Chromium doped forsterite Cr:Mg₂SiO₄ is well-known crystal for a tunable and femtosecond solid-state lasers emitting in the near infrared range (1.17-1.37 μm). It is using both in the optical communication and in biomedical field [1]. However, the wide usage of Cr: forsterite lasers is restrained by the parasitic absorption of Cr²⁺ ions. Earlier, in Refs [2, 3] there was reported that oxidizing annealing can, in principle, reduce the concentration of divalent chromium ions and at the same time the content of tetravalent chromium ions increases. The results of our attempts to optimize the characteristics of Cr:Mg₂SiO₄ laser crystals by the prolonged oxidizing annealing are presented in this talk. The dynamics of evolution of chromium content in different oxidation states during the series of prolonged oxidizing annealings of forsterite single crystals grown in different conditions and having initially different ratios of Cr²⁺: Cr³⁺: Cr⁴⁺ concentrations were studied. Forsterite single crystals were grown by the Czochralski technique [4] using iridium crucibles on seeds cut along the a crystallographic axis (Pbnm notation). The pulling and rotation rates of the crystals were 3 mm/h and 12 rpm, respectively. The stoichiometric mixture of oxides MgO and SiO₂ with 4N qualification was used as the charge. Chromium concentrations in the melt were 0.13 - 0.14 wt %. The growth atmosphere was extra-pure argon or a mixture of argon with oxygen (the partial oxygen pressure 2.2 kPa). This parameter was controlled in real time using the oxygen analyzer AKPM-01. Samples shaped as cube (5x5x5 mm³) were cut from single crystals and oriented using a DRON-4-13 X-ray diffractometer. The series of annealings of the samples in air was performed at 1573 K during 240 hours each treatment. Room-temperature polarized absorption spectra were measured before treatment and after each subsequent annealing by a Shimadzu UV-VIS-NIR scanning spectrophotometer. The obtained absorption spectra were approximated by sums of elementary Gaussians corresponding to elementary transistions between spin-orbital components of the ground and excited states of chromium ions of different oxidation states (Cr⁴⁺, Cr³⁺ and Cr²⁺). The spectra of the samples, in which the elementary band of the particular type is the best resolved, had been used to determine the peak position v^c_i and half-width w_i for the corresponding Gaussian. Then, using the set of Gaussians with determined in such a way fixed v^c_i and w_i values, we calculated the Gaussian amplitudes A_i and background levels y₀ for each spectrum, which provided the best approximation of the experimental curve. It was found, that the prolonged oxidizing annealing of Cr:Mg₂SiO₄ crystals leads to the fast decrease of the Cr²⁺ ion concentration and slow monotonic increase of Cr⁴⁺ ion concentration. The dynamics of both processes were studied and discussed.

REFERENCES:

1. A. A. Ivanov, M. V. Alfimov, and A. M. Zheltikov // Physics-Uspekhi, 2004, Vol. 47, No. 7, P. 687-704.

2. Y. Yamaguchi, K. Yamagishi, A. Sugimoto, Y. Nobe // OSA Proceedings on Advanced Solid-State Lasers. 1991, Vol. 10. P. 52-56.

3. W. Chen, G. Boulon // Optical Materials. 2003. Vol. 24. P. 163–168.

4. E.V. Zharikov, A.V. Gaister, V.B. Dudnikova, and V.S. Urusov // J. Cryst. Growth, 2005, Vol. 275, No 1-2, P. e871-e875.

1. POSTER: Influence of the prolonged oxidizing annealing onto the spectroscopic properties of Cr:Mg_{2}SiO_{4} crystals, Microsoft Office Document, 0MB

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