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

TLDR

Intense coherent deep‑UV light generation from nonlinear optical materials is essential for photolithography, micromachining, and synthesis, yet few materials combine large second‑harmonic generation with a short UV‑cutoff, and the only known example, KBe₂BO₃F₂, is toxic, highlighting the need for crystal‑engineering strategies that exploit polar‑axis displacements and cation polarizability. A microscopic model was developed that accounts for the unexpectedly large SHG response, challenging conventional anionic group‑theory predictions. The benign polar compound Ba₄B₁₁O₂₀F was synthesized, meeting the deep‑UV requirements and exhibiting the strongest SHG response among known borates lacking lone‑pair anions or second‑order Jahn‑Teller transition metals.

Abstract

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.