An Optimized KBe₂BO₃F₂‐Like Structure: The Unity of Deep‐Ultraviolet Transparency, Nonlinear Optical Property, and Ferroelectricity

Abstract

However, most of the above materials could not generate DUV coherent lights, because the prerequisites of DUV NLO crystals include a large bandgap (absorption cutoff edge < 200 nm), a sufficient second harmonic generation (SHG) response (>1 × KH 2 PO 4 , KDP), and an adequate birefringence that could achieve the birefringence phase matching (BPM) condition. [9] Unfortunately, these properties usually could not unite as a whole, due to their mutual restriction. [10] Therefore, up to now, KBBF is the only one achieving DUV coherent output. [11] Its specific structure could bring the large bandgap (≈8.44 eV), the sufficient SHG response (0.49 pm V -1 ) and the adequate birefringence (0.085@546 nm), simultaneously, [12] while it also results in the layered habit, which seriously hinders the growth of crystals in large sizes. Although the 177.3 nm DUV laser has been achieved by the prism coupling technique, [12] its wide application is still challenging. To overcome the layered characteristic, many strategies were proposed, such as the optimization of KBBF structure and the exploration of new KBBF-like DUV NLO crystals. As for the optimization of KBBF structure, crystals, such as Sr 2 Be 2 B 2 O 7 , [13] NaBe 2 BO 3 F 2 , [14] NH 4 B e 2 BO 3 F 2 , [15] and γ-Be 2 BO 3 F, [15] were obtained, while in the aspect of new KBBF-like DUV NLO crystals, such as Zn 2 BO 3 (OH), [16] CsAlB 3 O 6 F, [17] and C(NH 2 ) 3 SO 3 F, [18] were also found. However, the achievement of DUV coherent output is still difficult. Therefore, it is urgent to develop new DUV NLO crystals.Thus, a new strategy with an optimized KBBF-like structure based on the quasi-phase matching (QPM) principle is proposed. As for QPM, the modulation of NLO coefficients can be realized by the periodically structural design. [19] Besides, in contrast with BPM, QPM could make use of the maximum NLO coefficient, without any walk-off effect and short-wave matching limit in principle. [20] For QPM, the ferroelectric are always an optimal select, in which a periodic polarization structure can be obtained by an external electric field polarization method. [21] Therefore, this strategy aims at designing an optimized KBBF-like ferroelectric. First, the introduction of covalent bonds by replacing the triangular [BO 3 ] groups with tetrahedral groups could completely eliminate the layered characteristic, inheriting a large bandgap and an adequate SHG response from KBBF structure at the same time. Besides, the At present, only KBe 2 BO 3 F 2 (KBBF) can effectively realize a deep-ultraviolet (DUV) coherent output, while the layered growth habit seriously hinders its application. Here, a new design strategy with an optimized KBBF-like structure based on the quasi-phase matching principle is proposed. Thus, the first KBBF-like ferroelectric, Li 2 KRb(SO 4 ) 2 , is designed. It exhibits a short DUV absorption cutoff edge of 163 nm, a large nonlinear optical (NLO) effect of 1.7 × KH 2 PO 4 under a 1064 nm laser, and typical ferroelectricity. More importantly, it completely eli...

A series of novel phosphites/phosphates with deep-ultraviolet cut-off edges from 0D polar to 3D non-polar architectures

“…In recent years, the research on DUV NLO crystals has been mainly focused on phosphates/phosphites, 6 borates, 7 aluminates, 8 silicates, 9 germanates, 10 and iodates, 11 such as Ba 5 P 6 O 20 , 12 Li 3 [Al(PO 4 ) 2 (H 2 O) 1.5 ], 13 KBe 2 BO 3 F 2 , 14 CaCe(IO 3 ) 3 (IO 3 F)F, 15 Ba 4 Ag 5 (IO 3 ) 6 (I 3 O 8 ) 3 (I 4 O 11 ) 2 , 16 etc. Among them, several DUV NLO phosphate crystals have been reported and studied recently; 17 for example, LiHgPO 4 , 18 RbNaMgP 2 O 7 , 19 Cs 2 LiPO 4 , 20 K 2 Sb(P 2 O 7 )F, 21 and Ba 3 P 3 O 10 X (X = Cl, Br) 22 are found to have good NLO performance in the DUV region.…”

Tetrahedron-based deep-ultraviolet nonlinear optical crystal with optimized KBe2BO3F2-like structure and disordered cations

“…Furthermore, LARS crystals exhibit a strong SHG response of 2 × KDP (Fig. 3b), which is higher than that of most other DUV NLO sulfates, 36–42 as shown in Fig. 3c.…”

The Strategy to Improve the Phase Matching Ability in Tetrahedron‐Based Deep‐Ultraviolet Nonlinear Optical Crystals

“…Based on the above idea, a series of explorations have been carried out in our laboratory for developing the polar tetrahedronbased structures. Up to now, three tetrahedron-based DUV QPM crystals, LiKSO 4 , [78] Li 2 KRb(SO 4 ) 2 , [103] and LaBSiO 5 , [56] 8a) result in the outstanding optical and electrical properties, with a large bandgap (7.75 eV), a strong SHG response of 4.0 × KDP, and the typical ferroelectric polarization reversal characteristics. [78] In addition, LaBSiO 5 crystals isomorphic to the typical QPM crystals, LaBGeO 5 , are explored.…”