Study On Phonon-polariton In Undoped And Ion-doped PPLN Crystals

The phonon-polariton mode is a coupled mode of the electromagnetic (EM)field and an optic phonon in ionic or polar crystals. The existence of polaritons wasfirst predicted by Huang in an isotropic diatomic ionic crystal in1951, firstdenominated as polariton by Hopfield in1958, and first observed experimentallyby Henry and Hopfield in GaP in1965. Owing to the unusual properties, such asthe dielectric abnormality, polaritonic stop gap, and significantly reduced groupvelocities, phonon-polaritons have attracted much attention. Extensive studies havebeen devoted to phonon-polaritons in the infrared, terahertz, and microwavefrequencies. Several theoretical modes of phonon-polaritons, such as the surfacephonon-polariton, bulk phonon-polariton, and interface phonon-polariton, havebeen proposed to investigate properties of phonon-polaritons. In last decade,extensive studies have been devoted to phonon-polaritons in the periodically poledsuperlattice lithium niobate (PPLN), in which the periodicity of the lattice isartificially expanded from the atomic scale to microns. Correspondently, thefrequency position of the phonon-polariton is in the microwave region. In PPLN,the phonon-polariton, resulting in the polaritonic stop gap and dielectricabnormality, and so on, originates from the coupling between superlatticevibrations and EM waves due to the piezoelectric effect. In the previous studies,the exciting method of the phonon-polariton in PPLN is that the electric fieldvector of the EM wave is perpendicular to the acoustic propagation vector (thecrossed-field scheme). The phonon-polariton can also be excited by the otherscheme: the electric field vector of the EM wave is parallel to the acousticpropagation vector (the in-line field scheme), that has not been studied. In addition,lithium niobate is a ferroelectric crystal with the oxygen octahedron. This structurecharacteristic of the lithium niobate crystal makes it easy to introduce most metal ions into the crystallographic frame. The introduction of the doped ions into thecrystallographic lattice can tune the properties of crystal. The dopant ions can alsoinfluence the physical properties of phonon-polariton in PPLN. In this thesis weaim at a theoretical study of the phonon-polaritons of the in-line field scheme inPPLN and periodically poled superlattice ion-doped lithium niobate (PPLN:ion).Fist, in PPLN, we discuss the phonon-polariton of the in-line field scheme indepth. We calculate the dielectric function of the phonon-polariton by solvingequations of the coupling between the superlattice vibrations and EM waves. Weanalyze the mechanism for polariton coupling. Under the action of the electric fieldof EM waves, the positive and negative domains vibrate differently due to thepiezoelectric effect, which results in the appearance of the opposite charges on theinterface of two adjacent domains. The positive charges and the negative chargescancel each other out. And the opposite charges, which appear on the oppositesides of PPLN along the modulation direction of piezoelectric coefficients, makethe crystal as a whole polarized electrically synchronously. This transversepolarization will in turn emit an EM wave that couples strongly with the originalEM wave, resulting in the phonon-polariton. The mechanism for polariton couplingis different from that of the crossed-field scheme. In the crossed-field scheme, theopposite charges do not appear on the interface of two adjacent domains, butappear on he opposite sides of PPLN along the direction perpendicular tomodulation direction of piezoelectric coefficients. We discuss the influence of theduty cycle on the polariton stop gap, and give the solution of the widest andnarrowest polaritonic stop gap.Second, we study the phonon-polariotn of in-line field scheme in PPLN:ion.On the basis of the theoretical calculation of the phonon-polariton in PPLN wecalculate the dielectric function of the phonon-polariton in PPLN:ion. We analyzethe effects of the duty cycle and dopant ions on the polaritonic stop gap in crystals.From the solution of the polaritonic stop gap, we find the dopant ions influencesthe width and frequency position of the polaritonic stop gap by changing thedielectric properties, the piezoelectric properties, the elastic properties, and massdensity. We simulate numerically the effect of the variation of these properties onthe polaritonic stop gap. From the research of the phonon-polariton in PPLN:ion, we find that thedoping of ions into PPLN influences the properties of phonon-polaritons in thecrystals by changing the dielectric properties, the piezoelectric properties, theelastic properties, and mass density. So in order to obtain the properties ofphonon-poaritons in PPLN: ion, we need to know the influence of the dopant ionson these properties. We have investigated the influence of Mg doping on thedielectric properties of MgO-doped lithium niobate from the viewpoint of theoryand experiment. By solving the piezoelectric equations and motion equations, weobtain the dielectric function of MgO-doped lithium niobate, which is the functionof the circle frequency and Mg concentration. Based on the defect structure modesand our experimental data, we analyze the influence of Mg ions on the dielectricproperties of lithium niobate crystal.In PPLN and PPLN:ion, the polaritonic stop gap structure of phonon-polarit-on, which is created due to the coupling between the superlattice vibration and EM,is similar to the band gap structure in the photonic and phononic crystas. So thepolaritonic stop gap has potential practical values. In PPLN and PPLN: ion, thepolaritonic stop gap can be tuned by the duty cycle and the dopant ions. In PPLN,the polaritonic stop gap can be tuned by the periodicity of crystal and the dutycycle; while in PPLN: ion, the polaritonic stop gap can be tuned by not only theperiodicity of crystal and the duty cycle but also the dopant ions. We can tune thepolaritonic stop gap to satisfy different applications and requirement. These studiesopen a brand way for the design of polaritonic band structure, control of thepolariton, and research of the electromagnetic and ultrasonic devices.