Spectral-luminescent and laser properties of the (Y_1-x,Yb_x)₂O₃−Al₂O₃−B₂O₃ glasses

Highlights

• The limiting quantum yield of Yb³⁺ ions luminescence is close to 94%.

• Low efficiencies of the ions cooperative luminescence and photoreduction.

• The lasing threshold and differential efficiency amount to 60 W/mm² and 12.5%.

Abstract

Yttrium-alumina-borate glasses activated by the Yb³⁺ ions with compositions close to the huntite-like (Y_1−xLn_x)Al₃(BO₃)₄ crystals have been synthesized by conventional melt-quenching technique in a platinum crucible, and their spectral-luminescent and laser properties have been investigated. It is established that this activator forms in the given glass one type of optical centers having the radiation decay time of 870 ± 40 μs. The limiting quantum yield of its luminescence in the case of complete dehydration of the glass will amount to ≈94%. The main laser parameters have been calculated and the nonlinearity of the refractive index and the threshold of laser-induced destruction of the glass surface have been determined. The lasing has been obtained on the glass plate of 2.1 mm thickness with a threshold of ≈60 W/mm² estimated by specific absorbed power.

Introduction

Nowadays, rare-earth ions are the main activator both for solid-state laser media and for most of other optical materials. However, the luminescence of these ions, except Gd³⁺, is prone to concentration quenching, which substantially limits the value of specific output of radiant energy. And if for the cerium ions such quenching occurs mainly on the formation of the Се⁴⁺–О–Се³⁺ clusters [1], for other ions, cross-relaxation and cooperative interactions are the main quenching factor [2]. The effectiveness of such interactions, other things being equal, is in reverse power dependence on the distance between the donor and acceptor of excitations, in the case of multipole interaction, and decreases exponentially with the index of dimension order of their electronic shells, in the case of exchange interaction [2]. It is known [3] that with increasing Ln concentration the mean-statistical distance R(Ln–Ln) is lowered to the minimum possible value (R_min) for the given matrix, determined by specific features of its structure, whereas on further rise of Ln concentration only the number of ions disposed at this distance is increased. Therefore, the search for matrices ensuring large R_min is of priority value in creating high-doped laser media having large specific energy output.

Recently [4], by an example of the Sm³⁺ ions activated yttrium-alumina-borate glasses of composition close to the stoichiometry of huntite-like (Y_1−xLn_x)Al₃(BO₃)₄ crystals it was established that they are characterized by record high, for oxide matrixes, value of R_min(Ln–Ln) amounting to 0.67 nm. Moreover, it was shown that in passing from the huntite-like crystal to the glass the limiting quantum yield of the Sm³⁺ luminescence, that is limited by the intra-center exchange of excitations for vibrations of the trigonal BO₃ groups, increases from 55% to 80%.

Naturally, on increasing the spectral gap (ΔЕ) between the metastable and the closest lower state of the activator (for the Sm³⁺ ions ΔЕ ≈ 7500 cm⁻¹) the luminescence quantum yield will enhance. For example, the yield is enhanced practically up to 100% for Tb³⁺ ions (ΔЕ ≈ 14700 cm⁻¹) in the huntite-like glass [5]. In this connection, of interest are similar glasses activated by the Yb³⁺ ions for which ΔЕ ≈ 10000 cm⁻¹. Due to absence of excited-state-absorption, small Stokes losses, and availability of powerful sources of light-diode pumping at λ ≈ 975 nm, these glasses are the most preferable disordered material for the accumulation of high energy density on the metastable level, while the relatively broad band of their luminescence is suitable for the realization of sub-femtosecond mode of lasing [6]. However, the attainment of maximum efficiency of lasers using Yb-containing glasses is hindered by photo-darkening and photo-bleaching as well as cooperative luminescence. The first two effects, most probably, are due to photoreduction of Yb³⁺ to Yb²⁺ in Yb−Yb clusters [7], and the latter – to super-exchange interaction between the Yb³⁺ ions bonded by bridge oxygen [8]. It is logical to believe that the Yb-containing huntite-like glasses, ensuring large minimum distance between the activator ions, should be characterized by both weak photo-darkening and photo-bleaching, and by low efficiency of cooperative luminescence.

Huntite-like glasses have an additional advantage thanks to relatively high physico-chemical characteristics. Specifically, their heat conductivity by 2–3 times exceeds the heat conductivity of commercial phosphate glasses [9], [10] while the coefficient of linear thermal expansion is less by a factor of 0.5–0.7 [11].

In spite of the indicated potential advantages, we failed to find in the literature any information on the spectral-luminescent and laser properties of huntite-like glasses activated by the Yb³⁺ ions. It is to meet this deficiency was the aim of the present work.