Investigation Of Second Harmonic Generation From Metamaterials

Metamaterials are a kind of artificial composites composed of sub-wavelength geometric arrays.Due to the designability of its geometric structure,metamaterials can manipulate the amplitude,phase and polarization of electromagnetic waves.Thus,metamaterials could realize a variety of specific optical responses without violating the basic physical laws,such as negative refraction effect,perfect absorption,mode coupling and so on.Nonlinear optics is a subject that studies the interaction between strong light and matter.When the strong light interacts with medium,the polarization of the medium is no longer proportional to the electric field amplitude of light,but it is proportional to the square or higher order of the electric field amplitude of light.Thus,the medium will produce second harmonic,higher harmonic and even mixing wave.The generation of electromagnetic wave with new frequency means that the manipulation of electromagnetic wave becomes more difficult,and the traditional materials in nature cannot meet the control and design of nonlinear optical effect because of their limited variety.With the appearance of metamaterials,this problem is expected to be solved.Its designable geometric structure unit,various arrangements and adjustable size are helpful to realize the design of nonlinear effects.In this thesis based on the previous research,we have designed three kinds of metamaterials with different structure.We have studied the physical mechanism and tunability of second harmonic generation（SHG）from these metamaterials.The full text is divided into five chapters,and its main contents are as follows:The first chapter is the introduction.Firstly,we have introduced the origin of metamaterials.Secondly,we have introduced several quite unique electromagnetic properties which bring by the unique geometric structure of the metamaterials.Finally,both the research progress of nonlinear effects provided by the metamaterials and the study of the nonlinear effects manipulated by the design of the metamaterials geometric structure have been introduced.The main content of Chapter 2 is that we designed a chiral metamaterial,which provide the elliptically polarized SHG based on Lorentz force under the excitation of circularly polarized light.Remarkable SHG-circular dichroism（SHG-CD）is attributed to the local magnetic field induced by the magnetic resonance in the chiral gammadion and the current generated in the Ga As wire.The SHG-CD follows the parity conservation when the parity of the chiral metamaterial changes.The maximal modulation depth of the ellipticity and the polarization rotation angle of the SHG reach 91%and 72°,respectively,resulting from the change of the second harmonic field current density with different wire lengths.This work deepens the understanding of the nonlinear optical effect in metamaterials and puts forward the chiral metamaterials as efficient and helicity-tunable nonlinear terahertz sources.In chapter 3,we design a bilayer metamaterial consisting of two split-ring resonators（SRRs）with a 90°twist angle.Based on the hydrodynamic model,the elliptically polarized SHG is provided by the bilayer metamaterial excited by the linearly polarized light.The underlying physical mechanism is that the bright and dark magnetic modes in the top and bottom SRRs could be excited.Both magnetic modes will generate the second harmonic with a specific linear polarization,resulting in the elliptically polarized SHG.The 2-αpolarization dependence of the SHG signal amplitude is observed because the single SRR can emit SHG as both bright atoms and dark atoms.The polarization angle dependent of the SHG data have a lateral shift due to the SHG from bright and dark part of SRR have been coupled.Then,we calculate the SHG under excitation with different polarization angle.Its special polarization angle dependence shows that the bilayer metamaterial will provide SHG under the excitation with any polarization angle,and the second harmonic radiated by the top and bottom SRR has the contribution of the light dark magnetic mode,and the second harmonic radiated by the light dark magnetic mode will be coupled with each other in the same SRR.The second harmonics generated by the bright and dark magnetic modes in the SRR could be coupling and coexistent.The symmetry of SHG polarization ellipse with different twist angle have indicated that the whole system of light interacting with materials must follows the conservation of parity under linearly polarized excitation.This work deeply reveals the physical mechanism of the SHG from the bilayer SRRs and provides theoretical guidance for the subsequent design of nonlinear metamaterials.In Chapter 4,we design a WS₂-Au metamaterial.As a new two-dimensional semiconductor material,WS₂ has high second-order nonlinear polarizability.Therefore,WS₂as the nonlinear source provide the SHG,Au with geometric structure is used to enhance and manipulate the SHG.We find that this metamaterial could improve the second harmonic generation efficiency several times compared with a single WS₂ material,and we have analyzed that this enhancement effect comes from the near-field enhancement effect of nano structure.We also calculated the incident angle dependence,polarization angle dependence and twist angle dependence of the SHG from this metamaterial.The results show that the dependence of the SHG is not related to the symmetry of WS₂,but to the symmetry of Au nano structure.These results mean that metallic metamaterials can enhance and manipulate the nonlinear harmonics generated by other materials.In chapter 5,we have summarized and prospected the works of the full text.The main contents,main conclusions and core innovations of the three works have been summarized and we have pointed out the research significance and value of the three works.Then we have looked forward to the three works from both theoretical and experimental aspects and pointed out the follow-up research direction and ideas of each work,as well as the necessity and importance of combining experiments.