High-quality interference coatings for LBO and BBO crystals produced by the ion-beam technique

The technology of producing interference coatings for nonlinear crystals has some peculiarities depending on certain properties of such crystals. Besides, the otpical properties of these coatings depend on laser beam irradiation, temperature, and intracavity or extracavity conditions. Temperature in the vacuum chamber, reactive gas pressure, energy of charged particles, conditions of evaporated materials influenced not in the same way as on usual substrates. It was noted that the ion beam acted on pure surfaces of crystals improving adhesion and reducing mechanical stress of the dielectric film. The ion beam sources were used for pre-cleaning just before deposition as assistance in the deposition process. Adhesion, mechanical stress and laser damage threshold depend on the ion beam energy. The optical properties of pure and coated crystals were tested in relation to the ion beam energy. The threshold has been defined. Besides, the degradation of films has been investigated after exposure to the laser beam. Undoubtedly, the antireflection coatings are of the most interest for such crystals. In this work the following aspects of the Ar coatings have been considered: (i) calculation of the optimum construction of optical coatings for a specific spectral range; (ii) production of the required parameters of the films; (iii) study of the optical parameters; (iv) influence of the ion beam on the spectral characteristics of the AR coatings for LBO and BBO crystals. It was noted that only monoenergetic low-energy beams can improve the optical properties for these crystals having AR coatings. As a result of this work, the AR coatings have been produced at one, two or three wavelengths simultaneously.