Birefringent materials are of great
importance in optical communication
and the laser industry, as they can modulate the polarization of light.
Limited by their transparency range, few birefringent materials, except
α-BaB2O4 (α-BBO), can be practically
used in the deep ultraviolet (UV) region. However, α-BBO suffers
from a phase transition and does not have enough transparency in the
deep UV region. By introducing the relatively small alkali metal Na+ cation and the F– anion to keep the favorable
structural features of α-BBO, we report a new birefringent crystal
Na3Ba2(B3O6)2F (NBBF), which has the desirable optical properties. NBBF not only
maintains the large birefringence (Δn = n
o – n
e =
0.2554–0.0750 from 175 nm to 3.35 μm) and extends its
UV cutoff edge to 175 nm (14 nm shorter than α-BBO) but also
eliminates the phase transition and has the lowest growth temperature
(820 °C) among birefringent materials. These results demonstrate
that NBBF is an attractive candidate for the next generation of deep
UV birefringent materials.
Deep-ultraviolet (DUV) nonlinear-optical (NLO) materials generating coherent DUV light by a direct second-harmonic-generation (SHG) process have long been pursued as industrially useful lasers. For several decades, KBeBOF (KBBF) has been regarded as the best DUV-NLO material; it is characterized by a short DUV phase-matching edge of 161 nm and a large SHG coefficient of 0.47 pm/V. However, it suffers a strong layering tendency, hindering the growth of large crystals for commercial use. Here, we use a computer-aided swarm structure searching technique to design an alternative DUV-NLO material with a new atmospheric-pressure phase BeBOF with a P6̅2 c space group (γ-BBF) that outperforms the DUV-NLO properties of KBBF. The predicted DUV phase-matching edge and SHG coefficient of γ-BBF are 152 nm and 0.70 pm/V, respectively. The structure of γ-BBF reduces the layering tendency compared with KBBF because of the absence of K atoms in the γ-BBF crystal. Our work paves the way for superior DUV-NLO materials that can be grown as large crystals for commercial applications.
Searching for high performance materials for optical communication and laser industry in deep-ultraviolet (DUV) region has been the subject of considerable interest. Such materials by design from scratching on multi-component complex crystal systems are challenging. Here, we predict, through density function calculations and unbiased structure searching techniques, the formation of quaternary NaBeBO3 compounds at ambient pressure. Among the four low-energy phases, the P63/m structure exhibits a DUV cutoff edge of 20 nm shorter than α-BaB2O4 (189 nm) – the best-known DUV birefringent material. While the P-6 structure exhibits one time second-harmonic generation efficiency of KH2PO4 and possesses excellent crystal growth habit without showing any layer habit as observed in the only available DUV nonlinear optical material KBe2BO3F2, whose layer habit limits its wide industrial applications. These NaBeBO3 structures are promising candidates for the next generation of DUV optical materials, and the structure prediction technique will shed light on future optical materials design.
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