Effect of methylene blue dye on solubility, growth, optical, photoluminescence, and laser damage threshold properties of KAP crystal

Pure and 0.1 mol% methylene blue (MB)-doped potassium acid phthalate (KAP) single crystals were grown by the slow evaporation solution technique. The incorporation of the dye led to the formation of light bluish crystals with the dimensions of 19 × 18 × 3 mm³. Single-crystal XRD revealed that the crystals retain the orthorhombic structure (space group Pca₂₁) with slight lattice strain due to MB incorporation. The 0.1 mol% MB-doped KAP crystal did not have any extra peak, which did not change the KAP crystal’s fundamental structure. Pure and 0.1 mol% MB-doped KAP single crystals show positive solubility between 25 and 50 °C, with enhanced solubility for the doped crystal. The optical transparency increased from 74% (for Pure KAP) to 82% (for MB-doped KAP), with characteristic MB absorption between 530 and 720 nm observed for the 0.1 mol% MB-doped KAP. High-intensity luminescence is observed at 680 nm when excited at 530 nm for the 0.1 mol% MB-doped KAP crystal. Thermal analysis indicates the decomposition temperatures at 302 and 292 °C for pure and 0.1 mol% MB-doped KAP crystals, respectively. Laser damage threshold values improved from 2.3 GW/cm² (Pure KAP) to 7.5 GW/cm² (0.1 mol%-doped KAP) with MB doping. The chemical etching revealed reduced etch pit density from 1.764 × 10⁴ cm⁻² (Pure KAP) to 0.988 × 10⁴ cm⁻² (0.1 mol%-doped KAP). The inclusion of dye molecules into the KAP crystal lattice increases the piezoelectric coefficient from 0.60 to 1.63 pC/N.