A2 SnS5 : A Structural Incommensurate Modulation Exhibiting Strong Second-Harmonic Generation and a High Laser-Induced Damage Threshold (A=Ba, Sr).

Structural modulations have been recently found to cause some unusual physical properties, such as superconductivity or charge density waves; however, thus-induced nonlinear optical properties are rare. We report herein two unprecedented incommensurately modulated nonlinear optical sulfides exhibiting phase matching behavior, A2 SnS5 (A=Ba, Sr), with the (3+1)D superspace groups P21 21 2(00γ)00s or P21 (α0γ)0, featuring different modulations of the [Sn2 S7 ]∞ belts. Remarkably, Ba2 SnS5 exhibits an excellent second harmonic generation (SHG) of 1.1 times that of the benchmark compound AgGaS2 at 1570 nm and a very large laser-induced damage threshold (LIDT) of 8×AgGaS2 . Theoretical studies revealed that the structural modulations increase the distortions of the Sn/S building units by about 44 or 25 % in A2 SnS5 (A=Ba, Sr), respectively, and enhance significantly the SHG compared with α-Ba2 SnSe5 without modulation. Besides, despite the smaller Eg , the A2 SnS5 samples exhibit higher LIDTs owing to their smaller thermal expansion anisotropies (Ba2 SnS5  (1.51)

How To Maximize Birefringence and Nonlinearity of π-Conjugated Cyanurates.

Materials with large birefringence (Δn) are highly needed by fiber-optic isolators, whereas crystals showing strong second-order harmonic generation (SHG) are the key component for all-solid-state laser devices. Cyanurate constructed by the planar π-conjugated ring (C3N3O3, CY) is a class of fascinating materials exhibiting not only very large Δn (larger than that of calcite) but also strong SHG (2 times that of ß-BaB2O4, BBO). Here, we report five new cyanurates (I-V) and their single-crystal structures; among them, II realizes a Δn = 0.4, the maximum in the cyanurate family, and IV realizes a d33 = 6.69 pm/V, one of the highest values in the cyanurate family. We discover a dependence between Δn and the coplanarity of the CY rings that is explicitly described by a Boltzmann function, in which the coplanarity is defined by the dihedral angle (γ) between the CY plane and the principal optical axes XY plane. II realizes the maximum Δn due to its zero γ that indicates perfect coplanarity. Such a Δn-γ dependence also allows the Δn prediction of cyanurates. Furthermore, we uncover that the SHG intensity of cyanurates increases monotonically as the angle (θ) between the maximum hyperpolarizability (ßmax) vector and the crystal 21 polar axis decreases. We predict the dij to extend well beyond such a value and to maximize at θ = 0°. Our results have significant implications for the future rational design and discovery of high-performance materials of π-conjugated and other related systems.

Thioborates: potential nonlinear optical materials with rich structural chemistry.

Nonlinear optical (NLO) crystal materials with good performance are urgently needed. Various compounds have been explored to date. Metal chalcogenides and borates are common sources of potential NLO materials with desirable properties, particularly in the IR and UV regions, respectively. However, these two types of crystals have their specific drawbacks. Thioborates, as an emerging system, have unique advantages by combining the merits of borates and sulfides, i.e., the high laser damage thresholds and rich structural diversity of borates with large optical nonlinearity and the favorable transparency range of sulfides. However, only a limited number of thioborates are known. This paper summarizes the known thioborates according to structural motifs that range from zero-dimension to three-dimension, most of which are formed by sharing corners of the basic building units (BS3)3- and (BS4)5-. Although nearly one-third of the known thioborates are noncentrosymmetric, most of their properties, especially their NLO behaviors, are unexplored. Further attempts and additional investigations are required with respect to design syntheses, property improvements and micro-mechanism studies.