Enhanced quantum yield of yellow photoluminescence of Dy³⁺ ions in nonlinear optical Ba₂TiSi₂O₈ nanocrystals formed in glass

Abstract

Transparent crystallized glasses consisting of nonlinear optical Ba₂TiSi₂O₈ nanocrystals (diameter: ∼100 nm) are prepared through the crystallization of 40BaO–20TiO₂–40SiO₂–0.5Dy₂O₃ glass (in the molar ratio), and photoluminescence quantum yields of Dy³⁺ ions in the visible region are evaluated directly by using a photoluminescence spectrometer with an integrating sphere. The incorporation of Dy³⁺ ions into Ba₂TiSi₂O₈ nanocrystals is confirmed from the X-ray diffraction analyses. The total quantum yields of the emissions at the bands of ⁴F_9/2→⁶H_15/2 (blue: 484 nm), ⁴F_9/2→⁶H_13/2 (yellow: 575 nm), and ⁴F_9/2→⁶H_11/2 (red: 669 nm) in the crystallized glasses are ∼15%, being about four times larger compared with the precursor glass. It is found that the intensity of yellow (575 nm) emissions and the branching ratio of the yellow (575 nm)/blue (484 nm) intensity ratio increase largely due to the crystallization. It is suggested from Judd–Ofelt analyses that the site symmetry of Dy³⁺ ions in the crystallized glasses is largely distorted, giving a large increase in the yellow emissions. It is proposed that Dy³⁺ ions substitute Ba²⁺ sites in Ba₂TiSi₂O₈ nanocrystals.

Introduction

Photoluminescence (PL) of rare-earth (RE) ions in glassy or crystalline solids is one of the fundamental light-matter interactions and has been utilized for various photonic devices such as Nd³⁺-doped solid state lasers and Er³⁺-doped optical fiber amplifiers. Dy³⁺ ions in solids show yellow (∼570 nm) emissions resulting from the f–f transition of ⁴F_9/2→⁶H_13/2 and 1.3 μm emissions due to the ⁶F_11/2, ⁶H_9/2→⁶H_15/2 transition, and these emissions have been considered to be utilized as visible solid state lasers and optical amplifiers in broad band telecommunication systems. Since PL properties of RE ions depend on the structural and chemical bonding states of RE ions in a given host material, it is of interest and importance to search materials giving intense yellow or 1.3 μm emissions for photonic device applications of Dy³⁺ ions. Many studies on PL properties of Dy³⁺ ions in glasses have been reported so far [1], [2], [3], [4], [5]. For instance, Tanabe et al. [1] reported that the relative intensity of yellow emissions of Dy³⁺ ions in xNa₂O–(95−x)B₂O₃–5CaO glasses increases with increasing Na₂O content. Further studies on quantitative analyses for yellow emissions of Dy³⁺ ions in glasses are strongly required.

On the other hand, nanostructures are the gateway into a new realm in physical, chemical, biological, and materials science. Crystallization of glass is one of the effective methods for fabrication of nanostructures [6], and recently new optically transparent bulk nanocrystallized glasses (glass-ceramics) have been successfully fabricated in some glasses [7], [8], [9], [10]. Among them, transparent nanocrystallized glasses consisting of nonlinear optical/ferroelectric nanocrystals have received much attention, because such materials have a high potential for applications in photonic devices such as tunable waveguide and optical switching [11], [12]. In RE-doped nonlinear optical/ferroelectric crystals, laser emissions in the short-wavelength region have been expected through self-frequency doubling (SFD) phenomena [13], [14]. It is, therefore, of interest and importance to develop RE-doped transparent crystallized glasses with nonlinear optical nanocrystals and to clarify the features of RE ions in nanocrystals. It should be pointed out that the reports on incorporations and optical properties of RE ions in transparent crystallized glasses with nonlinear optical crystals are a few [15], [16], [17]. Recently, there have been numerous reports on incorporations and fluorescence properties of RE ions in fluoride nanocrystals such as LaF₃ or CaF₂ formed in transparent oxyfluoride crystallized glasses [18], [19]. But, fluoride nanocrystals reported so far are centrosymmetric crystals inducing no second harmonic generations (SHGs).

Very recently, Maruyama et al. [17] synthesized transparent crystallized glasses consisting of nonlinear optical Nd³⁺, Er³⁺-doped Ba₂TiSi₂O₈ nanocrystals in 40BaO–20TiO₂–40SiO₂ glasses and clarified the features of optical properties of Nd³⁺ and Er³⁺ ions. In particular, they proposed from the Judd–Ofelt analyses that the site symmetry of Nd³⁺ and Er³⁺ ions in nanocrystallized glasses is largely distorted due to their incorporations into Ba²⁺ sites in Ba₂TiSi₂O₈ nanocrystals. Since it is expected that PL intensities of RE ions being located at distorted sites are enhanced, it is of interest to clarify PL quantum efficiencies of RE ions quantitatively in Ba₂TiSi₂O₈ nanocrystals.

In this study, Dy³⁺-doped Ba₂TiSi₂O₈ nanocrystals are synthesized through the crystallization of Dy₂O₃-doped 40BaO–20TiO₂–40SiO₂ glasses and examined PL properties of Dy³⁺ ions in the visible region. In particular, the quantitative quantum efficiency for yellow (∼570 nm) emissions is measured directly by using a PL spectrometer with an integrating sphere. It is found that the quantum efficiency for yellow emissions of Dy³⁺ ions is largely enhanced through the incorporation into Ba₂TiSi₂O₈ nanocrystals in comparison with Dy³⁺ ions present in the precursor glass. It is known that the intensity of the ⁴F_9/2→⁶H_13/2 transition of Dy³⁺ ions giving yellow emissions in glasses depends largely on the local symmetry of coordination environments around Dy³⁺ ions [1], [2], [3], [4], [5]. It is also pointed out that studies on the direct measurements of PL quantum efficiencies of RE ions in the visible region are scarce.