Spectroscopy and energy transfer parameters of Tm3+- and Ho3+-doped Ba2NaNb5O15 single crystals
Introduction
The good electro-optical and non linear optical properties of Ba2NaNb5O15 (BNN) make it a very attractive crystal for technological applications like frequency doubling, optical parametric oscillation, and optical storage of information [1]. Presently, the broadband emission properties of BNN crystals activated with optically active rare-earth ions are extensively investigated in order to develop new materials for short pulse generation and tunable laser operation in the visible and NIR regions [2], [3], [4]. In this paper we present the results of a study carried out in order to evaluate the perspectives of applications of BNN crystals codoped with Tm3+ and Ho3+ as active media in widely tunable solid-state laser sources operating around 2 μm. These devices are interesting for applications in medical surgery, metrology, optical communications, detection of pollutants and coherent laser radars.
Section snippets
Experimental
The crystals were grown from sodium tetraborate flux [4]. Their optical quality was fairly good for spectroscopic measurements and their maximum size was 2 × 2 × 3 mm3. The doping levels of the samples used in this study were: Ho/Ba = 1 and 14% (nominal molar ratio) and Tm/Ba = 6.6% and 10.5% for the singly doped crystals, and Ho/Ba of 0.7%, Tm/Ba 7% for the codoped crystals. The X-ray fluorescence analysis of the singly doped crystals yielded a segregation coefficient of 0.53 for the Ho3+ and of 0.33
Structural data and absorption spectroscopy
BNN belongs to the tetragonal tungsten bronze (TTB) family, its space group is P4bm with cell parameters [5]a = 12.463 Å and c = 3.990 Å and Z = 2. Fig. 1 shows the room temperature visible absorption spectra of Tm:BNN (10.5%) and Ho:BNN (14%). The observed transitions have been assigned on the basis of the energy level diagrams of the Tm3+ and Ho3+ ions reported in the literature [6]. The absorption manifolds are characterized by a marked inhomogeneous broadening as a consequence of the modulation of
Emission spectra and energy transfer parameters
As previously discussed, we have focused our attention to the emission properties in the 2 μm region. The spectra of the singly doped crystals are shown in Fig. 3. The emission of Tm:BNN consists of a broadband ranging from 1.6 up to 2.0 μm with maximum of 0.63 × 10−20 cm−1 at 1872 nm for the σ (E ⊥ c) polarization. It is assigned to the 3F4 → 3H6 transition. On the contrary, the spectrum of Ho:BNN presents three distinct bands in the 1.8–2.2 μm range. They have a FWHM of the order of 200 nm, and the
Conclusions
The 2 μm spectroscopic properties of Tm–Ho codoped BNN crystals have been investigated. The (Tm) 3H4 ← 3H6 absorption transition, lying in the 780–800 nm range, is particularly suitable for diode laser pumping thanks to its marked inhomogeneously broadening. The emission spectra and decay profiles of the lowest excited levels of both Tm3+ and Ho3+ have been measured and used to evaluate the coefficients for the Tm → Ho direct and back-transfer processes. A comparison with the values reported in
Acknowledgements
The authors wish to thank Ilaria Grassini for her skills in preparing the samples. This work has been carried out with the financial support of the Italian Ministry of University and Scientific Research (Project COFIN 2005).
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