L-arginine phosphate (LAP) crystals are high-performance nonlinear optical crystals that have garnered significant attention due to their high laser damage threshold and high frequency conversion efficiency, particularly in comparison to analogous crystals. Numerous unique properties have been identified through research. This paper reviews the exploration and advancements of LAP crystals over several decades, with a focus on the structure of LAP crystals, the growth of high-quality LAP single crystals, the mechanisms underlying the optical properties of LAP crystals, the surface morphology of LAP crystal, the application domains of LAP crystals, and the novel nonlinear optical crystals derived from LAP. LAP crystals hold considerable application potential in the field of nonlinear optics and are anticipated to be utilized in inertial confinement fusion (ICF). However, challenges remain in their practical applications.

Research has demonstrated that the compositional elements of phosphor arginine (PA) and LAP crystals are entirely identical. PA serves as a carrier for energy storage and transport in invertebrates, exhibiting a characteristic of molecular conformational variability during the storage and transportation of bioenergy. When energy is stored in PA, the interaction between the arginine molecule and the phosphate group induces a conformational transition from an extended structure to a bent structure; conversely, during transport, the conformation reverts. Similarly, LAP crystals also display a propensity for conformational variability under laser irradiation. The application of laser radiation results in a reduction in the number of Raman vibrational splitting peaks caused by phosphate groups within the LAP crystals, leading to an increase in the uniformity of the phosphate groups within the LAP crystals. This phenomenon suggests a trend towards a transition from distorted tetrahedral to regular tetrahedral configurations. The transfer of energy through conformational changes within the molecules enhances their resistance to laser damage, thereby providing a new explanation for the exceptional laser damage resistance of LAP crystals. This paper explores the relationship between the structural design and the mechanism underlying the ultra-high laser damage thresholds, theoretically investigating the electronic interaction among various functional groups. The findings offer a theoretical basis for material improvement and provide guiding insights for the future exploration of novel nonlinear optical materials. The high damage thresholds of LAP crystals are summarized in Table 1, indicating that, in all cases, the damage thresholds of LAP and deuterated LAP (DLAP) crystals exceed those of potassium dihydrogen phosphate (KDP) crystals and fused quartz, thereby highlighting the significant potential of LAP crystals for applications in high-power laser systems.

L-arginine, an organic compound essential for various proteins, is required for the growth of LAP crystals. Prolonged crystal growth can lead to an increase in microbial populations within the solution, which may cause the mother liquor to become moldy. The incorporation of these impurities into the crystal lattice can adversely affect crystal quality. Therefore, preventing microbial contamination is crucial for the growth of high-quality, large-sized single crystals. The addition of substances such as liquid paraffin or n-hexane can effectively inhibit contamination and enhance crystal quality. The seed crystal method is employed to achieve large-sized LAP crystal growth, with stringent control over the saturation of the growth solution and meticulous regulation of the linear growth steps to improve crystal quality. Techniques such as solution overheating, continuous filtration, and thorough stirring are utilized to widen the metastable zone of the solution. This paper reports on the growth of large-sized LAP crystals measuring 170 mm×120 mm×60 mm by our research team.

In the field of high-energy lasers, the characteristics of stimulated Brillouin scattering (SBS) indicate the potential of LAP crystals to enhance beam quality as phase conjugate mirrors. The DLAP crystal further reduces absorption at a wavelength of 1064 nm, demonstrating a higher SBS gain. The reflection of SBS diminishes transmitted energy, thereby protecting the crystal from damage. The capability of the DLAP crystal for aberration correction in a 10 Hz laser system is demonstrated, with the experimental setup illustrated in Fig. 5. The experiment utilizes a Nd∶YAG laser (wavelength λ=1064 nm, pulse width τ=12?13 ns), where the introduction of aberrations results in a divergence of 6 mrad. The assessment of the DLAP crystal ability to correct for aberrations is conducted through an examination of the far-field intensity profile. Additionally, in the terahertz (THz) domain, significant application potential has been exhibited. Arjun et al. reported for the first time that LAP crystals can generate THz radiation in the range of 0.1 THz to 2.0 THz. As the input power increases from 500 mW to 1200 mW, the THz output power is correspondingly enhanced. The measured refractive index and absorption coefficient of the crystal in the 0.1 THz to 2.0 THz range vary from 1.2 to 1.65 and from 5 cm?1 to 40 cm?1, respectively, indicating another promising application prospect for LAP crystals.

LAP crystals, as a novel nonlinear optical material originating from China, have garnered significant attention both domestically and internationally due to their exceptional comprehensive performance. Their potential application value in Inertial Confinement Fusion systems is particularly noteworthy, as they hold promise for further enhancing the frequency conversion efficiency and the laser damage thresholds of existing materials.