Another candidate nonlinear optical crystal is entering the field: lutetium aluminum borate, or LuAl3(BO3)4 (abbreviated as LuAB). Engineers at the Chinese Academy of Sciences (Fuzhou, China) have characterized the optical properties of LuAB, finding that the UV cutoff wavelength of the crystal is 178 nm and its birefringence -(no - ne) is in the range of 0.067 to 0.088 between the near IR and the UV spectral regions, with example indices no and ne of 1.8563 and 1.7681, respectively, at a 266 nm wavelength.
The researchers grew the crystals themselves, using a top-seeded solution-growth (TSSG) method to produce colorless, transparent crystals weighing more than 40 g and on the order of 20 mm in size. The material is phase-matchable in the regions from 0.49 to 1.4 μm for type-I and 0.65 to 1.4 μm for type-II frequency-doubling, respectively. Under test; the material showed an effective nonlinear coefficient as high as 1.03 pm/V for type I frequency-doubling at longer wavelengths. The walkoff angle for type-I phase-matching varies between about 5 and 45 mrad for fundamental wavelengths between 0.49 and 0.68 μm. The crystal has a smaller walk-off angle and larger acceptance angle than beta-barium borate (BBO), an often-used conventional nonlinear optical material. In addition, LuAB has a wider range of phase-matching angles in the UV region and is resistant to thermal shock, moisture, and acids. The researchers are now trying to grow larger LuAB crystals. Contact Ning Ye at [email protected].