The low temperature beta phase of barium borate (β-BaB₂O₄, so called BBO) is a negative uniaxial non-centrosymmetric crystal with large birefringence and nonlinear optical (NLO) coefficient. It has a high transmission over a wide spectral range (190-3500 nm), extremely high damage threshold (about 50 times higher than that of the KTiOPO₄), and very large phase-matching temperature bandwidth (40-50°C). These properties make BBO the choice of NLO crystals for a generation of coherent UV light by higher harmonic generation using Nd:YAG solid lasers and near infrared radiation by optical parametric processes. Currently, reliable and reproducible processing for the fabrication of NLO films are a subject of intense research....[ Read more ]

The low temperature beta phase of barium borate (β-BaB₂O₄, so called BBO) is a negative uniaxial non-centrosymmetric crystal with large birefringence and nonlinear optical (NLO) coefficient. It has a high transmission over a wide spectral range (190-3500 nm), extremely high damage threshold (about 50 times higher than that of the KTiOPO₄), and very large phase-matching temperature bandwidth (40-50°C). These properties make BBO the choice of NLO crystals for a generation of coherent UV light by higher harmonic generation using Nd:YAG solid lasers and near infrared radiation by optical parametric processes. Currently, reliable and reproducible processing for the fabrication of NLO films are a subject of intense research.

In this thesis, I report our first effort to grow high quality BBO thin films by a pulsed laser deposition (PLD) technique. PLD is an attractive deposition technique because it involves a simple experimental operation, preserves good film stoichiometry, provides energetic flux, and allows easy addition of various gases during the deposition process. The aim of this project is to search for the optimum experimental conditions for the formation of crystalline BBO films. An emphasis is on the investigation of the effect of deposition (substrate) temperature and substrate selection in the control of the crystallinity and crystal orientation alignment. Extensive x-ray diffraction (XRD) characterization work has been performed for examining the crystalline structures and orientations of the deposited BBO films.

Though crystalline BBO films can be obtained on various substrates either by post-annealing after room-temperature deposition or by high temperature in-situ deposition, the best film obtained is on sapphire (001) at a deposition temperature around 800 ℃ and in the presence of 0.1 mbar of O₂ gas. Under such conditions, the BBO film grown on the sapphire (001) substrate is single c-axis oriented with domain sizes as large as several tens of micrometers. We have found that the single (001) orientation of the film is largely attributed to the unique feature of the planar anion (B₃ O₆)^3- ring in the BBO molecule, which can parallelly lie on the substrate at an elevated temperature once it gains sufficient thermal energy to adjust itself.

The film grown on the sapphire (001) substrate exhibits a maximum effective second harmonic generation coefficient, d_eff, of 2.2 pm/V, which is comparable to that of the bulk β-BaB₂O₄ single crystals.