Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors...

The mixed cation compounds Na_1-KAsSe₂ (x = 0.8, 0.65, 0.5) and Na_0.1K_0.9AsS₂ crystallize in the polar noncentrosymmetric space group Cc. The AAsQ₂ (A = alkali metals, Q = S, Se) family features one-dimensional (1D) ¹/_∞[AQ₂^-] chains comprising corner-sharing pyramidal AQ₃ units in which the packing of these chains is dependent on the alkali metals. The parallel ¹/_∞[AQ₂^-] chains interact via short As···Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As···Se interchain distance increases the band gap from 1.75 eV in γ-NaAsSe₂ to 2.01 eV in Na_0.35K_0.65AsSe₂, 2.07 eV in Na_0.2K_0.8AsSe₂, and 2.18 eV in Na_0.1K_0.9AsS₂. The Na_1-KAsSe₂ (x = 0.8, 0.65) compounds melt congruently at approximately 316 °C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of Na_1-KAsSe₂ (x = 0.8, 0.65, 0.5) and Na_0.1K_0.9AsS₂ suggest that Na_0.2K_0.8AsSe₂ and Na_0.1K_0.9AsS₂ have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on Na_0.5K_0.5AsSe₂ confirm its SHG response with the highest value of d₃₃ = 22.5 pm/V (χ₃₃₃⁽²⁾ = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be d_eff = 18.9 pm/V (χ_eff⁽²⁾ = 37.8 pm/V), which is consistent with the experimentally obtained value of d_eff = 16.5 pm/V (χ_eff⁽²⁾ = 33.0 pm/V). Three-photon absorption is the dominant mechanism for the optical breakdown of the compounds under intense excitation at 1580 nm, with Na_0.2K_0.8AsSe₂ exhibiting the highest stability.