplitude modulation propertiesof aforementioned material can also provide a new way to design other EMmodulators.Secondly, we designed and fabricated a multiple metal grooves (MMG)metamaterial to realize infrared waves and multi-band radar waves perfect compatible stealth, which utilize the all-metallic sub-wavelength unit cells in microwave rangebut geometrically much larger than infrared wavelength to realize simultaneouslyperfect microwave absorption and total infrared reflection. The influences of thegroove height h and gap g on radar absorption and infrared reflection have beenstudied. Results show that the radar perfect absorption peaks rshift to lowerfrequency with the increasing of h and six reflection loss peaks exceed-20dBappeared in8-20GHz when h=9.5mm. The linear relationship between h andresonant frequency rindicates that the absorption mechanism is standing waveresonance in the grooves, which causes the strong ohmic loss in the surface of metal.Meanwhile, the multiple absorption peaks can effectively integrate due to theinteraction between grooves with different heights when g <0.1mm. The absorptiondepth of MMG metamaterial can achieve below-10dB in the frequency of3.26-3.81GHz (B=0.55GHz) when h=24.5mm， g=0.05mm. The MMG radarmetamaterial exhibits reflection property of metal plane to incident infrared waveswithout depending on h and g due to the sizes of MMG structure are much larger thanthe infrared wavelength. The measured reflectivity of MMG metamaterial in8-14mis above95%when h=9.5mm，g=0.5mm, corresponding to the emissivity0.05.The aforementioned MMG metamaterial exceeded the bottleneck of traditionalinfrared/radar compound stealth material and may provide a new strategy to prepareinfrared/radar compound stealth material. |
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