Thermal, and optical features study of Dy:YAlO₃ and Dy/Tb:YAlO₃ crystals for yellow laser applications

Highlights

• Dy³⁺:YAP and Dy³⁺/Tb³⁺:YAP crystals were grown by Czochralski method. .

• XRD rocking curves, thermal behaviors were studied for the produced crystal.

• Anisotropic spectral behaviors were explored.

• The role of Tb³⁺ to enforce Dy³⁺ yellow emission was established.

Abstract

Optical features of Dy³⁺:YAP and Dy³⁺/Tb³⁺:YAP single crystals that were successfully fabricated by the Czochralski method were reported. The thermal behaviors and XRD rocking curves of the grown Dy³⁺:YAP crystal were carried out. The identified broad absorption bands of Dy³⁺:⁶H_15/2 → ⁴I_15/2 with FWHMs approximately to 11 nm and the moderate absorption cross-sections at 450 nm in both crystals, imply their high suitability to the blue LD pumping. Polarized emission spectra with yellow fluorescence peaks at 572 nm were acquired in both crystals by means of a 450 nm Xenon lamp excitation. As compared with Dy³⁺:YAP crystal, the larger emission cross-sections of 7.83 × 10⁻²¹, 9.08 × 10⁻²¹ and 9.01 × 10⁻²¹ cm² for E//a, E//b and E//c polarizations respectively, were determined in Dy³⁺/Tb³⁺:YAP crystal. Moreover, a slightly reduced lifetime of the upper laser level ⁴F_9/2 and a much shortened lifetime of the lower laser level ⁶H_13/2 were obtained in Dy³⁺/Tb³⁺:YAP crystal, implying the existent energy migration between Dy³⁺ and Tb³⁺, as well as an effective deactivation of Tb³⁺ to Dy³⁺. The role of Tb³⁺ to reinforce Dy³⁺:⁴F_9/2 → ⁶H_13/2 emission in YAP host was established. On the basis of the derived results, both crystals could be recognized as competitive materials for polarized yellow lasers, especially the Dy³⁺/Tb³⁺:YAP crystal.

Introduction

Nowadays, rare-earth (RE)-doped distinct materials that emit in yellow waveband for related solid-state laser fabrication have fascinated great notices from the scientists, on account of their comprehensive utilization in information storage, medical science, and telecommunications, etc [[1], [2], [3], [4], [5]]. Particularly, in ophthalmology, as compared to other waveband lasers, the yellow laser with the advantages of higher security and reliability is an important method for eyes treatment. Up to now, the established methods of yellow laser involve the following aspects: first, the self-frequency doubling of an infrared laser crystal, for instance, the microchip Yb³⁺:YAB self-doubled yellow laser accomplished by Burns et al. [6]. Second, the sum of frequency of two lasers, such as 1062 nm and 1338 nm lasers of Nd³⁺ in GAB crystal [7]. Third, the stimulated Raman scattering in crystals [8,9]. However, these frequency conversion techniques are prone to the disadvantages like large-sized equipment, complicated operation, and poor beam quality which prohibit their farther developments and applications. Nowadays, thanks to the availability of blue laser diodes (LDs), yellow laser can be produced from RE ions such as Dy³⁺ and Tb³⁺, directly [10]. Here, relying upon Dy³⁺:⁴F_9/2 → ⁶H_13/2 and/or Tb³⁺:⁵D₄→⁷F₄ transitions, the yellow laser can be generated using a blue LD as excitation source. This method does not require frequency conversion and has merits such as compact structure, high stability, good beam quality, and low signal-to-noise ratio, etc, and therefore plays a more and more critical role in yellow laser technique.

As an important activation center in luminescent materials, Dy³⁺ ion owns affluent energy levels to realize emissions from visible to mid-infrared (MIR) wavebands. In previous investigations, Dy³⁺ was brought into various host materials, such as Dy³⁺:CaYAlO₄, Dy³⁺:NaGd(MoO₄)₂, Dy³⁺:YAG, Dy³⁺/Tb³⁺:LiLuF₄ crystals, and Dy³⁺:ZBLAN glass fiber, etc [[11], [12], [13], [14], [15]] to dig out their application prospects in yellow laser, and the results implied that Dy³⁺ possesses favorable absorption band for yellow fluorescence under blue LD pumping. While, the absorption of Dy³⁺:⁶H_15/2 → ⁴I_15/2 around 450 nm is a spin-forbidden process, which demands strong pump energy and larger-sized crystal to prevail over its weak absorption capability [10]. Furthermore, as a four-level system, the rather long lifetime of Dy³⁺:⁶H_13/2 state, where the yellow fluorescence terminates on, is detrimental in minimizing atoms number during the lasing process. At present, with the assistance of blue LDs, yellow laser outputs have been demonstrated in crystals like Dy³⁺:YAG [13], Dy³⁺/Tb³⁺:LiLuF₄ [14], and Dy³⁺:Zn(WO₄)₂ [16], etc. But, in the case of fluorides, they not only suffer much energy waste, but also have poor laser stability as the pump power increases, on account of their poor thermal conductivity and mechanical strength. As an alternative choice, oxides present higher mechanical strength and better chemical stability for lasing action. Furthermore, as they own higher phonon energy, oxides are more likely to decrease the particles on ⁶H_13/2 state and populate them on ground state via non-radiative transition. Incorporation with Tb³⁺ ion as a deactivation center emerges as an effective method as well. Via energy migration between Dy³⁺:⁶H_13/2 and Tb³⁺:⁷F₄ states, the measured lifetime of ⁶H_13/2 state decreased rapidly from 294 μs to 58 μs in LiLuF₄ crystal [14].

YAlO₃ (YAP) is a member of ABO₃ family, which presents a contorted orthogonal perovskite structure and is classified to the Pnma space group. There are 20 atoms in a unit cell, where Al³⁺ cations, locating at 4b position are encircled by six O²⁻ anions to establish [AlO₆]^9- octahedron, and Y³⁺ cations locating at 4c position are encircled by eight O²⁻ anions to establish [YO₈]¹³⁻ polyhedron. Compared to YAG crystal which also derives from the binary Al₂O₃–Y₂O₃ system, the accommodation of RE ions in YAP crystal not only has a higher segregation coefficient but also has a more uniform distribution. In addition, the anisotropic properties and the intrinsic birefringence suggest that YAP crystal has distinctive advantages in laser operations. Overall, RE-doped YAP crystals can be highly rated as excellent candidates for solid-state laser and the precedent researches have confirmed this [[17], [18], [19]].

In this report, 2 at% Dy³⁺:YAP and 2 at% Dy³⁺/0.5 at% Tb³⁺:YAP bulk crystals were produced via the Czochralski (CZ) method. The XRD rocking curves were employed to inspect the quality of the produced crystals and the thermal behaviors were also studied concretely. More importantly, the spectral characters of the crystals, such as polarized absorption and emission features, fluorescence lifetimes, and the energy migration routes between Dy³⁺ and Tb³⁺ were displayed and explained for exploring their laser prospects.