Electromagnetic Characteristic Of Metamaterials Under Oblique Incidence

Metamaterials,artificial structure of subwavelength periodic array,have shown promising applications in optical communications,chemical-chiral sensing and holographic imaging due to their unique electromagnetic properties.Metamaterials with specific structures and electromagnetic responses can not only realize the flexible control to amplitude,phase,polarization and propagation of electromagnetic waves,but also greatly improve the performance and quality factor of devices.In addition,the unit cells with subwavelength size provide a new platform for studying quantum-like phenomena in classical systems.Therefore,tunable metamaterials are always hotspots of electromagnetic research.However,many researches on the electromagnetic responses were carried out under normal incidence,the physical mechanism of electromagnetic responses in metamaterials under oblique incidence has not been clearly understood yet.The change of incident angle breaks the symmetry of the incident electromagnetic field,so more linear and nonlinear responses can be obtained in metamaterials.In this thesis,the metamaterial structures are theoretically designed through numerical simulation software,and the resonance responses of metamaterials are regulated in different methods by period of unit cells,the stacked method,the intrinsic chirality and the spatial complementarity.Then,we fabricated the metamaterials by micro-nano processing technology,and compared the measured data from terahertz(THz)emission spectroscopy system and time-domain spectroscopy system with the theoretical simulation results.By theoretical design and experimental optimization,the response efficiency and quality factor of metamaterials under oblique incidence are further improved.In this thesis,different metamaterials have been studied from experiment and theory,respectively.The specific content and innovation points are as follows:(1)Electromagnetic induced transparency in symmetric metamaterials under oblique incidence.Under normal incidence,the metamaterial has one broad stop-band,while a novel transmission peak appears under oblique incidence.According to the distribution of electromagnetic field and surface current,the oblique incidence breaks the symmetry of the incident electromagnetic field,resulting in the asymmetric surface current on the surface of the metal metamaterial.The above results are verified theoretically through the influence of geometric parameters and incident angles.In addition,we have investigated the sensing performance and quality factor of the metamaterials under oblique incidence.The resonance position of the novel transmission peak linearly shifts with the change of the refractive index of the surrounding environment.The quality factor of novel transmission peak is 30 times larger than that under normal incidence.It is important for the development of angle-dependent metamaterials and improvement of quality factor of devices.This work has been published in Europhysics Letters.(2)Rabi splitting phenomenon in complementary metamaterials under oblique incidence.By changing the incident angle,a strong coupling between surface plasmon mode and local waveguide mode has been realized in complementary metamateria.Rabi splitting with an energy of about 49 me V occurs when the incident angle reaches 64°.According to the distribution of the z-component of magnetic field and surface current,the momentum compensation of the electromagnetic field leads to energy exchange between two modes,thus realizing the Rabi splitting phenomenon.Besides,the coupling strength can be controlled by the incident angle,and thus generates significant phase shift of splitting peaks.As the increase of incident angle,the delay of group velocity of the splitting peaks decreases monotonically and reaches the maximum value of 0.72 ps.The group velocity delay based on mode coupling has potential application of slow light devices in THz band.This work has been published in Journal of Physics: Condensed Matter.(3)Circular dichroism and asymmetric transmission of chiral metamaterials under oblique incidence.The chiral metamaterials achieve asymmetric transmission of circularly polarized light excitations under normal incidence,which is caused by vertical interactions between magnetic dipoles and electric dipoles.The asymmetric strength of the complementary metamaterial is much larger than that of the original metamaterial.The induced magnetic dipoles and electric dipoles have coplanar components under oblique incidence,and thus achieve circular dichroism.As the increasing of incident angle,the absolute value of chiral parameter extracted from the original structures increases from 345 to 363,which is one hundred times larger than reported values in previous work.In addition,the chiral structures are sensitive to the surrounding environment and incident angle,which has important value for chiral sensing and environmental detection.This work has been published in Annalen der Physik as “Front Cover”.(4)Elliptically polarized THz emission spectroscopy of chiral THz metamaterials.We have studied the radiation characteristics of elliptically polarized THz waves in chiral metamaterials,and have analyzed the contribution of in-plane and out-of-plane photocurrent under oblique incidence.On one hand,the polarities of THz waves reverse under opposite helicity excitations,which results from in-plane propagation of surface plasmon with different spin angular momentum.On the other hand,the second-order intensity dependence of the THz amplitude on the pump fluence proves the contribution of the out-of-plane ponderomotive field.In addition,the ellipticity of the THz radiation can be tuned by the azimuthal angle of the chiral metamaterials.The polarization state of THz waves keeps a right-handed ellipse under either left-handed circularly polarized or right-handed circularly polarized excitations.It indicates the structural selectivity of the THz chiral metamaterials for highly efficient and stable polarized THz emitter.This work is in submission.