Preparation and stress evolution of sol-gel SiO₂ antireflective coatings for small-size anisotropic lithium triborate crystals

Lithium triborate (LiB₃O₅, LBO) crystal is now one of the most useful nonlinear optical materials for frequency conversion of high power lasers. The use of the crystal, however, has been hampered by the unavailability of antireflective (AR) coatings with high laser damage resistance. In this work, a "point contact" dip-coating method is developed to prepare sol-gel SiO₂ AR coatings on small-size LBO crystals. Using this approach, we obtain a homogenous coating surface on an 8 mm×8 mm×3 mm LBO crystal. The stress measurements show that the stresses in sol-gel SiO₂ coatings vary with the time of natural drying, which is beyond our expectation. The anisotropic Young's modulus of the LBO crystal and the different evolution tendency of the stress in the different SiO₂ coating layers are found to be responsible for the crack of the double-layer AR coatings on anisotropic LBO crystal. Meanwhile, the resulting coatings on LBO crystal achieve a LIDT of over 15 J/cm² (532 nm, 3ns) and the coated LBO is expected to have a transmittance of over 99% at 800 nm.