Structural and optical properties of ZnO–Al₂O₃–SiO₂ system glass–ceramics containing Ni²⁺-doped nanocrystals

Abstract

Transparent glass–ceramics were prepared from Ni²⁺-doped ZnO–Al₂O₃–SiO₂ (ZAS) system glasses by annealing. The color of the glasses drastically changed from brownish yellow to green by annealing. The crystalline phase being precipitated by annealing was identified as solid solutions of spinel compounds. The crystalline sizes estimated by transmission electron microscope observation in Ni²⁺-doped post-annealed glasses were about 10–15 nm in diameter, while that in non-doped post-annealed glasses were 5–10 nm. This suggests that Ni²⁺ ions in ZAS glass were incorporated into the crystalline phases as nucleation agents of crystallization. It was confirmed from optical absorption measurements that Ni²⁺ ions occupied fivefold sites in the as-cast glass, while Ni²⁺ ions occupied octahedral sites in the post-annealed glass. The Ni²⁺-doped post-annealed glasses exhibited near-infrared emission. It is thought that the crystals containing Ni²⁺ ions act as emission centers.

Introduction

The demand to increase the transmission capacity of wavelength division multiplexing (WDM) system is indispensable due to the rapid development of the telecommunications industry. Considerable effort has been devoted to materials and design for optical fibers, waveguide devices and optical fiber amplifiers to revolutionize the telecommunication systems. Especially, many attempts have been made toward broadening and flattening of gain spectra of optical fiber amplifiers such as Er-doped fiber amplifiers (EDFAs) [1], tellurite-based EDFA [2], Tm-doped fiber amplifiers (TDFAs) [3], fiber Raman amplifiers (FRAs) [4].

However, the expansion of optical amplification bandwidth due to one rare earth ion is limited, because the bandwidth of the 4f–4f optical transition is narrow by nature. FRAs can realize broadband amplification, but they require multi-wavelength pumping schemes and have low gain efficiency. Broadband FRAs have complex structures and require high power consumption. If ultra-broadband amplification is realized with high gain efficiency by the single wavelength pumping, a drastic evolution could be expected to occur in the WDM technology.

If transition metals were active in glasses, ultra-broadband optical amplifiers could be realized. Cr⁴⁺ doped in glasses have been extensively investigated so far [5], [6], [7]. As chromium ions can easily take multi-valent states, such as Cr³⁺, Cr⁴⁺ and Cr⁶⁺, strict valence control is required to obtain Cr⁴⁺. In contrast, nickel takes the divalent state in almost all the hosts, as the divalent nickel is extremely stable. There would be no serious need to control the valence state of nickel doped in glasses to obtain Ni²⁺.

Transition metal-doped glasses are not applicable to gain media for lasers and optical amplifiers, since relaxation of the excited states of transition metals via lattice relaxations is considerably large in glasses. Non-radiative decay processes dominate the relaxations of the excited states of transition metals and the quantum efficiency of transition metals is very low in glasses, though they can have sufficiently large quantum efficiency in single crystals. However, it is difficult to fabricate single crystal fibers. In contrast, glasses can be easily obtained as large size bulk and they can be used as fiber and waveguide materials.

Glass–ceramics are of interest as hosts for transition metal ions, since they have the advantages of both crystalline and glass materials. Glass–ceramics possess mechanical properties like glasses which can be fabricated into fibers with low attenuation. If the sizes of crystals in glass–ceramics are far less than the wavelength of interest, light scattering caused by the crystals would be negligibly low. Furthermore, if transition metal ions are successfully included in crystalline phase in glass–ceramics, the quantum efficiency of the broadband emissions could become high like in crystalline materials. In addition, glass–ceramics can be fabricated on a massive scale. It is thought that transition metal-doped glass–ceramics have a great potential for broadband optical amplification media if transition metals are successfully incorporated into the crystalline phase.

We have obtained an efficient broadband near-infrared emission from a Ni²⁺-doped zinc-alumino-silicate (ZAS) glasses [8]. However, the structural and optical properties of the glasses have not yet been clarified. Understanding of these properties is a clue to improve the emission efficiency of the glass. We have performed transmission electron microscopy and X-ray diffraction and thermal analysis as well as optical measurements of ZAS glasses in order to investigate the structural and optical properties of ZAS glasses in detail.