"L-Threonine and Beyond: Amino Acids as Promising SHG Materials"

Many optically active amino acids exhibit highly efficient optical second-harmonic generation (SHG), making them promising candidates for coherent blue-green laser generation and frequency doubling applications. Notably, amino acids such as L-alanine, L-histidine, and L-threonine possess unique features: (i) molecular chirality, (ii) wide transparency in the visible and UV ranges, and (iii) zwitterionic character. Molecular chirality compels these molecules to crystallize in noncentrosymmetric space groups—an essential criterion for SHG materials. Meanwhile, their zwitterionic nature contributes to high electro-optic parameters as well as good mechanical and thermal strength in the resulting crystals.

Among these compounds, L-threonine stands out as an important amino acid with SHG efficiency surpassing that of many other amino acids and their family crystals.

Over the years, amino acid family crystals have been extensively investigated by researchers for their nonlinear optical (NLO) properties. Nonlinear optical crystals capable of generating second-harmonic frequencies play a crucial role in optoelectronics and photonics. For numerous device applications, NLO crystals with high second-harmonic conversion efficiency and transparency in the visible and ultraviolet ranges are required.

The development of new nonlinear optical frequency conversion materials has significantly impacted fields such as optical computing, laser technology, optical communication, data storage, signal processing, and instrumentation. Most amino acids and their complexes belong to the family of organic and organometallic NLO materials. For example, L-alanine is a naturally occurring chiral amino acid, and organometallic crystals based on amino acids—such as L-alanine cadmium chloride, L-arginine phosphate, L-histidine tetrafluoroborate, L-alanine hydrogen chloride, L-threonine cadmium chloride, L-alanine magnesium chloride, L-alanine sodium nitrate, L-proline lithium bromide, L-alanine potassium chloride, L-valine cadmium chloride, bis L-proline cadmium iodide, L-proline magnesium chloride, alanine barium chloride, and glycine barium chloride—have been studied. These materials exhibit large nonlinearity, high resistance to laser-induced damage, low angular sensitivity, and good mechanical hardness. Thiourea, another nonlinear optical material, is often used to enhance the physical and chemical properties of organometallic crystals like L-alanine cadmium chloride.

However, most organic crystals suffer from inadequate transparency, poor optical quality, and a low laser damage threshold. Additionally, growing large single crystals suitable for device applications can be challenging. In contrast, inorganic crystals offer excellent mechanical and thermal properties but generally have only modest nonlinearity. Organometallic materials are often more suitable for device fabrication due to their wide transparency window, high second-harmonic generation efficiency, and mechanical and chemical stability. In these materials, the metallic component often focuses on group IIB metals, which typically provide high transparency in the UV region.