Photoluminescence enhancement of rare earth ions related to interface in glasses (August/1998)

Zhidong YAO

< Summary >

Rare earth ion doped glasses have found numerous applications. In order to raise the radiative quantum efficiency more effectively of rare earth ions in glasses, and to gain an insight into the luminescence behaviors of rare earth ions in the presence of a dielectric interface, in this thesis, photoluminescence characteristics of rare earth ions were investigated systematically in phase separated and crystallized glasses for the first time.

In Chapter 1, general background for the theoretical research in rare earth optics was reviewed. The present thesis was also outlined.

In Chapter 2, two glasses in Na₂O.B₂O₃.SiO₂ System were isothermally heat treated for phase separation, where Er₂O₃ was contained as dopant. After heat treatment, the droplet and intercornnected morphologies generate respectively in both glasses. The effects of heat treatment condition and the phase separated morphology on photoluminescence (PL) intensity of 4S_3/2←4I_15/2 transition of Er³⁺ were studied. It was found that the PL intensity increased with phase separation developing in both glasses. Moreover, the maximum PL intensities obtained by the phase separation in both glasses are 8.5 and 4.1 times as high as those in the respective untreated samples, the droplet morphology being superior to the intercornnected one. The mechanism of augmentation of the PL intensity was discussed in terms of the derived expression of the PL intensity. It was supposed that the induced interface after phase separation, causing notable increase in the radiative transition probability from the 4S_3/2 to 4I_15/2 levels, was responsible for the significant enhancement in the PL intensity.

The luminescence properties of rare earth ions are sensitive to the local structure around them. As a powerful technique to obtain information about the local structure around rare earth ions in glasses, the analyses of phonon sideband spectra of Eu³⁺ have succeeded in clarifying the local structure of rare earth ions in a series of glass systems. To our knowledge, however, there are few reports describing the application of the phonon sideband spectra to phase separated glasses. In Chapter 3, therefore, the phonon sideband spectra of Eu³⁺ associated with the 5D₂←7F₀ transition were investigated for two glasses with the same compositions as those in Chapter 2, where Eu₂O₃ Substituted for Er₂O₃ as dopant. The isothermal heat treatment was also carried out for phase separation. The evolutions of the phonon sideband spectra with heat treatment condition and the phase separated morphology were studied. Compared with corresponding IR and Raman bands in the system, it was observed in the glasses that the phonon modes coupled with Eu³⁺ were B-O^- and B-O bonds of BO₃ units, Si-O-Si, B-O-Si, boroxol ring as well as tetraborate group regardless of phase separation. Among them, it was suggested that the B-O^- bonds of BO₃ units are present only around Eu³⁺. Moreover, by analyzing the phonon sideband spectra of the phase separated glasses, two mechanics of immiscibility (spinodal decomposition and nucleation & growth mechanisms) can be distinguished and Eu³⁺ was deemed to retain in the borate-rich phase after phase separation.

In order to gain an insight into the luminescence behaviors of Er³⁺ in Na₂O.B₂O₃.SiO₂ system, in Chapter 4, the compositional dependences of Judd-Ofelt intensity parameters W_t with t=2,4,6 for Er³⁺ in xNa₂O.(40-x)B₂O₃.60SiO₂ glasses were investigated systematically. The value of W₂ reveals a maximum around 10 mol% Na₂O, inconsistent with a maximum found in some physical properties of sodium borosilicate glasses at a Na:B ratio of about 1. Whereas those of W₄ and W₆ decrease monotonically with an increase in Na₂O content. The local structure around Er³⁺ in the system was also explored by phonon sideband spectra associated with Eu³⁺. The highest theoretical microscopic optical basicity on the oxygen atoms in various silicate and borate units was suggested as a criterion of coordination preference of the oxyanion unit to rare earth ions, which successfully elucidated a series of coordination preference phenomena occurring in silicate, borate and borosilicate glasses. Compared to the compositional dependences of the intensity parameters in silicate and borate glasses, it is found that the borosilicate glass compositions can be tailored to obtain those intensity parameter values and related optical properties in both silicate and borate glasses.

Eu³⁺ holds even number electrons. The degenerated levels are lifted as single ones in ligand field due to Jahn-Teller effect. The transition characteristic between energy levels depends strongly on the site symmetry around Eu³⁺. Also in view of its simple energy level structure, Eu³⁺ was often utilized as the structural probe ion. The doping of Eu³⁺ instead of Er³⁺ in the phase separated glasses was expected to be favorable to obtain some information that is impossible of getting in the case of Er³⁺. In Chapter 5, two glasses with the same compositions as those in Chapter 2 and Eu₂O₃ Substituted for Er₂O₃ as dopant, were subjected to different phase separated heat treatments. PL characteristics of Eu³⁺ were investigated systematically. It was found that PL intensity of 5D₀→7F₂ transition of Eu³⁺ increases with phase separation developing, furthermore, the maximum increment of the PL intensity after phase separation depends on the morphology induced by phase separation, where the droplet morphology has the advantage over the interconnected one. the mechanism of enhancement of the PL intensity was then discussed on the basis of the derived mathematical expression of the PL intensity. It was concluded that the induced interface after phase separation, engendering significant increase in light scattering intensity and thereby notable increase in the population of Eu³⁺ on the 5D₀ energy level, accounts for considerable increase in the PL intensity. On the other hand, from the emission intensity ratio of 5D₀→7F₂ to 5D₀→7F₁ transitions of Eu³⁺, the phases into which Eu³⁺ segregates after various phase separation processes were estimated.

In the above study, we observed the intensified PL of rare earth ions in phase separated borosilicate glasses, where the induced interface after phase separation was believed to account for the increase in PL intensity. The difference in the refractive index, however, is a little for the silicate-rich and borate-rich phases in the glasses. The more striking the refractive index contrast is, the more significant the enhancement of PL intensity may be. The ferroelectric crystals with perovskite-type lattice are known to have higher refractive index. Accordingly, in Chapter 6, the xNaO_0.5.xNbO_2.5.(100-2x)SiO₂ glass system was chosen, in which NaNbO₃ crystalline phase will precipitate following crystallization heat treatment. PL behavior of Er³⁺ doped in the glasses was investigated before and after crystallization heat treatments. It is revealed that following the precipitation of NaNbO₃ Crystalline phase in these glasses, PL intensity of 4S_3/2→4I_15/2 transition of Er³⁺ significantly increases. A maximum over 100-fold increment in the PL intensity can be obtained after crystallization as compared with that of the uncrysta11ized glass. The mechanism of augmentation of the PL intensity was also discussed in terms of the expression of the PL intensity. It was then assumed that the remarkable difference in the refractive index between the residual glass and crystallized phases, was a dominant factor causing the notable enhancement of the PL intensity of Er³⁺ after crystallization.

It follows that the presence of a dielectric interface has a pronounced effect on luminescence behaviors of rare earth ions in glasses. PL intensity will increase to a different extent accompanying the generation and development of the interface. The increases in the population of rare earth ions on the upper level and the radiative quantum efficiency both result in the enhancement of PL intensity. The latter undoubtedly make a dominant contribution to the noteworthy augmentation of PL intensity. The refractive index contrast at the interface has a most striking effect on the increase of the radiative quantum efficiency.

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