The nonlinear refractive index n ₂ 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 Fe³⁺-doped KH₂PO₄ single crystals were grown using the conventional temperature cooling method. The Fe³⁺ ion content in the as-grown crystals and the corresponding transmission spectrum were measured, respectively. With an increase of Fe³⁺ 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 n₂ is positive, indicating the presence of a self-focusing effect for the as-grown crystals. Specially, nonlinear absorption and refraction have a heavy dependence on the Fe³⁺ ion content and crystal directions (II and z). With increasing Fe³⁺ ion concentration, a significant variation has been observed with respect to the nonlinear absorption coefficient β, nonlinear refractive index n₂ 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 H₂PO₄ ⁻ and (FeO₄)²⁻ groups.

High-quality KDP and DKDP crystals with 12%, 70% and 80%-deuterium content were grown by the conventional temperature cooling method. The nonlinear optical characteristics of K(D_xH_1-x)₂PO₄ crystals, including nonlinear absorption and refraction, were systematically investigated through the Z-scan method at λ = 532 nm. The results demonstrate the existence of nonlinear absorption and refraction, and a nonlinear refractive index with a positive sign indicates the generation of self-focusing effect. Moreover, the results indicate that nonlinear absorption and refraction were associated with crystal direction and structure, especially the distribution of H₂PO₄^- or D₂PO₄^- groups. For K(D_xH_1-x)₂PO₄ crystals, a similar rule of β was z > I > II, while n₂ (or χ_R⁽³⁾) was z > II ≈I. These results will evoke more attention to KDP and DKDP applications, especially frequency conversion and electro-optic switch.

A series of potassium dihydrogen phosphate (KDP) crystals and Fe³⁺ doped KDP crystals were grown at different temperatures.

The phenomenon of the cracking of KDP z-cut seed crystals with a series of sizes due to the temperature difference of the growth solution was explored in detail.

Nonlinear optical absorption effect in KDP and 70%-DKDP crystals, which were grown by the conventional temperature cooling method, was systematically studied using picosecond pulse laser excitation. Using open aperture Z-scan measurements, the dependence of nonlinear absorption effect on sample orientations (I, II, and z) as well as laser intensity was systematically measured at λ = 1064 and 532 nm. According to the experimental results, the nonlinear absorption effect at λ = 532 nm was confirmed, while at λ = 1064 nm no nonlinear absorption was observed for KDP and 70%-DKDP crystals. In addition, the optical absorption along I- and II-type affected by laser intensity was larger than that along the z-direction. The important nonlinear absorption coefficients β and ${\chi }_I^{\left(3\right)}$ (esu) measured along different orientations were exhibited in detail at wavelengths of 1064 nm and 532 nm. The results indicate that nonlinear absorption coefficients increase first and then decrease with the increment of laser intensity for KDP and 70%-DKDP crystals.