Effect of sensitizing Cr 3+ ion on optical parameters of Nd-doped yttrium calcium oxyborate crystals
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M. Guo, J.Y. Wang, J.Q. Wei, X.B. Hu, and Y.G. Liu Institute of Crystal Materials & National Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China (Received 27 December 1999; accepted 28 July 2000)
Pure, Nd doped, and Nd–Cr bidoped yttrium calcium oxyborate YCa4O(BO3)3 (YCOB), NdxY1−xCa4O(BO3)3 (NYCOB) with x ⳱ 0.86, and Cr:NdxY1−xCa4O(BO3)3 (Cr:NYCOB) with x ⳱ 0.45 and a small amount of Cr3+ ions crystallized with a fluorapatite-type structure in the monoclinic system. The unit cell constants were a ⳱ 0.8076(7), b ⳱ 1.6020(10), and c ⳱ 0.3527(2) nm with the angle  of 101.23° for the NYCOB and z ⳱ 2. The measured absorption spectra of NYCOB and Cr:NYCOB were compared to Judd–Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agreement. The experiments showed that the Cr3+ ions were introduced into the crystalline lattice as a sensitizer to absorb the excitation energy and transfer it to the Nd3+ ions in the lattice structure. I. INTRODUCTION
It is important to discover rare earth (RE) borates and their solid solutions, such as NdxGd1−xAl3(BO3)4 and Yb x Y 1−x Al 3 (BO 3 ) 4 1,2 and YCa 4 O(BO 3 ) 3 (YCOB), NdxY1−xCa4O(BO3)3 (NYCOB), YbxY1−xCa4O(BO3)3 (YYCOB) and Nd x Gd 1−x Ca 4 O(BO 3 ) 3 , 3–5 following –BaB2O4 and LiB3O5.6,7 These RE borates are suitable as a laser material for miniature lasers. The RE calcium oxyborate family has a large effective nonlinear coefficient and a high damage threshold and exhibits a small beam divergence and a large angular acceptance. They are nonhygroscopic and chemically stable with good mechanical properties allowing easy optical polishing and can be grown to large sizes. These oxoborates are not only good laser host materials but also allow the developments of efficient self-frequency doubled lasers.8,9 Selffrequency doubling in the NYCOB and YYCOB was demonstrated in continuous wave and pulsed lasers,8,10–12 with a broad transparency range and a low laser threshold. Our works have shown that NYCOB is promising as a potential material for green and blue laser radiation by second harmonic generation (SHG) in a proper wavelength range, which takes place at room temperature and enables noncritical phase matching for the SHG types I and II. However, in NYCOB, the larger Nd3+ concentration in the lattice structure, which affects the measured SHG conversion efficiency, will allow the concentration quenching of the Nd3+ ion and crystal defects to occur, and as a frequency double crystal, there is a narrow absorption peak at 812 nm for a semiconductor J. Mater. Res., Vol. 15, No. 10, Oct 2000
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laser. To improve its spectral characterization, an attempt was made in our laboratory to examine the effect of transition metal chromium as a sensitizer doped in the NYCOB, i.e., Cr and Nd bidoping YCOB (Cr:NYCOB), on the optical parameters of the crystal. The replacement of Y with Cr in the
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