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Deep-UV Nonlinear Optical Crystal
KBBF is one of the best known DUV NLO crystals and remains the best-performing. Its shortest PM SHG output wavelength of 161 nm is the theoretical limit for DUV NLO crystals (Table 1); it also produces a 177.3-nm DUV laser of 200 mW65,68.
To overcome this limitation, DUV NLO researchers are constantly exploring new DUV NLO crystals with shorter SHG wavelengths and higher SHG conversion efficiency. However, despite decades of material exploration, no DUV NLO crystals have been discovered that can exceed the performance of KBBF and its derivatives.
The DUV NLO performance of KBBF is due to directional covalent bonding within its structure. The anisotropic nature of KBBF is reflected in its refractive index and optically polarized spectra.
This anisotropy is caused by the difference in polarization of intralayer and interlayer light that is produced by the cation exchange between the A-site K+, Rb+ and Cs+ ions and the O-site O+. The resulting birefringent phase matching in the (Be2BO3F2) layers leads to increased interlayer polarization, while keeping the intralayer polarization almost unchanged.
Recently, a group of Chinese researchers has shown that a KBBF family crystal is capable of producing a tunable deep-UV laser with an average power tunable from 180 to 232.5 nm. This work has opened up the possibility of tunable DUV lasers at much lower wavelengths than previously possible.
Specifically, the deep-UV nonlinear optical crystal KBe2BO3F2 was grown using hydrothermal growth. The crystal was then activated by plasma activation and mechanical polishing. Its optical properties were then characterized by XRD and characterization by fluorescence.