Synergistic effects of amino acid additives in KDP and ADP single crystals: a review of enhanced NLO performance

Potassium Dihydrogen Phosphate (KDP) and Ammonium Dihydrogen Phosphate (ADP) single crystals are widely utilized nonlinear optical, electro-optic, and piezoelectric materials. However, their practical applications are constrained by moderate laser-induced damage thresholds, limited mechanical strength, and modest optical non-linearities. Doping these host lattices with zwitterionic amino acids—such as L-alanine, glycine, L-lysine, L-arginine, and L-proline—has emerged as an effective crystal-engineering strategy to overcome these limitations. This review provides a comprehensive overview of recent progress in the synthesis, characterization, and property optimization of amino acid-doped KDP and ADP crystals. The influence of dopants on crystal structure, lattice distortion, morphology, and intermolecular bonding is critically evaluated using X-ray diffraction and Fourier-transform infrared spectroscopy. Key enhancements in optical transparency, second-harmonic generation efficiency, dielectric behavior, mechanical hardness, and thermal stability are systematically discussed, alongside existing technical challenges and prospective opportunities for next-generation high-power photonic and optoelectronic applications.