Structure And Optical Properties Of Silicate Glass-ceramics Doped With Valence-variable Ions

With the rapid development of information technology,photonic glass and fiber play a crucial role in the optoelectronic information industry,such as large-capacity information transmission,high-precision laser processing and high-precision flat touch display.As an important type of photonic glass,photonic glass-ceramic combines the advantages of crystals and glass.For example,it possesses unique properties including enhanced radiative transition probablity,high doping concentration,and easy fabrication.In addition,it also owns the advantages of high mechanical and chemical stability,high resistance to laser and thermal damage threshold.Therefore,it is of great significance to study the structure and optical properties of photonic glass-ceramic,which can provide design criteria and theoretical support for the development of nanostructured materials.This thesis provides a comprehensive review of the crystallization theory and classification of transparent glass-ceramics,gives an overview of several applications for photonic.As we know,valence-variable ions are difficult to be activated in amorphous glass structure.To overcome this limitation,the typical valence-variable ions such as transition-mental and rare-earth ions?e.g.Cr,Fe and Eu?were chosen as the optical centers.Based on crystal-field theory and selective doping mechanism,we expected the selective activation of valence-variable ions in multiple lattice sites.Several new types of photonic glass-ceramics have been developed and prepared successfully.Differential Scanning Calorimeter?DSC?,X-ray diffraction?XRD?,high-resolution transmission electron microscopy?HRTEM?,X-ray absorption spectroscopy,transmission/absorption spectra,photoluminescence spectroscopy were used to study the structure and optical properties.Several types of transparent glass-ceramics with unique optical properties were obtained,including optical temperature sensing,ultra-broadband near-infrared?NIR?luminescence,multicolor luminescence and nonlinear optical properties.Two types of transparent Cr-doped B₂O₃-Al₂O₃-SiO₂ glass-ceramics containing Al₆Si₂O₁₃nanocrystals and dual nanocrystals(Al₆Si₂O₁₃ and Ga₂O₃)were prepared successfully.Base on the mechanism of selective doping,Cr³⁺is more likely to enter into the octahedral coordination environment of Ga₂O₃ nanocrystals.We suppose that the strong preference of active center for special coordination,octahedral position for Cr³⁺here,might be the underling driving forces for doping.It is interesting that 0.2 mol%Cr³⁺ions inhibit the precipitation of Al₆Si₂O₁₃nanocrystals and promote the precipitation of Ga₂O₃ nanocrystals.The values of crystal-field strength Dq/B for Cr³⁺-doped glasses and glass-ceramics were adjustable from 2.17 to 2.74.Another interesting point observed in Cr³⁺-doped glass-ceramic containing dual nanocrystals is that the luminescence intensity increases with the enhancement of ambient temperature.It is supposed that the trap centers in Ga₂O₃ semiconductor can be excited at high temperature,and then the energy transfer occurs between Ga₂O₃ and Cr³⁺.The optical temperature sensing performance of the paired thermal coupling levels of ²E and ⁴T₂ was characterized by fluorescence intensity ratio?FIR?technique.The results show that the relative sensitivity S_R reaches a maximum of 1.60%K^-1 at 423 K.Transparent Cr-doped MgO-Al₂O₃-SiO₂ glass-ceramic containing Mg₂SiO₄ nanocrystals was prepared successfully and characterized.Replacing the K₂CO₃ by using KNO₃ in glass composition does not change the structure of glass-ceramic,but it can greatly promote the oxidation from Cr³⁺to Cr⁴⁺and increase the content of Cr⁴⁺ions.As a result,the glass-ceramic shows interesting luminescent characteristics with four emission bands originated from high crystal field Cr³⁺and Cr⁴⁺ions respectively.The overlapping of these emission bands leads to the ultra-broadband NIR luminescence with the large full width at half maximum?FWHM?of about 340 nm.The Z-scan optical setup was employed for measurement of the nonlinear absorption of glass-ceramic.The fitted values of?_gs and?_es were 1.39×10^-16 and 1.20×10^-16cm²,respectively.Notably,this is the first trial,to the best of our knowledge,to use Cr⁴⁺-doped glass-ceramic as a saturable absorber,and the passive Q-switched pulse generation of Nd³⁺ions at 1064 nm was realized.The repetition frequency was 250 kHz and the pulse width was 176ns.Glass-ceramic fiber with core-clad configuration was successfully prepared by melt-in-tube method.Highly transparent BaO-Al₂O₃-SiO₂ glass-ceramic,in which,most of LaF₃ nanocrystals assembled in micro-sized BaAl₂Si₂O₈ single crystals was prepared successfully and characterized.The multiscale structure can induce efficient multiple scattering and generate optimal optical modes to confine light inside the matrix.Because Eu³⁺and Eu²⁺ions selectively occupy the octahedral positons of LaF₃ and BaAl₂Si₂O₈ crystals,a significant decrease in the energy-transfer rate can be realized and it ensures an efficient emission output.Multiscale structured glass-ceramic presents unique bright and tunable multi-color luminescence,including blue,red and even white,simply by switching the excitation wavelength from 324,393 to 376 nm.In addition,the multi-color luminescence can also be tuned by changing the ambient temperature.The effects of low doping concentration?less than 2.0 mol%?of Fe on the crystallization behavior and intrinsic optical properties of K₂O-Al₂O₃-SiO₂ glass-ceramics embedded with ZnO were investigated.The non-isothermal crystallization kinetics of glass was characterized by the modified JMA formula.The results show that the doping concentration affects the crystallization behavior of the glasses,and the greatest tendency is observed in the samples doped with 0.6 mol%Fe.The strong infrared localized surface plasmon resonance?LSPR?absorption in glass-ceramic can be attributed to the increase in carrier concentration caused by Fe³⁺ions doping in ZnO.With the increase of the concentration of Fe³⁺,the dopant may capture a large amount of free electrons,and the deep levels introduced by impurities and defects can also promote the recombination of electrons and holes.Above effects collaboratively result in a significant reduction in carrier concentration.In addition,the exchange coupling of s-d and p-d between the localized d electrons of Fe can be enhanced,resulting in a significant reduction in optical band gap.The carrier decay process,which can be characterized by transient absorption spectroscopy,consists of two parts:?1?fast decay process with the time constant of¹0² fs is originated from the electron-phonon interactions;?2?slow decay process with the time constant of ps magnitude is originated from the phonon-phonon interactions.Benefited from the broadband LSPR absorption feature,0.2 mol%Fe-doped glass-ceramic shows excellent infrared radiance and photo-thermal conversion efficiency.