Comparative review of natural and modified amino acid crystals: structural features and SHG efficiency with reference to KDP

Amino acid derivatives are promising nonlinear optical (NLO) materials due to their structural diversity, biocompatibility, and tunable optical properties. The zwitterionic nature of amino acids, along with strong hydrogen-bonding interactions and inherent molecular chirality, promotes non-centrosymmetric crystal packing and enhanced optical nonlinearity. This review compares the crystallographic features and second-harmonic generation (SHG) efficiencies of twenty amino acids and their derivatives. Most crystals were grown by the slow evaporation method and characterized using single-crystal X-ray diffraction, confirming non-centrosymmetric structures with aligned dipoles. SHG studies using the Kurtz–Perry technique shows that amino acids with polar or aromatic side chains exhibit higher efficiencies, often 2–3 times that of potassium dihydrogen phosphate (KDP), while nonpolar systems show weaker responses. Structure – property relationships highlight the role of side-chain functionalities in enhancing charge transfer and molecular polarizability, confirming the potential of amino acid-based crystals for photonic and laser applications.