Progress on Growth and Nonlinear Optical Properties of La₃Ga_5.5Nb_0.5O₁₄ Crystal

As an important mid-infrared (at 2–20 μm) nonlinear optical material, La₃Ga_5.5Nb_0.5O₁₄ (LGN) crystal has a wide spectral transmission range (at 0.28–7.40 μm), large second-order nonlinear coefficients (d₁₁ = 3.0±0.1 pm/V), and high laser damage thresholds (>1.41 GW/cm²@1064 nm), which makes it an excellent crystal material for high-power mid-infrared laser systems. In this review, we systematically summarize the structural properties of the LGN crystal, the large-size growth technology, the performance characterization, and the research progress of laser applications. To solve some problems of thermal stress cracking and volatilization of components during the growth process, a large-size and high-quality single crystal with a diameter of 80 mm is prepared via optimizing the temperature gradient and improving the growth parameters. Also we propose a synergistic optimization strategy of birefringent dispersion and nonlinear coefficient through component modulation, and develop La₃(Nb_1–xTa_x)_0.5Ga_5.5O₁₄ (LGNT_x) series of solid-solution single crystals, greatly improving the effective nonlinear coefficient. The LGN crystal has 4.4–5.7 μm mid-infrared output using the difference frequency generation technique, and the LGNT_x hybrid crystals can extend the difference frequency output wavelength to 6.84 μm, which is the longest wavelength for the continuous tuning output of oxide crystals. The results above provide an important theoretical basis and application prospect for designing new mid–infrared nonlinear optical materials and the large–scale use of LGN crystal in the future.

This review represents a comprehensive and in-depth summary of the La₃Ga_5.5Nb_0.5O₁₄ (LGN) crystal, covering important achievements in various key aspects, such as crystal structure and basic properties, large-size growth technology, nonlinear optical properties, mid-infrared nonlinear optical properties, and applications. In terms of large-size crystal growth, some technical problems of thermal stress cracking and component segregation during the growth of LGN are solved, and the preparation of large–size single crystals with the diameters of 60–80 mm is stablized. Meanwhile, the LGN crystal has nonlinear optical properties and applications, which provides a support for the development of high-power mid-infrared laser systems.

In the future, we need to optimize the crystal growth to obtain more accurate temperature regulation, further shorten the growth cycle, improve the growth efficiency, reduce the internal defects, and improve the annealing process, to obtain larger size (≥4 inches) high-quality LGN crystal, providing a better material base for high-performance mid-infrared laser. In addition, it is also necessary to investigate the performance regulation mechanism of LGN hybrid crystals in depth, and further optimize their nonlinear optical properties via utilizing elemental doping and structural fine-tuning, such as improving the nonlinear coefficient and broadening the gain bandwidth, to provide nonlinear optics crystals with excellent performances for the realization of high-power OPCPA applications. Developing mid-infrared laser sources and optoelectronic devices can meet a demand for mid-infrared wavelength lasers in biomedicine, environmental monitoring, national defense, security, and etc..