Recent advances in rare earth-based borate single crystals: Potential materials for nonlinear optical and laser applications

Due to an increase in the requirement for ultraviolet and visible laser sources nonlinear optical crystals are gaining importance. Borate-based nonlinear optical crystals such as lithium borate LiB₃O₅ (LBO), CsLiB₅O₁₀ (CLBO), SrBe₂B₂O₇ (SBBO), KBe₂BO₃F₂ (KBBF) and K₂Al₂B₂O₇ (KAB) have excellent transmission ranges extending from the ultraviolet into the infrared region. But their hygroscopic nature retards their use in real-time applications. Rare earth-based borate crystals such as rare earth calcium oxy borate RECa₄O(BO₃)₃ (RECOB) and rare earth calcium borate RE₂CaB₁₀O₁₉ (RECB) have excellent nonlinear optical behaviour and also allow the substitution of ‘laser-active’ ions to create ‘laser crystals’. Further they are non-hygroscopic which favours them in practical applications. A review on RECOB and RECB crystals is presented along with a discussion on our attempts to grow yttrium calcium oxy borate YCa₄O(BO₃)₃ (YCOB), lanthanum calcium oxy borate LaCa₄O(BO₃)₃ (LCOB), yttrium calcium borate Y₂CaB₁₀O₁₉ (YCB) and lanthanum calcium borate La₂CaB₁₀O₁₉ (LCB) crystals. These crystals possess excellent transmission properties combined with nonlinear optical behaviour. They also exhibit high laser damage threshold (LDT) values.