Linear and nonlinear properties of ultraviolet fluorine crystal: A first-principles study

• The electronic and optical properties of ultraviolet crystals BaMgF4 and KBe2BO3F2 are reported.
• GW approach and hybrid functionals are used to describe the band structure of both crystals.
• The calculated optical gap base on GW corrected eigenstates are in good agreement with experimental results.
• Strong excitonic effects were found for both crystals, which are important for well description of the linear optical properties.
• Nonlinear optical properties are calculated as well by taking advantage of 2n+1 theorem.

The electronic structure and optical properties of Deep Ultraviolet (DUV) nonlinear optical crystal BaMgF4 and KBe2BO3F2 have been studied in the framework of many-body perturbation theory as well as hybrid functionals. For the electronic properties, the hybrid functionals method makes significant improvement on description of electronic band gap for both crystals. However, the results still underestimate the band gap comparing with the GW results. By considering the self-energy of electrons, electronic band gap and the optical band gap are both described well comparing with the experimental results, which is crucial for prediction the performance of DUV crystals in the DUV region. In addition to the remarkable self-energy effect, the macroscopic dielectric function and related optical properties, such as refractive index n(ω), excitation coefficient k(ω), absorption coefficient α(ω), energy-loss function L(ω), and reflectivity R(ω) have been calculated by solving Bethe–Salpeter equation (BSE). By comparing the RPA results and BSE results, we found that the excitonic effects play an important role in description of optical properties, which is absence in the previous work. Furthermore, taking advantage of 2n+1 theorem, the nonlinear optical susceptibility have been calculated as well. As all key factors that determine the performance of DUV nonlinear crystals have been addressed theoretically, our present work pave the way using first-principle theory to assist the crystal engineering for other candidates of DUV nonlinear optical crystal.