Two-dimensional Electron Gas And Hybridized Surface Plasmon Polaritons In LiNbO₃/ITO Heterojunction

Advancing of the information age stimulates the research activities of photonics and propels photonics technology rapidly,which has given birth to the new field of nanophotonics.The development of this field is closely dependent on the maturity and perfection of thin film fabrication in preparing smaller optical devices with better performance.In recent years,the study of new and exotic physical properties at the interface of the two materials has derived many frontier research fields,and also injected vitality into nanophotonics.Surface plasmon polariton(SPP)refers to the electromagnetic mode propagating at the interface of a conductor and a dielectric.An SPP is a collective mode of electromagnetic field and electron gas.The electromagnetic field of SPP is of "evanescent field",which can confine the photonic energy in subwavelength scale,and raise the amplitude of SPP near the interface by several orders of magnitude.In traditional plasma systems,isotropic dielectrics are used as the substrates,and metallic materials(especially noble metals,Au and Ag)are used as conducting films.Due to the huge loss caused by the electronic interband transitions in metal materials,which restricts the development of plasma devices based on metal constituents,it has become a hot research topic to seek a low loss plasmonic materials.Among them,transparent conductive oxide film and two-dimensional electron gas(2DEG)formed at dielectric interface are widely studied as low loss infrared band plasmonic materials,yet much less works on visible responsive plasmonic materials apart from metals were reported.In this thesis,the hybrid SPP characteristics of the visible regime supported by the two-dimensional electron gas formed of the electrostatic modification in the Li Nb O3/ITO system and the asymmetric scattering of the incident light are studied in depth,and extended to the general ferroelectric/transparent oxide(FO/TCO)system,which provides a new approach to preparation of ultra-low plasmonic systems.Firstly,using Thomas Fermi semiclassical screening model,the mechanism of the formation of two-dimensional electron gas through the electrostatic modification of ITO by LN spontaneous polarization field is analyzed systematically.It is found that the nonuniform two-dimensional electron gas is mainly distributed in nanometer scale.Based on the physical image of electrostatic modification,we got the electron density distribution in the two-dimensional electron gas layer,and calculated the dielectric function and other optical parameters of the modified layer according to the Drude model.The results show that the LN/ITO heterostructure is metallic-like and can support the SPPs in the visible and near-UV regimes.Secondly,according to the physical picture of the electrostatic modification,the LN/ITO heterojunction system can be seen as the sandwiched structure of anisotropic LN substrate/modified ITO nano-layer/unmodified ITO layer.In order to fill in the blank in dealing with this kind of structure theoretically and meet the demand of two-dimensional electron gas produced by polarity catastrophe,the dispersion relation of SPP in this kind of system is obtained from Maxwell equation and boundary condition.Considering the anisotropy of LN medium,SPP supported in LN/ITO system has hybrid polarization property(i.e.hybrid SPP,HSPP).The HSPP can be excited either by a TM mode light wave,or TE mode,or even a TE and TM mixed mode.On the one hand,it improves the flexibility of SPP excitation and the efficiency of SPP excitation by mixed polarization light.On the other hand,it reduces the scattering loss of SPP propagation.Thirdly,due to the significant photorefractive effect in LN,a bulk phase grating can be written,and owing to the photovoltaic effect,a sinusoidal carrier density can be formed on the surface of LN,forming a thin phase grating.The quasi wave vector provided by two phase gratings can compensate the phase mismatch between the propagating light and SPP wave vector.It was verified directly that the two-dimensional electron gas system can support SPP in the visible light band,and the optimal excitation efficiency can be over 80%.The absorption valley excited by SPP is supported by the dark band in the colorful diffraction spots observed in the experiment.Compared with the number of diffraction spots in the transmission field,it can be seen that the excitation efficiency of HSPP is closely related to the polarization of the incident light due to the anisotropy of LN.Finally,we expanded the special case of electrostatic modification of LN/ITO heterojunction to a general FO/TCO combination.When Zn O is used in place of ITO,and other ferroelectric oxides such as lithium tantalate,potassium niobate,barium titanate and lead lanthanum zirconate titanate are used in place of LN,2DEG with visible light response is obtained likewise.This provides a new approach to the design of the FO/TCO heterojunction plasmonic system,and serves as guideline for the design and development of nanophotonics devices,such as nonlinear optics devices and all-optical modulators,based on the FO/TCO platform.