All-solid-state deep UV lasers (266nm and shorter) have many potential applications including metrology, LIBS, imaging, chem/bio standoff detection, ARPES spectroscopy, and laser surgery. There are few suitable direct laser emission lines at such short wavelengths, making the use of frequency generation by harmonic generation an important tool for UV solid state laser development. The primary limitation of the field is the development of suitable single crystals for nonlinear frequency conversion. The materials must be acentric, have bandgaps substantially wider than the desired conversion line and and be capable of phase matching to these short wavelengths. Several borate crystals have been reported as possible candidates for deep UV nonlinear applications. These include compounds with the formula ABBF (ABe2BO3F2) where A = K, Rb or Cs, and SBBO (Sr2Be2B2O7). Both compounds have very attractive properties for deep UV nonlinear applications including acentric crystal structures, wide band edges, moderate birefringence and reasonable nonlinear coefficients. Both classes are difficult to crystallize and process however. They have two dimensional structures making them somewhat soft, difficult to cut and polish at a critical angle, and making them prone to disorder. The ABBF phases have very attractive optical properties with wide bandgaps and are capable of frequency conversion to 175nm. They are extremely difficult to grow as single crystals however and are extremely soft and difficult to process. To date these are debilitating limitations. The SBBO crystals appear to have nearly as attractive properties with somewhat less of a bandgap than KBBF, but appear to have greater NLO coefficients. They are considerably harder than KBBF but are prone to severe disorder.
Recently the application of hydrothermal methods led to the crystal growth of both of these classes of compounds. The hydrothermal growth of KBBF and RBBF led to formation of large single crystals that can be used for new cutting and polishing methods enabling their use in applications such as ARPES. The hydrothermal growth of SBBO greatly reduces the disorder in the crystals enabling a more confident determination of their optical properties. The recent structural redetermination of SBBO and the examination of their optical properties will be discussed.
Joseph W. Kolis, Colin D. McMillen, and Henry Giesber, "Single crystals of borates for deep UV nonlinear optics (Conference Presentation)," Proc. SPIE 10902, Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII, 1090202 (Presented at SPIE LASE: February 05, 2019; Published: 4 March 2019); https://doi.org/10.1117/12.2514740.6008595913001.
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