Thick-Film Crystal Growth of Highly Bi-Substituted Garnet for Optical Isolators

We studied the melt composition to obtain pit-free, highly Bi-substituted (BiLu)3 Fe5 O12 crystals by the LPE method. It was confirmed that adding B2 O3 to the melt increases the equilibrium segregation coefficient of Bi K0(Bi), and we found that a decrease in garnet composition ratio R4 acts to increase the growth rate dependence of the normalized effective segregation coefficient of Bi Keff(Bi)/ K0(Bi). It was also found that crystals with mirror-smooth surfaces could be obtained by adding Gd2 O3 to the melt. Using a melt with Gd2 O3/Lu2 O3 ¿ 1, the pits which usually appear in LPE growth of (BiLu)3 Fe5 O12 were eliminated, and pit-free, highly Bi-substituted (BiLuGd)3 Fe5 O12 crystals were obtained. The optical properties of 250 ¿m thick (BiLuGd)3 Fe5 O12 crystals were measured in order to evaluate their applicability to optical isolators for use at 1.3 ¿m. A sufficient Faraday rotation of ¿F = ¿1800 deg/cm and an extinction ratio of ¿40 dB were obtained.