Black phosphorous (BP), the most thermodynamically stable allotrope of phosphorus, fills up the lacuna left by other two-dimensional materials with a band gap from 0.3 to 2 eV.

A series of K(DxH1−x)2PO4 (DKDP; x = 0, 12, 70, and 80%) crystals were grown using the point-seed technique. A positive nonlinear refractive index is exhibited, implying a self-focusing effect. The nonlinear refraction along the z-direction is greater than that along the II-direction, indicating that the distribution of the H2PO4− and D2PO4− groups have a critical influence on the nonlinear refraction. The nonlinear refraction effect of “point-seed” crystals is greater than that of traditional growth crystals, demonstrating that the crystalline quality is a major factor. Compared to the pyramid region, the prism region has a larger nonlinear refraction effect. The vital parameters n2 and have been calculated and the results allow for DKDP applications.

The nonlinear refractive index n2 is an important parameter for the nonlinear optical properties of a medium. Especially for the anisotropic media, the nonlinear refractive index is closely related with the direction.

A series of trace Fe 3+ -doped KH 2 PO 4 single crystals were grown using the conventional temperature cooling method. The Fe 3+ ion content in the as-grown crystals and the corresponding transmission spectrum were measured, respectively. With an increase of Fe 3+ ion concentration, the transmittance of the crystal gradually decreases within the wavelength range of 200 nm to 370 nm. Under picosecond pulse laser irradiation, third-order nonlinear optical characteristics were systematically measured at λ = 532 nm. The existence of nonlinear absorption and nonlinear refraction was demonstrated. The results show that the nonlinear refractive index n 2 is positive, indicating the presence of a self-focusing effect for the asgrown crystals. Specially, nonlinear absorption and refraction have a heavy dependence on the Fe 3+ ion content and crystal directions (II and z). With increasing Fe 3+ ion concentration, a significant variation has been observed with respect to the nonlinear absorption coefficient β, nonlinear refractive index n 2 and third-order nonlinear susceptibility χ (3) . A similar pattern for II < z has been presented for nonlinear absorption and refraction. The investigations suggest that nonlinear absorption and refraction caused by distortion of the electron cloud might be associated with the H 2 PO 4 − and (FeO 4 ) 2− groups.

Topological Kondo insulators have been attracting great attention from the condensed-matter physics community due to their fascinating topological and strongly correlated properties. Here, we report angledependent c-axis magnetoresistance (MR) oscillations in a Kondo insulator, SmB 6 single crystal, in a magnetic field of up to 13 T rotated in the ab-plane. Four-fold symmetric MR oscillations are first observed above 8 K, which result from the four-fold (C 4 ) degeneracy of the bulk Fermi surface of SmB 6 . With decreasing temperature down to 2.3 K, the C 4 symmetry of the MR oscillations gradually weakens and C 2 symmetry appears. This demonstrates a crossover from three-dimensional bulk states to two-dimensional surface states and implies the possible emergence of topological nematic states. Our experimental observations shed new light on the metallic surface states and nematic states in the Kondo insulator SmB 6 .

Abstract:In this work, using large-sized single crystals of high optical quality, the optical properties of Ba 2 TiSi 2 O 8 were systematically investigated, including transmission spectra, refractive indices and nonlinear absorption properties. The crystal exhibits a high transmittance (>84%) over a wide wavelength range from 340 to 2500 nm. The refractive indices in the range from 0.31256 to 1.01398 µm were measured, and Sellmeier's equations were fitted by the least squares method. The nonlinear absorption properties were studied by using the open-aperture Z-scan technique, with a nonlinear absorption coefficient measured to be on the order of 0.257 cm/GW at the peak power density of 16.4 GW/cm 2 . Such high transmittance and wide transparency indicate that optical devices using the Ba 2 TiSi 2 O 8 crystal can be applied over a wide wavelength range. Furthermore, the small nonlinear absorption observed in Ba 2 TiSi 2 O 8 will effectively increase the optical conversion efficiency, decreasing the generation of laser damage of the optical device.

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Organic–inorganic hybrid Pb-halide perovskites have attracted tremendous interest due to their potential application in photovoltaic and optoelectronic fields. However, the toxicity and poor stability of Pb-halide perovskites have become key bottlenecks toward their future commercialization and industrialization. Therefore, in this work, two novel hybrid lead-free perovskite nonlinear optical (NLO) crystals (2-AMP)2BiX7·H2O (X = Cl, Br) with high stability were successfully synthesized. Both the crystals belong to the orthorhombic P212121 space group, displaying a zero-dimensional perovskite structure. The thermal, environmental, and solvent atmospheric stabilities were comprehensively investigated, with high thermal stability up to 170 °C and high environmental and high solvent atmospheric stability observed for (2-AMP)2BiBr7·H2O. First-principles calculations were carried out to study the relationship of the structure and properties. A large birefringence of 0.1368@1064 nm was determined for (2-AMP)2BiBr7·H2O, which was derived from the strong aeolotropic conjugated π-electron distribution of planar 2-aminomethylpyridium. A second-harmonic generation (SHG) effect that was 0.25 and 0.32 times that of KH2PO4 (KDP) was measured for (2-AMP)2BiCl7·H2O and (2-AMP)2BiBr7·H2O, respectively, and the stronger SHG response of bromide was attributed to the larger distortions of {BiBr6} octahedrons. This work may offer new guidance for exploration of low toxicity and high stability of perovskite NLO materials.