BBO crystal (β-Barium Borate, β-BaB₂O₄), a trigonal non-centrosymmetric nonlinear optical crystal, is renowned for generating 2nd–5th harmonics of Nd:YAG lasers with efficiencies up to 70% (SHG, 532 nm), 60% (THG, 355 nm), 50% (FHG, 266 nm), and a record 200 mW at 213 nm (FFHG). Its superior performance stems from a broad transparency range (189–3500 nm), high nonlinear coefficient (d₁₁ = 2.6 pm/V at 1064 nm→532 nm), low photorefractivity, and non-hygroscopicity, making it critical for UV lithography, laser micromachining, and medical optics.
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