New nonlinear optical crystal K_2Al_2B_2O_7

The new nonlinear optical crystal K2Al2B2O7 is discovered with the molecular engineering approach on the basis of anionic group theory. An optically perfect single crystal with space group P321, free of moisture and hygroscopy, is readily grown by the top-seeding flux method. Its transparence range covers 180 to 3600 nm. The refractive indices are measured with the minimum-deviation method, based on which the Sellmeier equation is obtained. The measured nonlinear optical coefficient d11 is 0.45 pm/V. The moderate walk-off angle and angular bandwidth, together with the high optical homogeneity, make it a promising candidate for the fourth- and the fifth-harmonic generation of a Nd:YAG laser.