Investigation of third-order optical nonlinearity in KBe₂BO₃F₂, NaSr₃Be₃B₃O₉F₄, β-BBO, LiB₃O₅, and La₂CaB10O19 crystals by Z-scan

KBe 2 BO 3 F 2 (KBBF), NaSr 3 Be 3 B 3 O 9 F 4 (NSBBF), [Formula: see text]-BBO(BBO), LiB 3 O 5 (LBO), and La 2 CaB[Formula: see text]O[Formula: see text](LCB) are nonlinear optical (NLO) crystals that are commonly used to achieve UV solid-state laser output by NLO frequency conversion. The second-order NLO properties of these crystals have been widely studied, but the third-order NLO properties have rarely been paid attention to. Here, nonlinear absorption coefficient [Formula: see text] and nonlinear refractive index [Formula: see text] of KBBF, NSBBF, BBO, LBO, and LCB are investigated by the Z-scan technique employing two different laser sources under different energies at the wavelengths of 532[Formula: see text]nm and 355[Formula: see text]nm, respectively. The results show that the KBBF crystal has no nonlinear absorption at either 532[Formula: see text]nm or 355[Formula: see text]nm. NSBBF, BBO, LBO, and LCB crystals have multi-photon absorption, and the value of [Formula: see text] will increase with increasing laser energy. Not only that, the nonlinear refractive index [Formula: see text] of the crystals will increase with the increasing energy, also, this is supported by both experimental and theoretical simulation results. More importantly, it is the first time that we found NSBBF, LBO, and BBO crystals can realize conversion from self-defocusing effect to self-focusing effect with the increasing laser energy. In addition, the damage of the crystals was observed by high-energy open-aperture Z-scan.