Manipulating The Electromagnetic Waves Based On Metamaterials And Metasurfaces

In recent years, the research of metamaterials and metasurfaces has aroused the tremendous attention in academic community. This thesis focuses on three aspects, including the manipulation of EM waves in transmission, absorption and reflection. The main results are as follows:1. Electromagnetic transparency phenomenon of microwave metamaterials.We realize the transmission manipulation of EM waves via two kinds of metamaterials design. One design is cascading two identical metallic plates perforated with deep subwavelength slit arrays to achieve the extraordinary optical transmission(EOT) phenomenon. Both the LC-circuit-based microscopic picture and the effective-mediumbased macroscopic model are established to capture the essential physics behind such phenomenon. Another design is employing the cascading parallel metal pieces to realize the total transmission at a set of Fabry-Pérot(FP) resonant frequencies when illuminated TM waves. We found such structure can be used to design various transformation optics devices for a series of frequencies. Its basic mechanism is the FP resonance along the length direction of the slits can be formed and the phase of EM waves inside the device can be matched again with the phase of EM waves outside the device.2. Electromagnetic absorption in ultrathin transparent conductive films based ohmic metasurfaces.Based on the free-carrier absorption mechanism, the conducting films can dissipate the EM waves in deep subwabelength scale. Thus, it can also be known as ohmic metasurface(or ohmic sheet). According to the requirements of manipulation, the surface impedance(square resistance) can be designed as uniform or spatial gradient. By studying the absorption properties of ultrathin conductive films in the microwave regime, we find a moderate absorption effect which gives rise to maximal absorbance 50% if the sheet resistance of the film meets an impedance matching condition. In order to break the absorption limit, the concept of coherent perfect absorption(CPA) can be employed. In experiment, the transparent conductive film sample with 180Ω sheet resistance is illuminated coherently by two counterpropagating light beams, which has the same amplitude, phase and polarization. Finally, the almost perfect(~100%) absorption is frequency-independent in the spectrum which spans 6-18 GHz in our microwave experiments. Remarkably, it occurs in films with thicknesses that are at the extreme subwavelength scales, ~λ/10000 or less. Also using the CPA method, we experimentally realize the perfect electromagnetic absorption in the one-atom thick graphene. Employing coherent illumination in the waveguide system, the absorbance of the unpatterned graphene monolayer is observed to be greater than 94% over the microwave X-band(7-13GHz), and to achieve a full absorption, >99% in experiment, at ~8.3GHz.Furthermore, we have also experimentally and numerically demonstrated that the coherent perfect absorption(CPA) can equivalently be accomplished under single beam illumination. Instead of using the counter-propagating coherent dual beams, we introduce a perfect magnetic conductor(PMC) surface as a mirror boundary to the CPA configuration. Such a PMC surface can practically be embodied, utilizing high impedance surfaces, i.e., mushroom structures. By covering them with an ultrathin conductive film of sheet resistance 377Ω, the perfect microwave absorption is achieved when the film is illuminated by a single beam from one side. Employing the PMC boundary reduces the coherence requirement in the original CPA setup, though the present implementation is limited to the single frequency or narrow band operation.3. Applying ohmic metasurfaces(ultrathin transparent conductive films) in the study of anti-reflection.We provide a unified theory for a class of antireflection coatings in the deep subwavelength scale, with either tangential electric or magnetic fields being almost constant inside the coating. Our theory gives the parameters of the ultrathin antireflection coatings, which reveal that various types of media including conductive films, gain media and zeroindex media, can all realize antireflection under certain conditions. Especially, we point out that antireflection coatings made of conductive films are perfect candidates in the radio wave and microwave frequency regimes and we experimentally demonstrated extremely broadband(5-17GHz), wide-angle and polarization-insensitive anti-reflection coatings by using ultra-thin conductive films(<λ/10000) in microwave regime. For different substrates, we can adjust the thickness and conductivity of the conductive film to satisfy the antireflection condition. Interestingly, FP resonances inside the dielectric slab can be eliminated by attaching with the conductive anti-reflection coating, resulting in and frequencyindependent and thickness-independent reflectance and transmission. And such characteristics are found to have the ability to improve the radiation patterns of antennas with dielectric shells. Moreover, we theoretically point out that 10-25 nm titanium films are promising candidates for broad-band and wide-angle in the infrared frequency regime.