Investigations On The Electromagnetic Properties Of Optical Metamaterials And Chiral Metamaterials

Metamaterials are a kind of materials composed of artificial meta-atoms, of which theproperties are obtained from the accurately designed geometric structures, but not from thecomposition of the meta-atoms. By artificially designing different meta-atoms, metamaterialsare capable of exhibiting numerous extraordinary properties that can not accomplish bynatural materials, for instance, negative refraction, reversed Cherenkov radiation, reversedDoppler effect, super-resolution imaging, invisible cloak, giant polarization rotation power,and so on. The appearance of metamaterials extremely extends people‘s knowledge ofmaterial science. Because of the promising applications in areas such as energy,electromagnetism, optoelectronics, information storage, sensor detection, and public safety,metamaterials quickly become one of the most attractive scientific forefront and hot spot. Inthe past decade, metamaterials have undergone an explosive development. Concomitant withthe considerable progress in the field of metamaterials, a larger number of metamaterials withdifferent functionalities have been proposed, and in the science community metamaterialshave twice been regarded as one of the top ten significant discoveries. So far, metamaterialshave gradually become an emerging interdisciplinary discipline including electromagnetics,life science, condensed matter physics, material science, acoustics, microwave and antennaengineering, classical optics, nanoscience, and so on.In this dissertation, on the basis of the basic principle of electromagnetic metamaterials,we proposed several kinds of optical metamaterials and chiral metamaterials, theelectromagnetic properties of which were investigated via simulations and experiments indetail. The main results of the dissertation are as follows:1. On the basis of high-order resonance, a metamaterial composed of U-shaped cells wasproposed. The numerical results confirmed that the proposed metamaterial could realizemulti-band negative refractive index with very low loss at visible frequencies. Themechanism of multi-band negative refractive index could be well explained by analyzingthe current distribution, effective LC circuit models and kinetic energy of electrons. Theinfluences of the thicknesses of silver cladding and dielectric substrate on theelectromagnetic properties of the metamaterial were studied. Further simulationsconfirmed that this U-shaped metamaterial could still accomplish multiband negativerefractive index at terahertz frequencies, showing extensive frequency flexibility.2. Based on the disordered silver dendritic cells, a kind of metamaterial model operating at visible light frequencies was proposed. By using the―bottom-up‖electrochemicaldeposition, the visible metamaterial consisting of double layers of silver dendritic cellswas prepared. The effects of the reaction conditions in the electrochemical depositionprocess on the morphology and geometry parameters of the silver dendritic structureshave been studied. Finally, the best conditions to prepare the silver dendritic structureswere obtained. The optical properties of the metamaterial were investigated. Theexperimental results demonstrated that the present visible metamaterial could achievemultiband transmission and flat slab focusing behaviors at the frequencies of transmissionpeaks.3. A kind of chiral metamaterial composed of conjugated fylfot-like was proposed.Simulations and experiments were carried out to investigate the electromagneticproperties of the chiral metamaterial at normal and oblique incidences. The resultsindicated that the polarization rotation power of the present chiral metamaterial was aslarge as3400°/λ, simultaneously showing strong circular dichroism. Due to the giantchirality, the chiral metamaterial could easily realize negative refractive index at theresonant frequencies. In addition, the CMM was able to achieve90°polarization rotationin a very wide range of incident angles (0～45°).4. A composite chiral metamaterial composed of cut-wire pair and conjugated gammadionresonators was designed via the combination of the traditional resonators and pure chiralresonators. The numerical and experimental results demonstrated that the suggestedcomposite chiral metamaterial exhibited considerably stronger polarization rotatorypower than the pure chiral metamaterial just consisting of conjugated gammadionresonators, and it could stably function as a90°polarization rotator regardless of theincident angles. In addition, the composite chiral metamaterial with certain geometricparameters could perfectly accomplish90°polarization rotation for a linearly polarizedincident wave, even if the permittivity of the dielectric substrate was altered.5. Based on the composite chiral metamaterial mentioned above, an ultrathin andfrequency-tunable90°-polarization rotation device consisting of a composite chiralmetamaterial and two auxiliary dielectric slabs was constructed. By mechanically varyingthe distance between the CCMM and auxiliary slabs parallel to its surfaces, the operatingfrequency of the proposed90°-PRD could be dynamically adjusted and widened by about1.02GHz with a relative bandwidth of about17%. Moreover, further experimentsconfirmed that this mechanical tuning process was completely repeatable and reversible.6. A conjugated complementary windmill-shaped chiral metamaterial model was proposed. The simulations confirmed that chiral metamaterial possessed very strong polarizationrotation power and was capable of realizing nearly perfect cross-polarization transmissionconversion at9.06GHz; the transmitted waves in the whole simulated frequency regionwere all linear polarization due to the small circular dichroism, and in the frequencyrange of8.43~9.26GHz the designed chiral metamaterial could achievecross-polarization rotation for a linearly polarized incident wave.7. A chiral metamaterial consisting of two layers of split-ring resonator array with the splitdirections mutually perpendicular to each other was designed. The numerical resultsshowed that the proposed chiral metamaterial could achieve highly efficientcross-polarization conversion and exhibit considerable asymmetrical transmission effect.The absolute operating bandwidth of the chiral metamaterial was as large as about2.9GHz (12.8~15.7GHz), with a relative bandwidth of about20.4%. It was found that thestrong cross-coupling effects of the sample resulted from the magnetic resonances byanalyzing the surface current distributions of the sample.