An outstanding second-harmonic generation material BiB2O4F: exploiting the electron-withdrawing ability of fluorine

It is well known that the incorporation of fluorine usually blue-shifts the absorption edge of a crystal due to its large electronegativity. Herein, we validate the new functionality of fluorine that leads to the strong second harmonic generation (SHG) effect in a unique nonlinear optical (NLO) bismuth borate, BiB2O4F, where F coordinates to both B3+ and Bi3+ cations. The F-incorporation not only forms a more asymmetric unit [BO3F] compared to the almost optical isotropic [BO4], but also withdraws the non-bonding electrons of Bi3+, increasing its local polarization; thus the total SHG efficiency is increased significantly. Indeed, experimental measurements and first-principles calculations both support our anticipation that BiB2O4F is a phase-matchable NLO material with an exceptionally large SHG response. In order to further consolidate the functionality of fluorine, a comparison study with the structurally similar compound BiB2O4(OH) was performed.