Jiyong Yao

The family of metal thiophosphates is an important but long-ignored compound system of the nonlinear optical (NLO) materials with desirable properties for the mid-infrared (mid-IR) coherent light generation. In the present work, the mid-IR NLO capabilities of metal thiophosphate crystals are systematically investigated based on their structure–property relationship. The linear and nonlinear optical properties of these crystals are predicted and analyzed using the first-principles calculations. In particular, several metal thiophosphate compounds are highlighted to exhibit good mid-IR NLO performances, as supported by the primary experimental results. These candidates would greatly promote the development of the mid-IR NLO functional materials.

The new compound BaGa(4)Se(7) has been synthesized for the first time. It crystallizes in the monoclinic space group Pc with a = 7.6252 (15) Å, b = 6.5114 (13) Å, c = 14.702 (4) Å, β = 121.24 (2)°, and Z = 2. In the structure, GaSe(4) tetrahedra share corners to form a three-dimensional framework with cavities occupied by Ba(2+) cations. The material is a wide-band gap semiconductor with the visible and IR optical absorption edges being 0.47 and 18.0 μm, respectively. BaGa(4)Se(7) melts congruently at 968 °C and exhibits a second harmonic generation response at 1 μm that is approximately 2-3 times that of the benchmark material AgGaS(2). A first-principles calculation of the electronic structure, linear and nonlinear optical properties of BaGa(4)Se(7) was performed. The calculated birefractive indexΔn = 0.08 at 1 μm and the major SHG tensor elements are: d(11) = 18.2 pm/V and d(13) = -20.6 pm/V. This new material is a very promising NLO crystal for practical application in the IR region.

Structure matters: Owing to the structure and arrangement of the [Be3B3O12F]10− group (see picture, left, Be blue, B olive, F green, O red), the mixed-cation fluorine beryllium borate NaSr3Be3B3O9F4 (right) exhibits a large second harmonic generation effect and a short UV absorption edge. Its crystals show no layering tendency, making it promising for applications in deep-UV nonlinear optics. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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.

The four compounds BaGa(2)MQ(6) (M = Si, Ge; Q = S, Se) have been identified as a new series of IR nonlinear optical (NLO) materials and are promising for practical applications. They are isostructural and crystallize in the noncentrosymmetric polar space group R3 of the trigonal system. Their three-dimensional framework is composed of corner-sharing (Ga/M)Q(4) (M = Si, Ge; Q = S, Se) tetrahedra with Ba(2+) cations in the cavities. The polar alignment of one (Ga/M)-Q2 bond for each (Ga/M)Q(4) tetrahedra along the c direction is conducive to generating a large NLO response, which was confirmed by powder second-harmonic generation (SHG) using a 2090 nm laser as fundamental wavelength. The SHG signal intensities of the two sulfides were close to that of AgGaS(2) and those for the two selenides were similar as that of AgGaSe(2). The large band gaps of 3.75(2) eV, 3.23(2) eV, 2.88(2) eV, and 2.22 (2) eV for BaGa(2)SiS(6), BaGa(2)GeS(6), BaGa(2)SiSe(6), and BaGa(2)GeSe(6), respectively, will be very helpful to increase the laser damage threshold. Moreover, all the four BaGa(2)MQ(6) (M = Si, Ge; Q = S, Se) compounds exhibit congruent-melting behavior, which indicates that bulk crystals needed for practical applications can be obtained by the Bridgman-Stockbarger method. The calculated birefringence indicates that these materials may be phase-matchable in the IR region and the calculated SHG coefficients agree with the experimental observations. According to our preliminary study, the BaGa(2)MQ(6) compounds represent a new series of promising IR nonlinear optical (NLO) materials which do not belong to the traditional chalcopyrite-type materials such as AgGaQ2 (Q = S, Se) and ZnGeP(2).