Abstract:

In the 1960s, Maiman constructed the first laser. Pulsed lasers with high repetition rates and short pulse widths have extensive applications in fiber optics, military applications, spectroscopy, laser ranging, materials processing, medicine, and frequency conversion, etc. For instance, short pulse lasers with high repetition rates are desirable for material processing, in which the processing speed depends upon the repetition rate of the laser source. Electro-optic Q-switching has numerous advantages in many fields because of its better hold-off ability, larger pulse energy, and more controllable repetition rates. In 1961, Franken et al. first applied a ruby laser directly to quartz crystals and observed double-frequency radiation. Afterward, Bloembergen et al. analyzed the principle of nonlinear optical parametric generation theoretically. Since then, nonlinear optics has been playing an increasingly vital role in human society. Mid-infrared (mid-IR) lasers using nonlinear optical (NLO) crystals have essential applications in science as well as in daily life (e.g., infrared remote sensing, biological tissue imaging, environmental monitoring, and minimally invasive medical surgery). For generating mid-IR lasers in the spectral range of 3–20 µm, NLO materials are indispensable for optical parametric oscillation (OPO) or difference frequency generation. It is common for the available wavelength range to be limited by multiphonon absorption in the oxide crystal, and the damage threshold for semiconductors is relatively low. At present, the most widely used NLO crystal materials in the mid-IR band are semiconductor crystals represented by ZnGeP₂. However, their laser damage thresholds are low, which limits their application range. Therefore, one of the key issues in the field of NLO materials at present is to explore new mid-IR NLO crystal materials with excellent performance that are applicable to high-power lasers. Langasite materials are famous for their multifunctionality in optoelectronic applications, such as in piezoelectric convertors, electro-optic Q-switched laser generation, and surface acoustic wave devices. Their structure without central symmetry endows the crystal with electro-optic, piezoelectric, and NLO properties, and their laser damage threshold is high because they are oxides. The phonon energy of the crystal is low and the transmission range is wide owing to their composition, which may have important applications for mid-IR high-power lasers. The Langasite family comprise a set of perfect electro-optical crystals with an electro-optical coefficient of 2.3 × 10⁻¹² m∙V⁻¹, a broad transmission spectral range, and a high optical damage threshold of 950 MW∙cm⁻². Besides, their small piezoelectric coefficient (6 × 10⁻¹² C∙N⁻¹) reveals the possibility for Q-switching under high repetition rates without a piezoelectric ring effect. In this brief review, three important compounds—La₃Ga₅SiO₁₄, La₃Ga_5.5Nb_0.5O₁₄, and La₃Ga_5.5Ta_0.5O₁₄—are investigated and analyzed based on available experimental data. The electro-optical Q-switch and mid-IR OPO applications are summarized in detail. Finally, promising search directions for new metal oxides that have good mid-IR NLO performances are discussed.

Key words: Langasite, Mid-infrared, Electro-optical, Nonlinear, Optical parametric oscillation