Research On The Electromagnetic Response And Active Control Characteristics Of Polarization-dependent Terahertz Metamaterials

Metamaterials refer to a new class of engineered materials that are generally composed of a lot of subwavelength ‘artificial atoms',which have drawn significant attention over the years due to their exotic properties that nature materials cannot possess.However,it is difficult to change the electromagnetic response of the metamaterials that have been fabricated.Importantly,multiple electromagnetic responses and functionality can be realized in polarization-dependent metamaterials under different polarization incidence,which plays an important role in constructing photonic integrated devices.Besides,dynamic manipulation of the incident terahertz waves can be achieved through integrating active materials(e.g.semiconductors,liquid crystals,and graphene et al.)in metamaterials.In this thesis,we demonstrated two works on polarization-dependent metamaterials at terahertz frequencies,and further investigations show that active controlling the incident terahertz waves can be obtained.The details of the design,simulation,fabrication method and experimental measurement were described.The main content is as follows in short:1.We introduced the development history and significance of terahertz technology,and the sources and detectors of terahertz waves.Then,we presented the background and current status of terahertz metamaterials,emphasizing the development of the active metamaterials.To clearly explain the coupling mechanism,we introduced the general theories in detail.2.We numerically and experimentally demonstrated polarization-dependent terahertz responses in a proposed metamaterial with A-shape resonators.With the horizontal polarization incidence,the observed transmission window is formed by the superposition of two resonance dip tails,corresponding to the inductive-capacitive resonance at the lower frequency and the high-order antisymmetric resonance at a higher frequency,respectively.When the incident wave is perpendicularly polarized,the transmission window arises from the electromagnetically-induced transparency spectral response.The origin of the polarization-sensitive resonance properties is revealed by mapping the electric field and terahertz-induced surface current in theproposed metamaterials.Moreover,the influence of the geometry of the A-shape microstructures on the transmission spectra is analyzed.These polarization dependent metamaterials may provide more degrees of freedom in tuning the electromagnetic responses,thus offering a path toward robust metamaterials design.3.We presented a novel class of light-driven active terahertz metamaterials that enable a dynamic manipulation to the polarization-dependent electromagnetically induced transparency responses.The proposed planar metamaterial is composed of central square ring and two split ring resonators bridged by semiconductor Si inclusions.The coupling between the adjacent two rings led to strongly transmitted windows at different frequencies under different polarization incidences.In the simulation,the dynamic modulation of the amplitude of the electromagnetically induced transparency windows was achieved through changing the conductivity of the silicon bridges.This work provides a profound understanding of the fundamental coupling mechanisms in metamaterials and would certainly benefit the active and passive device designs with desirable optical properties.