Depolarized Metalloid Layer And Plasmon Characteristics On Niobate Crystal Surface

The development of photonic devices has led the change of the information technology.However,the traditional photonic devices are restricted by the diffraction limit,which seriously hinders the further improvement in transmission rate and in miniaturization and integration.The study of surface plasmon polariton(SPP)is quite promising in further device miniaturization.As a collective electromagnetic mode propagating at the interface between a conductor and a dielectric,SPP can not only break the diffraction limit of photons,but also can localize energy at the subwavelength scale.It has been widely applied to the fields of information transmission,photocatalysis and sensors.The current SPP systems are all based on metal materials with huge losses,and the corresponding fabrication processes are relatively complex,which seriously hinders scale application of the related devices.Therefore,it is urgent to find cheap and low-loss plasmonic materials instead of metals.Recently,two dimensional electron gas(2DEG)and two dimensional hole gas(2DHG)with extremely high concentration and mobility at the oxide interface resulted from polarity catastrophe have attracted much attention globally,which provide a good opportunity in realizing SPP excitation in insulator materials.In this thesis,based on carrier accumulation by the spontaneous polarization field of ferroelectric niobate crystals,the electrostatic modified layer of metallization is obtained.Phase gratings recorded by the photorefractive effect of niobate can serve as mediation in SPP excitation of the modified metallic-like layer.A series of surface phenomena stemming from SPP excitation are systematically studied.The SPP systems apart from metal constituents based on niobate crystals has obvious advantages in lowering power dissipation due to their extremely thin responsive layers,and the writing of neighboring phase grating can keep compactness at the same time.Therefore,it has a broad application potential in conceiving and designing compatible new nanophotonic devices and circuits.Firstly,the surface electrostatic modification based on spontaneous polarization field of ferroelectric niobate is taken into account.Utilizing Thomas-Fermi semi-classical charge screening model and density functional theory respectively,the distribution of surface carrier density is calculated.The results of the two calculation methods are compared.The results show that the surface carriers will screen the spontaneous polarization field of the niobate crystal,thus forming a modified layer of non-uniform 2DEG and 2DHG within a sub-nanometer scale on different surfaces.The electron density in the modified layer is comparable to that in noble metals such as gold and silver,namely,the modified layer can exhibit metal-like properties.By calculating the permittivity of the modified layer,it shows negative real part of the permittivity even in visible region.Thus,it is suitable to the corresponding SPP excitation theoretically.By analyzing several niobates,it is confirmed that the surface metallized layer seems universal in ferroelectric crystals.Secondly,phase gratings were written by using the strong photorefractive effect in niobates to match the wavevector of SPP excitation,and the subwavelength energy coupling of two beams as well as the difference of scattering due to SPP excitation in graphene coated lithium niobate were analysed.The experimental results indicate that n-doped monolayer graphene contributes to recording the surface phase grating and exciting SPP.The low-loss SPP excitation would enhance the subwavelength energy coupling of two laser beams and form strong scattering bands,and hence greatly affect the optical response of lithium niobate.Thirdly,further studies of diffraction and subwavelength energy coupling in lithium niobate crystal slab,indium tin oxide coated lithium niobite crystal slab,and potassium sodium strontium barium niobates bulk crystal also demonstrated 2DEG supported SPP excitation and the resultant significant optical changes.Remarkable two dimensional high diffraction orders and three orders of magnitude two-beam-coupling exponential gain coefficient increase were observed,which indicates that surface thin gratings formed in the modified layer of accumulated carrier play dominant roles in the process of SPP excitation and the gratings formed on the opposite surfaces of the niobate crystal have a π phase shift.Finally,the enhancement of third-order nonlinear effect and the variation of parametric scattering caused by SPP excitation are investigated.The depolarization field intensity and field gradient on the surface of lithium niobate are simila r to those of the internal atomic field,which can significantly enhance the surface nonlinearity.Owing to the field confinement of SPP,there is a large refractive index change within the half wavelength range.The formation process is affected by pyroelectric and photovoltaic effects of the lithium niobate crystal.The singular four-wave mixing parametric scattering patterns and concentric diffraction rings beyond conventional photorefractive theoretical frame were observed.The phenomena were all concerned with the non-uniform 2DEG accumulation and SPP excitation on the niobate surface.In this thesis,low-loss SPP excitation in the visible region was realized on the basis of composite systems of three niobates.By analyzing the influence of SPP excitation on optical response of the crystal,the important roles of surface effects were confirmed.Our study provides a new approach to conceiving and developing plasmonic devices.