Investigation On Optical Properties Of Ca²⁺-doped Tungsten Bronze Strontium Barium Niobite Crystals

Doped and un-doped Strontium barium niobate crystals have been studied extensively for their important electro-optic,piezoelectric,pyroelectric,and photorefractive applications.For several vacancies lying in the inner structure of tungsten bronze niobites,crystal quality and other proprieties can be improved by means of ions dopant or further molecular design.Recently,works with these ideas have been developed rapidly.Calcium doped Strontium barium niobate crystal were grown by the Institute of Crystal Materials of Shandong University of China are ferroelectric and photoelectric crystals having the structure of ferroelectric tetragonal tungsten bronze-type (TTB).The elementary 45 atoms cell of CSBN((Ca_0.28Ba_0.72)_x(Sr_0.60Ba_0.40)_1-xNb₂O₆)has tetragonal system with the space group P4bm(C_4v²)and the Ca²⁺partly replaces the Ba²⁺ at A2 site of the un-filled TB structure.Optical proprieties of CSBN crystals,which including lattice vibration spectra (Raman and infrared)and thermal proprieties;birefringence and polarized light interference;photorefractive nonlinear characteristic and photorefractive effect center; crystal diffraction and optical phase conjugation etc,were mainly performed in this dissertation.Main contents are listed as following:1)The space group theoretical analyses and assignment of the lattice vibration modes of the CSBN crystals with various geometries have been performed.The lattice vibration spectra of this crystal are studied by Raman scattering and infrared absorption measurements.At the same time,the lattice vibration spectra with various geometries were compared and analyzed with different Ca²⁺-doped crystals.Although,in theory,120 or 119 Raman peaks and 53 Infrared activity bands can be observed in experiments,only 55 normal modes and 5 Infrared spectrums were acquired.Characteristic peaks arising mainly from internal vibrations of the Nb-O anionic cluster were identified from vibration spectra except three-fold degeneracy anti-symmetric stretching vibration modesv₃.The reasons why fewer modes were observed attribute to the preparation of samples,the relative stability of complex crystal of Nb-O octahedron and randomicity of crystal vacancies. 2)Thermal properties of CSBN single crystals were systemically studied.CSBN crystals have its own brand of thermal properties:Experimental results suggest that negative thermal expand property along c axis of CSBN crystals below the Curie temperature while positive thermal expand above the Curie temperature;When the measurement was along a axis of crystals,crystals tend to positive thermal expand with the temperature increasing.Thermal conductivity of CSBN crystals tend to be the similar near the Curie point whether the measurement is along c axis or a axis and its appear biggish aeolotropism deflecting the Curie point.The thermal properties of CSBN crystals were considered of thermal motion of Nb atom,which is vertical to the O-Nb-O lines to decrease its separation,in such a way that negative thermal expand result.3)Birefringence as well as its polarized lights interference were systemically studied in this dissertation.The extinction angles of CSBN crystals under the incident with polarized lights were affirmed with the formula asφ=kÏ€+θand the refraction indexes, birefringence of crystals were measured by the method of light center distill,which the measurement accuracy is up to 10^-4.In addition,the value k of extinction angles at 532nm were calculated as k_CSBN75≈270;k_CSBN50≈259;k_CSBN25≈255.The study on polarized lights interference of crystals were executed by self-designed optical systems instead of the traditional polarizing microscope technique.The following parts were discussed based on conoscopic interference patterns:the origin of order of interference change,influence on Ca²⁺dopen to crystals,the change regularity of conoscopic intereference patterns with the incident angle and the optical homogeneous of CSBN crystals,which are affirmed to be good optical homogeneous and the birefringence grads is up to 10^-5.The conoscopic intereference patterns were simulated accoding to the theoretic analysis and calculating,which valadated the theoretic analysis and calculating in this paper.4)The photo-refraction center of CSBN crystals were discussed based on the transmission spectrums:The influence on Ca²⁺dopen to the photo-refraction properties of crystals were analyzed through transmissions spectrums(including:absorption edge of CSBN crystals,attenuation index and transmission curves along a and c axes of crystals).It comes to the conclusion that photo-refraction effects of CSBN crystals were enhanced in specific wavelength scopes with the Ca²⁺dopen instead of Ca²⁺acting as the centre of the photo-refraction crystals.5)Photo-refractive nonlinear characteristic of CSBN crystals were studied in different ways.Z-Scan curves and nonlinear refractive index were acquired based on improved Z-Scan technique.Photo-refractive characteristic were measured under Michelson interferometer with the incident of un-polarized lights and an important characteristic about photo-refractive characteristic was drawn:Refractive index change of CSBN25 represent self-focusing and CSBN75 is self-defocusing,while CSBN50 represents dynamic change properties from self-focusing to self-defocusing.When the experiment was executed under polarized lights,the same conclusion could be acquired. The change from self-focusing to self-defocusing could be explained based on the Feinberg's theory-Thermal self-focusing induced photo-refractive asymmetry self-defocusing.Furthermore,dynamic change properties of birefringence in CSBN crystals were firstly studied and its temporal change curves with input power were plotted in this investigation.6)Body-grating diffraction charicreristic was discussed with some novel ways in this disquisition.Measurements of diffraction efficiencises of body-grating showed that the diffraction efficiency of extraordinary rays are much more than that of ordinary rays,but they shares the samilar change regularity coinciding with theories of Kukhtarev and Hong. Investigation on self-diffraction witnesses the fact that the direction of the Light-induced self-diffraction spot changes with different Ca²⁺-doped CSBN crystals,indicating that the formed noise-grating wave vector direction changes with different ion-doped.Based on the wave optics theory and light-induced self-diffraction under the incident of plane monochromatic light wave in CSBN crystals,the theoretical model of noise-grating is analyzed,constructed and validated in this work firstly.Obviously,it can provide a theoretical foundation for retraining noise to study the noise grating.Going on this premise,the photo induction and scattering of CBN28 crystal was discussed in this paper: The photo induction and scattering of CBN28 was the result of the nestification of noisegrating wave plane.Although,the photo induction and scattering could not be found in CSBN crystals,which suggested that stoichiometric proportion dope can directly influence the photo-refractive characteristic of crystals.The far-field diffraction of CSBN was distinctly observed in experiments,which is considered as the result of lens-like effect being same like as of LiNO3 crystals.7)The self-pumped phase conjugation in CSBN75 was firstly studied based on an anti-traditional method,which using part laser spot to irradiate crystal.In the case,when the site of the laser spot on the crystal is less than0.85mm,the deformed laser spot and its complementary pattern are distinctly displayed on the screen,also the deformed laser spot was observed along the inverse direction of the incidence beam,which affirmed the existence of the conjugation mechanism in this investigation.Theories analysis basing self-pumped conjugation succeeded in explaining these phenomena.The experimental results demonstrate that the CSBN75 crystals will be widely applied in optical information processing,such as the edge enhanced of optical correlation pattern effect realized in this paper.Furthermore,the self pump phase conjugation reflectance ratio and response time of CSBN crystals had been measured in this paper,experiments showed that wide response field with these crystals to realize self pump phase conjugation and doping content can affect photo refractive effect of crystals deeply.