Deep-UV coherent light generated by nonlinear optical (NLO) materials possesses highly important applications in photonic technologies. Beryllium borates comprising anionic planar layers have been shown to be the most promising deep UV NLO materials. Here, two novel NLO beryllium borates Na2Be4B4O11 and LiNa5Be12B12O33 have been developed through cationic structural engineering. The most closely arranged [Be2BO5]∞ planar layers, connected by the flexible [B2O5] groups, have been found in their structures. This structural regulation strategy successfully resulted in the largest second harmonic generation (SHG) effects in the layered beryllium borates, which is ∼1.3 and 1.4 times that of KDP for Na2Be4B4O11 and LiNa5Be12B12O33, respectively. The deep-UV optical transmittance spectra based on single crystals indicated their short-wavelength cut-offs are down to ∼170 nm. These results demonstrated that Na2Be4B4O11 and LiNa5Be12B12O33 possess very promising application as deep-UV NLO crystals.

A novel series of alkali and alkaline earth metal combined fluorine beryllium borates NaCaBe2B2O6F, KCaBe2B2O6F, and KSrBe2B2O6F were successfully synthesized through molecular engineering design and grown in crystals by spontaneous nucleation technique from self-flux systems. The idea, introduction of relatively small alkali and alkaline earth metal cations and the fluorine anion, successfully resulted in the novel UV NLO crystal NaCaBe2B2O6F, the following substitution of cations directed to two centrosymmetric compounds KCaBe2B2O6F and KSrBe2B2O6F. In all of their structures, the a–b plane is the infinite lattice layer (Be3B3O6F3)∞ made up of BO3 and BeO3F anionic groups, and for the first time, it was found that the adjacent layers are connected with fluorine bridge atoms to form (Be6B6O12F3)∞ double layers, instead of oxygen bridge atoms usually occurred in other oxides. This structural characteristic is greatly beneficial to improve the layering-growth habit and eliminate polymorphism of a crystal. Optical measurements on the nonlinear optical crystal of NaCaBe2B2O6F reveal that this crystal is phase-matchable and its short-wavelength absorption edge is down to deep UV (below 190 nm). Theoretical calculations on electronic structure were carried out to explain the experimental results. Our preliminary results indicate that NaCaBe2B2O6F has promising applications in the UV spectrum region.Keywords: crystal growth; deep uv; fluorine beryllium borates; nonlinear optical crystal;

K2Al2B2O7 (KABO) is a new nonlinear optical crystal capable of laser harmonic generation in the UV range. However, abnormal UV absorption prevents its application in effectively generating UV light with wavelength shorter than 300 nm. The transmittance spectra of the grown crystals show distinct absorption bands at 216 nm and 264 nm. It is observed that the UV absorption is strongly correlated with iron impurity at a parts per million (ppm) level. Furthermore, electron paramagnetic resonance (EPR) spectra of the absorbing crystals show a strong signal at g = 2.00 position corresponding to a Fe3+ center. A new crystal growth method which reduces the iron content has been proposed and results show that the new KABO crystal is free from the Fe3+ UV absorptions.The UV absorptions for KABO crystals were assigned to an O2− → Fe3+ charge-transfer process in which Fe3+ substituting Al3+ sites. A new growth method to reduce the iron content in the grown crystal was adopted and a new KABO crystal shows negligible Fe3+ UV absorption was obtained for the first time.