Elsevier

Optical Materials

Volume 33, Issue 3, January 2011, Pages 501-505
Optical Materials

Intensity parameters of Tm3+ doped Sc2O3 transparent ceramic laser material

https://doi.org/10.1016/j.optmat.2010.10.032Get rights and content

Abstract

This work is focused on spectral investigations of Tm3+ doped Sc2O3 transparent ceramic as potential material for diode-pumped solid-state laser emitting around 2 μm. In the context of the Judd–Ofelt (J–O) theory a series of spectroscopic parameters such as J–O intensity parameters, oscillator strengths, radiative transitions probabilities, and radiative lifetimes as well as branching ratios are evaluated. The gain cross-sections which lead to an estimation of the probable operating laser wavelength for the 3F4  3H6 Tm3+ laser transition were also calculated.

Introduction

The high-melting sesquioxides of scandium, yttrium, and lutetium are attractive laser systems because their favorable properties, especially high thermal conductivity [1] and low phonon energies [2] and the possibility of doping with rare earth ions. Due to their very high melting point [1] >2400 °C, it is very difficult to grow crystals with high optical quality and large sizes. However, laser operation of sesquioxides crystals doped with various RE3+ (Ho, Tm, Er, Yb) ions has been demonstrated [3], [4], [5], [6]. The possibilities to overcome the crystal growth problems of sesquioxides by using transparent polycrystalline materials produced by ceramic technologies were also exploited [7], but the papers refer mainly to Nd3+ or Yb3+ doped or co-doped ceramics, with very promising results on Yb3+ emission in high-power laser experiments [8], [9], [10], [11], [12].

Diode-pumped solid-state lasers operating in the eye-safe spectral region near 2 μm and based on Tm3+-doped solid-state media have a number of applications, first of all medical, because of the strong water absorption in this wavelength region. Surgical tissue treatment using such systems can be performed with minimal local thermal damage, which is especially important in ophthalmologic surgery. Due to water–vapor transparency and the presence of absorption lines of a number of chemical compounds in the 1.9–2.0 μm spectral region, two-micron tunable lasers offer the possibility to be used for such applications as a coherent source for laser radar, atmospheric sensing, and also for laser photo acoustic spectrometry where free-running multimode operation can be successfully used. Tm3+ doped crystalline materials can generate laser emission from blue to 2 μm. The ∼2 μm 3F4  3H6 emission can be achieved by diode lasers pumping in the 3H4 level (800 nm range), followed by the cross-relaxation (3H4  3F4)–(3H6  3F4). The visible emission is difficult to stimulate by direct pumping; there are no suitable sources. It was, however, proved that the visible (blue) emission from the 1G4 Tm3+ level can be obtained by up conversion of near infrared radiation (using the existing efficient diode lasers in the 800 or 940–980 nm ranges), especially in co-doped systems. Thus, blue Tm3+ up conversion emission was obtained in Yb, Tm systems under Yb3+ 980 nm excitation [13]; even higher up conversion efficiency was reported by Tm3+ excitation in the 800 nm range, with participation of Yb3+ [13], [14]. Blue up conversion was also studied under excitation in the 800 nm range in (Nd, Tm) [15] or (Nd, Tm, Yb) co-doped systems [16], [17], [18].

Few spectral data have been reported on Tm3+ in Sc3O3 single crystals [3] and recently on transparent ceramics [19] and no J–O analysis has been published for these materials. In the previous study [19] of Tm:Sc2O3 ceramics, an extended energy level scheme of Tm3+ has been obtained from spectral measurements at various temperatures, and the concentration quenching mechanisms of the 3H4 level were investigated. In the present study we analyze in details the spectral characteristics of Tm3+:Sc2O3 transparent ceramics determining a series of spectroscopic parameters such as J–O intensity parameters, oscillator strengths, radiative transitions probabilities, and radiative lifetimes as well as branching ratios.

Section snippets

Experimental methods

Transparent Sc2O3 ceramic doped with Tm3+ ions were prepared at the World Lab. Co. Nagoya, Japan by by the solid-state synthesis followed by isostatic compression (∼2 Mpa) and vacuum sintering at 1750 °C. The Sc2O3 – sesquioxide forms at room temperature a cubic C-type structure, belonging to the IA3¯ space group [20]. The unit cell contains two types of centers: C2 site is an eightfold cubic structure with two oxygen vacancies on a face diagonal, while C3i site corresponds to a cube with two

Absorption data and Judd–Ofelt analysis

The sample of 5 at.% Tm doped Sc2O3 transparent ceramic used in spectral measurements have good optical quality as show the photograph (Fig. 1) of a 5 mm thick sample (not polished at laser quality). Figure 2 show the transmittance of Tm3+:Sc2O3 transparent ceramic, it reaches about 75% for 1 mm thick sample in the ∼2 micron range.

In our preliminary study [19] on Tm:Sc2O3 transparent ceramics, Tm3+ energy level scheme (Fig. 3) and the main mechanisms involved in 2 μm emission have been

Conclusions

The spectroscopic properties of Tm3+ in Sc2O3 transparent ceramic have been investigated at room temperature. The J–O theory has been applied to the analysis of the room temperature absorption spectra, and the intensity parameters Ωt were estimated to be Ω2 = 2.92 × 10−20 cm2, Ω4 = 1.3 × 10−20 cm2, and Ω6 = 0.9 × 10−20 cm2. The highest estimated value of the emission cross-section of Tm3+ in Sc2O3 ceramic corresponding to the 3F4  3H6 transition of is 0.97 × 10−20 cm2. The results of these investigations

Acknowledgments

The author wish to thank to P. Aschehoug from the Laboratoire de Chimie de la Matière Condensée, CNRS-UMR 7574, ENSCP, Paris France for the lifetime measurement. This work was supported by CNCSIS –UEFISCSU, project number PNII – IDEI 503/2008.

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