Synthesis and optical characterizations of Nd, Y: CaF₂ transparent ceramics

Highlights

• Highly transparent of Nd, Y co-doped CaF₂ ceramics were fabricated by hot pressed method.

• The microstructure of Nd, Y co-doped CaF₂ ceramics nanopowders and ceramics were characterized.

• The optical properties were discussed by comparing with Nd: CaF₂ transparent ceramics.

Abstract

Highly transparent Nd, Y co-doped calcium fluoride (Nd, Y: CaF₂) ceramics with different Y³⁺ ions doped concentrations were fabricated by hot-pressed method using Nd, Y: CaF₂ nanopowders synthesized by co-precipitation method. According to the XRD calculations and SEM observations, the average grain size of nanopowders was about 22 nm. From the SEM micrograph of the nanopowders, it clearly shows that the nanoparticles exhibit nearly spherical morphology and agglomerated slightly. For 2 mm thickness sample, the transmittance of the as-fabricated Nd, Y: CaF₂ (1 at.% Nd and 2 at.% Y) ceramic at 1400 nm reached up to 87%. The microstructure, absorption spectra and emission spectra of the Nd, Y: CaF₂ ceramics were measured and discussed. Compared with the Nd: CaF₂ ceramic, the Nd, Y: CaF₂ ceramics fluorescent intensity increased drastically with the increase of Y³⁺ ions doped concentration.

Introduction

Since the first transparent ceramic laser: Dy-doped CaF₂ ceramic was made by Hatch, transparent ceramics have gained much attention over the last decades [1]. Especially, Akio Ikesue fabricated the Nd: YAG transparent ceramics in 1995 and achieved an oscillation threshold and a slope efficiency of 309 mW and 28%, respectively [2]. Up to 2009, Nd: YAG ceramic laser breaking the 100 kW mark by Northrop Grumman [3]. During recent years, many researchers have been devoted to the study of transparent ceramic and made some important progress, like the study of Nd: Y₂O₃, Alumina transparent ceramics and others [4], [5]. Nowadays, the transparent ceramics have been applied in various fields, such as solid-state laser device [6], ICF fast ignitor [7], the medical detection [8] and other fields. Compared with glass, glass ceramics and single crystal, transparent ceramics exhibit much more advantages: scalability to large sizes, excellent optical transparency, high melting point, tougher and stronger and higher laser damage value, superior optical uniformity, and controllable dopant concentration [9]. In addition, ceramics processing can easily form complex shapes with a lower cost and allow exploration of new hosts and dopant elements which may revolutionize laser design and performance [9], [10], [11].

Among these transparent ceramics, fluoride ceramic present much more special advantages than other kinds of transparent ceramics including lower phonon energy, melting point and refractive index, good thermal conductivity as well as the higher optical transmittance and broader transmittance range, which may be more appropriate for the practical application in the solid state lasers [12], [13], [14]. Nowadays, many researchers have been devoted to the research of CaF₂ transparent ceramics and got some improvements, such as Yb: CaF₂ [15], Er: CaF₂ [16], and Nd: CaF₂ [17] transparent ceramics. According to M. Mortier’s work [15], the 4 at.% Yb-doped CaF₂ ceramic sample has been diode-pumped to deliver output power of 1.6 W with optical-to-optical efficiency reaching 25%, a slope efficiency of 43%, a gain of 1.4, and wavelength tunability from 1015 to 1060 nm in 2015. Rare-earth doped transparent ceramics have been regarded as a kind of attractive materials for the laser application. Among the lanthanide rare-earth ions, the Nd³⁺ ion has been regarded as a promising element, because it can operate as a four-level system in either pulsed or continuous wave mode and present a metastable level of high quantum efficiency [17]. Nd³⁺-doped solid state laser hosts have low laser output threshold, high quantum efficiency, and high stimulated emission cross section [18]. Furthermore, Nd³⁺ ions doped CaF₂ crystals always have very broad and smooth absorption and emission spectra due to heterovalent substitution of Ca²⁺ and the charge compensations [19]. However, according to our work and some literature reports, it is a pity that a very detrimental concentration quenching effect, which results from the clustering of the rare-earth ions and some cross-relaxation type energy transfers, kills their emission quantum efficiency greatly. Codoping with Y³⁺ ‘buffer’ ions has already been considered to improve the situation in the past [20]. In 2013, L. B. Su successfully fabricated the Nd, Y: CaF₂ single crystal, which showed that codoping Nd: CaF₂ with Y³⁺ ions can manipulate the spectroscopic properties of Nd³⁺ ions significantly [21]. Unfortunately, to our best knowledge, there have been few papers reported on the Nd, Y: CaF₂ transparent ceramics. In this work, we synthesized the Nd, Y: CaF₂ nanopowders with 1 at.% Nd-doped concentration and different Y-doped concentration (0, 2, 5, 10 at.%) by co-precipitation method. Then we measured the microstructure of these powders by SEM and XRD. The highly transparent Nd, Y: CaF₂ laser ceramics were successfully fabricated by hot pressed method. The optical properties of Nd, Y: CaF₂ and Nd: CaF₂ samples were investigated and discussed.