Designing a Deep-Ultraviolet Nonlinear Optical Material with a Large Second Harmonic Generation Response

The
generation of intense coherent deep-UV light from nonlinear
optical materials is crucial to appli­ca­tions ranging from
semiconductor photolithography and laser micromachining to photochemical
synthesis. However, few materials with large second harmonic generation
(SHG) and a short UV-cutoff edge are effective down to 200 nm. A notable
exception is KBe2BO3F2, which is
obtained from a solid-state reaction of highly toxic beryllium oxide
powders. We designed and synthesized a benign polar material, Ba4B11O20F, that satisfies these requirements
and exhibits the largest SHG response in known borates containing
neither lone-pair-active anions nor second-order Jahn–Teller-active
transition metals. We developed a microscopic model to explain the
enhancement, which is unexpected on the basis of conventional anionic
group theory arguments. Crystal engineering of atomic displacements
along the polar axis, which are difficult to attribute to or identify
within unique anionic moieties, and greater cation polarizabilities
are critical to the design of next-generation SHG materials.