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
Fe
₅
O
₁₂
crystals by the LPE method. It was confirmed that adding B
₂
O
₃
to the melt increases the equilibrium segregation coefficient of Bi K
_0(Bi)
, and we found that a decrease in garnet composition ratio R
4
acts to increase the growth rate dependence of the normalized effective segregation coefficient of Bi K
_eff(Bi)
/ K
_0(Bi)
. It was also found that crystals with mirror-smooth surfaces could be obtained by adding Gd
₂
O
₃
to the melt. Using a melt with Gd
₂
O
₃
/Lu
₂
O
₃
≥ 1, the pits which usually appear in LPE growth of (BiLu)
₃
Fe
₅
O
₁₂
were eliminated, and pit-free, highly Bi-substituted (BiLuGd)
₃
Fe
₅
O
₁₂
crystals were obtained. The optical properties of 250 μm thick (BiLuGd)
₃
Fe
₅
O
₁₂
crystals were measured in order to evaluate their applicability to optical isolators for use at 1.3 μm. A sufficient Faraday rotation of θ
_F
= -1800 deg/cm and an extinction ratio of -40 dB were obtained.