BaB2O4

BaB2O4, commonly known as beta-barium borate (BBO), is an inorganic nonlinear optical crystal widely used for frequency conversion and related electro-optic applications. It is valued for its broad optical transparency, extending from the ultraviolet into the infrared, and for its relatively high resistance to laser-induced damage.

The crystal enables second-order nonlinear optical processes such as second-harmonic generation, sum-frequency generation, and optical parametric generation because its structure lacks a center of symmetry. BBO exhibits birefringence, which allows phase matching for a range of wavelengths. By selecting appropriate crystal cuts and orientations, both type-I and type-II phase matching can be achieved, enabling efficient frequency conversion across visible and near-infrared regions. Its nonlinear coefficients provide strong second-order responses, contributing to its popularity in solid-state laser systems.

BaB2O4 can be grown by melt-growth methods, such as the Czochralski technique, as well as by solution-growth approaches. High-quality crystals can be produced in various sizes and cut geometries tailored for specific phase-matching needs. The material's combination of broad transmission, sizable nonlinear response, and robust laser damage tolerance makes it a common choice for ultraviolet generation (for example, frequency doubling of lasers around 1064 nm) and for new ultraviolet and visible light sources, including ultrafast and tunable systems.

In practice, BBO crystals require careful orientation and temperature management to maintain phase matching, particularly in high-power or broadband applications. While not the most nonlinear NLO material available, BBO offers a favorable balance of efficiency, wavelength coverage, and durability that underpins its widespread use in research and industry.