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.

The electronic structure and geometric distribution of neutral and charged states of interstitial oxygen in potassium dihydrogen phosphate ͑KDP͒ are investigated using a first-principles method. The energy gap is lowered to about 4.2 eV from the density-functional theory ͑DFT͒ value of 5.9 eV by a neutral O interstitial, which corresponds to a two-photon absorption of 364 nm ͑3.4 eV͒ after correction of 1.3 eV due to the underestimation of the band gap using the DFT. The addition of a single electron lowers the gap to 1.4 eV. The addition of two electrons leads to the formation of an isolated interstitial H 2 O molecule in KDP accompanied by the breaking of two hydrogen-bonded chains, and thus no defect states appear in the energy gap. The results can be utilized to explain the recently reported decomposition of KDP during a laser-induced breakdown.

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.

The face growth rate and critical supersaturation of {100} face were in situ measured using the laserpolarization-interference technique in the presence of potassium pyrophosphate, trimetric sodium phosphate and sodium hexametaphosphate impurities. The polyphosphate impurities inhibit the growth rate of prismatic faces. The face growth rate as a function of supersaturation at different impurity concentrations, as well as critical supersaturation as a function of impurity concentrations, was found in good agreement with a twodimensional nucleation model in the pure system and Kubota and Mullin's model in the presence of impurities. The average distance L between active sites available for impurity adsorption as well as the edge free energy was calculated.

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.

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