Finding an excellent deep-UV nonlinear optical material in fluoroborophosphate system

This study is led by Prof. Fangfang Zhang (Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences) and Prof. Shilie Pan (Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences). Deep-UV nonlinear optical (NLO) crystals are key materials for the all-solid-state lasers. Generally, there are three basic conditions that need to be satisfied for deep-UV NLO crystals, i.e., large second harmonic generation (SHG) response (> 1 × KDP), short UV cutoff edge (λ_cutoff ≤ 200 nm) and suitable birefringence (Δn: 0.05-0.10) to meet the phase matching conditions in the deep-UV region. To date, KBe₂BO₃F₂ (KBBF) is the sole practical crystal that can output coherent light below 200 nm via SHG process. With the development of laser technology, it is urgent to find new deep-UV NLO crystals with excellent performances. To solve the key scientific problem of how to achieve the comprehensive performance balance of "large SHG response, deep-UV cut-off edge and suitable birefringence" in the design of deep-UV NLO materials, Pan's group proposed that the fluorinated borate groups are optimal groups in the design of deep-UV NLO materials (J. Am. Chem. Soc., 2017, 139, 10645； Angew. Chem. Int. Ed., 2017, 56, 3916; Angew. Chem. Int. Ed., 2017, 56, 14119).

Recently, Pan's group extended the "fluorination strategy" to the borophosphate system and synthesized a new deep-UV NLO crystal (NH₄)₃B₁₁PO₁₉F₃ (ABPF) with complex anionic units. ABPF has a unique fundamental building block [B₅PO₁₄F], and its anionic framework presents a KBBF-like layer [B₅PO₁₀F]_∞ (Figure 1). The introduction of strong B-O-P covalent bonds leads to the reduction of interlayer spacing, which enhances the interlayer interaction, thus avoiding the layered growth habit.

ABPF has excellent optical properties including a short cutoff edge (183 nm), a large SHG effect (1.2 × KDP), and a sufficient birefringence (0.088 at 1064 nm) for deep-UV phase-matching. The effect of the anionic groups, [BO₃], [BO₄], [BO₃F] and [PO₄] on the linear and nonlinear optical properties of ABPF are elucidated through the structural analysis and the first-principles calculations. The introduction of π-conjugated units [BO₃] is beneficial to the large SHG effect and sufficient birefringence, which enables ABPF to break through the so-called “200 nm” wall. Meanwhile, the introduction of non-π-conjugated units [BO₄], [BO₃F] and [PO₄] with large HOMO-LUMO gaps helps to eliminate the dangling bonds of the [BO₃] units, thereby obtaining a deep-UV transparency. Therefore, the excellent comprehensive performances of ABPF are mainly derived from the unique KBBF-like structure composed of the π-conjugated and non-π-conjugated units. The current work demonstrates the advance of the "fluorination strategy" and the effectiveness of using multiple anionic groups in the exploration of high-performance deep-UV NLO crystals.

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See the article:

(NH₄)₃B₁₁PO₁₉F₃: a deep-UV nonlinear optical crystal with unique [B₅PO₁₀F]_∞ layers

https://doi.org/10.1093/nsr/nwac110

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