Ho³⁺ doped Na₅Y₉F₃₂ single crystals doubly sensitized by Er³⁺ and Yb³⁺ for efficient 2.0 μm emission

Highlights

• The Na₅Y₉F₃₂: Er³⁺/Ho³⁺/Yb³⁺ single crystals were prepared by a Bridgman method.

• The 2.0μm emission was greatly improved by the doubly doping Er³⁺ and Yb³⁺.

• The energy transfer mechanism between Ho³⁺, Er³⁺ and Yb³⁺ was studied.

• The lifetime at 2.0 μm emission of the single crystal can reach ~21.82 ms.

Abstract

The 2.0 μm spectral characteristics of the 980 nm LD pumped Er³⁺/Ho³⁺/Yb³⁺ tri-doped Na₅Y₉F₃₂ single crystals grown by an improved Bridgman method were investigated with the help of their absorption spectra, emission spectra and decay curves. The introduction of Yb³⁺ and Er³⁺ greatly improves the 2.0 fluorescence intensity through joint energy transfer processes from Yb³⁺ to Er³⁺, and Er³⁺ to Ho³⁺. The doping concentrations of 0.5 mol% Er³⁺ and 1.0 mol% Yb³⁺ ions were optimized to obtain the maximum 2.0 μm fluorescence intensities for the fixed 0.8 mol% Ho³⁺/1 mol% Yb³⁺ and 0.8 mol% Ho³⁺/0.5 mol% Er³⁺ doped Na₅Y₉F₃₂ single crystals. Besides, according to the absorption and fluorescence spectra of the crystals, the absorption and emission cross-sections were calculated, and excellent gain performance was also obtained. Meanwhile, the maximum 2.0 μm emission lifetime of Er³⁺/Ho³⁺/Yb³⁺ tri-doped Na₅Y₉F₃₂ single crystal can reach ~21.82 ms. The energy transfer mechanisms among the three rare-earth ions were further investigated by calculating energy transfer micro-parameters (C_DA) and efficiencies. The excellent properties in both intense emission at 2.0 μm and chemical stability obtained in the Er³⁺/Ho³⁺/Yb³⁺ tri-doped Na₅Y₉F₃₂ single crystals make them promising candidate material for 2.0 μm laser.

Introduction

Recently, infrared (IR) lasers at ~2.0 μm have been extensively investigated for their applications in fields of medical surgery, monitoring of atmospheric pollutants, remote sensing, medical imaging and human eye safety laser radar due to the especial characteristics of its absorption wavelength matching atmospheric components and the safe area of the human eye in the spectrum [[1], [2], [3], [4], [5]]. For examples, the 2.0 μm laser scalpel has a strong ability to absorb OH in water and other water-rich substances, which can be used to condense, carbonize and vaporize organic tissues, thereby reducing skin injuries and bleeding [6,7]; it has been successfully used for monitoring agricultural food storage and air pollution in systems by measuring CO₂ concentrations [8,9].

So far, a large amount of reports on 2.0 μm emission have mainly based on Tm³⁺ or Ho³⁺ ions doped solid state materials by Tm³⁺: ³F₄ →³H₆ or Ho³⁺: ⁵I₇→⁵I₈ transitions [4,[10], [11], [12], [13]]. Compared with Tm³⁺, Ho³⁺ ion has a wider emission band and a higher emission cross section [13]. Moreover, the long fluorescence lifetime of Ho³⁺ is beneficial to Q-switched lasers [14,15]. Unfortunately, these Ho³⁺ single-doped solid-state lasers require special pump light sources instead of the low-cost and easily commercially available 808 nm or 980 nm laser diodes because Ho³⁺ ions lack a suitable absorption band [16]. As is known, Yb³⁺ is a commonly used sensitizer to solve this problem because it has a strong absorption band around the 980 nm wavelength and can efficiently transfer energy to Ho³⁺ ion by energy transfer process from Yb³⁺: ²F_5/2 to Ho³⁺: ⁵I₆ level [12]. However, previous studies have shown that the transfer efficiency of Yb³⁺ to Ho³⁺ is as low as 17% [17]. Therefore, the development of an effective method to improve the indirect energy transfer efficiency of Yb³⁺ to Ho³⁺ will be beneficial to improve the emission of 2.0 μm of Ho³⁺ ions.

It has been confirmed that Er³⁺ ion can act as an acceptor ion to efficiently transfer Yb³⁺ energy in Er³⁺/Yb³⁺ co-doped glass pumped by 980 nm laser diode (LD) due to the well energy match between Yb³⁺: ²F_5/2 and Er³⁺: ⁴I_11/2 levels [18]. At the same excitation, Er³⁺ is used as the donor ion for efficient energy transfer to Ho³⁺ in the Ho³⁺/Er³⁺ co-doped glass because the Er³⁺: ⁴I_11/2 level can be well matched with the Ho³⁺: ⁵I₆ level, which is conducive to achieving 2.0 μm emission [19]. Therefore, it is expected that the energy transfer efficiency from Yb³⁺ to Ho³⁺ can be enhanced by introduction of Er³⁺ into Yb³⁺/Ho³⁺ system through the indirect energy transfer processes of Yb³⁺ to Er³⁺, and Er³⁺ to Ho³⁺ solely pumped by 980 nm LD. Recently, Zhang et al. [20] has confirmed that the addition of Er³⁺ ions increased the emission of 2.0 μm in Er³⁺/Ho³⁺/Yb³⁺ triply-doped germanate-tellurite glasses. However, the glasses suffer from poor mechanical, thermal and chemical properties, and low luminous efficiency to rare earth ions. These disadvantages hinder their application in infrared lasers and other optical devices. Therefore, it becomes vital and important to find a suitable principal matrix with excellent comprehensive performance. Na₅Y₉F₃₂ single crystal has been considered as one of the excellent materials for infrared emission due to its low minimum matrix phonon energy, high chemical stability and low absorption coefficient at ~2.0 μm [21]. To the best of our knowledge, preparation of the Er³⁺/Ho³⁺/Yb³⁺ tri-doped single crystals and 2.0 μm fluorescence properties in the single crystals pumped by 980 nm LD have not been investigated.

In the present study, the Ho³⁺, Er³⁺, and Yb³⁺ tri-doped Na₅Y₉F₃₂ single crystals were grown by Bridgman method and the effects of Er³⁺, and Yb³⁺ concentrations on the 2.0 μm emission of Ho³⁺ doped crystals are presented. A significantly enhanced 2.0 μm emission of Ho³⁺ doped Na₅Y₉F₃₂ single crystals under 980 nm excitation was obtained by introduction of Er³⁺ and Yb³⁺ ions. This study implies that the single crystal under 980 nm excitation is considered as a potential material for 2.0 μm laser output.