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Growth and characterization of YAG single crystals doped with large concentration of vanadium.

Andrzej L. Bajor ¹, Jaroslaw Kisielewski ¹, Krzysztof Kopczynski ², Jadwiga Mierczyk ², Dorota A. Pawlak ¹, Marek A. Swirkowicz ¹, Tadeusz Łukasiewicz ¹

1. Institute of Electronic Materials Technology (ITME), Warszawa 01919, Poland
2. Military University of Technology, Institute of Optoelectronics (IOE), Kaliskiego 2, Warszawa 00-908, Poland

Abstract

Yttrium Aluminium Garnet (YAG) doped with V ions is an interesting optical material, especially useful as non-linear absorber (Q-switch) working in the spectral range of approx. 1100 to 1400 nm (1300 nm being the optimal wavelength). Besides, it has a noticeable absorption band around 800 nm which makes it applicable in lasing applications (diode pumped lasers). YAG:V single crystals were grown by the Czochralski method using Cyberstar Oxypuller 05-03 equipment. Thermal system consisted of iridium crucible of 50 mm diaameter, and a passive iridium afterheater of 60 mm diameter. Inductive heating with Hüttinger generator was used. It is very important that the vanadium ions are located in defined sites in the crystal structure (oxygen tetrahedrons). To obtain this, different growth atmospheres were applied (nitrogen without and with admixture of oxygen), and, besides, the as-grown boules were annealed in reducing atmospheres after the growth processes. Finally, good quality single crystals, [111]-oriented, with vanadium content of 0.6 to 3.5 at. % were obtained. They were up to 25 mm in diameter and up to 70mm in length.

The initial concentrations of vanadium (0.6 to 1.5 at. %) were found to be a little bit too small for optimal Q-switching operation. Therefore, we have also tried to grow crystals with larger concentrations, namely of 1.5 and 3.5 at. %. One needs to remember that the larger the doping is, the larger is also the potential concentration of V⁺³ ions. However, this process of vanadium reduction into V⁺³ ions doesn’t seem to be linear. Besides, as a rule, the larger the doping is, the larger the crystal imperfections can become. So, a certain optimal value of vanadium doping should be expected.

We have used several techniques of vanadium reduction into V⁺³ ions, and the thermal reduction in vacuum was found to be optimal. By spectroscopic and polarimetric investigations we have also determined that the crystals with increased doping concentrations were optically homogeneous. The absorption coefficient at 1300 nm wavelength was found to be as large as 1.75 cm^-1 in crystals doped with largest concentrations of vanadium, i.e. it was approx. 20 % larger than those reported elsewhere¹. This makes our crystals a better source for Q-switching operation.

References :

1. Z. Mierczyk, Z. Frukacz, “YAG:V⁺³ – new passive Q-switch for lasers generating radiation within near infrared range”, Opto-Electronics Rev., 1 (2000) 67-74.

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