p–(p,π*) interaction mechanism revealing and accordingly designed new member in deep-ultraviolet NLO borates LinMn−1B2n−1O4n−2 (M = Cs/Rb, n = 3, 4, 6)

Abstract

Exploration on the compounds in the complex alkali metal borate system had resulted in the discovery of a class of deep-ultraviolet second-order nonlinear optical (NLO) materials, including Li₃Cs₂B₅O₁₀ (L3CBO), Li₄Cs₃B₇O₁₄ (L4CBO), and Li₆Rb₅B₁₁O₂₂ (L6RBO), which can be reduced to a general formula of Li_nM_n−1B_2n−1O_4n−2 (M = Cs/Rb, n = 3, 4, 6). All of the three crystals exhibit a short UV cutoff edge (below 190 nm) and share a class of topologically similar BO groups interconnected by LiO_n (n = 4, 5), Rb/CsO_n (n = 8, 9, 10). The mechanism of NLO properties of this class of crystals was studied using band-resolved and SHG-density methods based on the first-principles theory. The results reveal that the “charge-transfer excitation” from the non-bonding 2p occupied states of O atoms to the π* and 2p unoccupied states of the BO₃ substructure in BO groups is the key mechanism of NLO properties of this material family. Through systematic analyses on the relationship between crystal structure and NLO effects, a new crystal, Li₄Rb₃B₇O₁₄, was designed and subsequently synthesized through solid state reaction, which is isomorphic with Li₄Cs₃B₇O₁₄ and exhibits a short UV cutoff edge (below 190 nm) and a SHG response of 2/3 × KDP.