New ultraviolet nonlinear crystal is transparent to 155 nm
Common nonlinear materials such as b-barium borate (BBO) and lithium borate (LBO) cannot generate harmonics shorter than 205 nm. Chuangtian Chen and colleagues at the Fujian Institute of Research on the Structure of Matter (Fuzhou, Fujian, China) last year developed potassium beryllium borate fluoride (KBBF, KBe2BO3F) that can generate second harmonics to 184.7 nm. It has better ultraviolet transparency and birefringence properties than either BBO or LBO, but is difficult to grow because of weak bonds between layers.
Recently, Chen`s grou¥developed another borate crystal that retains the good optical properties of KBBF, but interlayer oxygen bonds make it easier to grow. The borate groups in strontium beryllium borate (Sr2Be2B2O7, SBBO) are also packed closer together, making its nonlinear coefficient about double that of KBBF. The researchers have grown SBBO crystals as large as 7 ¥ 7 ¥ 3 mm, transparent between 155 nm and 3.78 µm; they are still measuring minimum second-harmonic wavelength. Limin Zhu, R&D manager of Fujian Castech Crystals, the commercial venture of the institute, says it plans to announce SBBO at CLEO `95 in Baltimore, MD, in May, after completing physical and chemical tests. For more on nonlinear crystals, see p. 67.