Coherent Control Of Transmission In Metamaterials

Metamaterials can achieve many exotic optical properties such as negative refraction,perfect lens,gradient index and optical cloaking.A variety of applications were realized with metamaterials,such as perfect absorption,broadband chirality,anomalous refraction,holograms and polarization manipulation.More recently,there has been increasing awareness of coherently controlled metamaterials that allow modulation of intensity,propagation direction and polarization of light with advantages such as high contrast,THz bandwidth and arbitrarily low intensity levels down to single photons.But in such coherent schemes,well-engineered metamaterials are prerequisite.In this thesis,coherent control based on metamaterials will be demonstrated.Coherent control based on a kind of hybrid metamaterial consisting of square split wires and cut-wires on a dielectric substrate is used to exhibit a controllable broadband cross-polarization conversion.And coherent control based on an ultrathin trapezoid-shaped slit metasurface is used to manipulate one anomalous refracted beam and two deflected anomalous beams.The main contents and acquired innovative achievements are as follows:We designed a broadband cross-polarization conversion which was constituted by a kind of metamaterial and two counterpropagating coherent beams.The proposed metamaterial has a periodic hybrid structure constructed by square split wires and cut-wires,which can be etched from copper cladding covering the surface of the FR4 PCB substrate.The metamaterial can exhibit a broadband cross-polarization conversion for both reflected and transmitted waves in the frequency range of about 10 GHz,and its effect of polarization conversion is about 25%.We can coherently modulate the broadband linear and circular polarization conversion in the standing wave formed by two counterpropagating coherent beams.At electric antinodes and nodes,the metamaterial undergoes enhanced or vanishing polarization conversion due to interferometric effect.In particular,the broadband polarization conversion can be freely applied to different polarized waves and its efficiency can be easily accomplished via phase modulation between two input beams.The coherent control scheme enables the metamaterial to achieve a dynamic broadband modulation of the polarization state.We propose an ultrathin gradient metasurface with single trapezoid-shaped slot antenna as its building block that allows one normal and two deflected transmitted beams.It is numerically demonstrated that such metasurface with multiple scattering modes can be coherently controlled to modulate output intensities by changing the relative phase difference between two counterpropagating coherent beams.Each mode can be coherently switched on/off and two deflected anomalous beams can be synchronously dictated by the phase difference.At an electric anti-node of the standing wave,the trapezoid-shaped slit metasurface is excited in-phase and accordingly scattering intensities of the anomalous and opposite anomalous output beams increase.For anti-phase excitation at an electric node of the standing wave,the interaction of the metasurface with the standing wave is negligible,thus the output intensities of the anomalous and opposite anomalous beams are completely suppressed to zero.And thus the output beam is a perfect plane wave.The coherent control effect in the trapezoid-shaped slit metasurface will offer a promising opportunity for multichannel signals modulation,multichannel sensing and wave front shaping.