Design And Characteristics Of DAST-based All-Dielectric Metamaterials

In the past decades,the research fields of metamaterials range from microwave to visible light,or from fundamental to applied research.Initially,metamaterials were basically composed of a metal-dielectric-metal three-layer sandwiched structure,known as metallic metamaterials.Recently,owing to the advantages of simple structures and low loss,all-dielectric metamaterials have gradually replaced the metallic metamaterials and become the research hotspot of metamaterials.Unfortunately,the emergence of dielectric metamaterials does not promote the research of tunable metamaterials in high frequency band such as visible light and most previous results were related to the metallic metamaterials.This is because the previous dielectric metamaterials were basically constructed by inorganic dielectric materials,such as silicon or titanium dioxide,whose structural responses are not adjustable.Consequently,seeking for the appropriate dielectric to realize tunability of dielectric metamaterials is attracted.4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate(DAST)is a artificially-designed and synthesized organic nonlinear material.Owing to its excellent electro-optical effect,birefringence effect,transverse optical phonons,etc.it holds great potentional applications in the fields of detection,high-speed electro-optic modulation,and metamaterials.Accordingly,novel DAST-based metamaterials were investigated in this thesis.The main conclusions of this work include:(1)A novel all-dielectric perfect reflector based on organic DAST crystal was designed.This reflector is composed of single-layer DAST cylindrical array.The electromagnetic parameters of the metamaterial were extracted by S-parameter retrieval method,and the electromagnetic field distributions of Mie resonance modes were calculated by CST simulation.Consequently,the response characteristics of the reflector were deeply analyzed.The effects of the radius r and height H of the DAST cylinders and the periodic constant P of the array on the response of the DAST reflector were systematically investigated.The related physical principles were explored,and thus the variation regular of the structural parameters of the metamaterial reflector were concluded.(2)According to the characteristic that DAST has a large electro-optical coefficient in the visible light range,an electrically tunable all dielectric reflector based on DAST was further designed.The structural parameters of the metamaterials were optimized,and the influences of the applied voltage on the reflection spectrum of metamaterials were emphatically investigated.In addition,the effects of the voltage on the basic electromagnetic resonance mode were also studied.(3)A dual-band flexible absorber based on DAST was also designed.By designing the size parameters of the DAST cylindrical array,the resonator's HE and EH mixed modes are coincident in frequency,thus achieving the purpose of high absorption.In addition,the formation mechanism of two resonance absorption peaks were analyzed and discussed.(4)4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate(DAST)was synthesized,purified,and characterized.Then,DAST single crystals and polycrystalline thin films were prepared by melt recrystallization method and spraying process,respectively.Based on this,DAST polycrystalline thin films were measured by ellipsometry,by which the optical parameters of refractive index and permittivity of the DAST films were yielded.