Deep-Ultraviolet Nonlinear-Optical Material K3Sr3

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Deep-Ultraviolet Nonlinear-Optical Material K3Sr3Li2Al4B6O20F: Addressing the Structural Instability Problem in KBe2BO3F2 Hongping Wu,† Hongwei Yu,‡ Shilie Pan,*,† and P. Shiv Halasyamani*,‡ †

Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, 40-1 South Beijing Road, Urumqi 830011, China ‡ Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204, United States S Supporting Information *

KBBF. The former has been accomplished in a few materials.22,23 The latter is necessary to grow large crystals. Such crystals are required in order to determine the birefringence and, subsequently, the phase-matching (PM) wavelength range. Crystals are also needed to determine the individual SHG coefficients, dij’s. In fact, for an ideal DUV NLO material, a moderate birefringence (0.05−0.08) and a strong SHG response (dij > 0.39 pm/V) are necessary. These are in addition to a wide transparency range and large laser damage threshold (>5 GW/cm2). Herein, we report on the design, synthesis, crystal growth, optical properties, and PM capabilities of K3Sr3Li2Al4B6O20F. The reported material retains the positive optical attributes of KBBF but does not require BeO in the synthesis and exhibits strong interlayer bonding, which facilitates crystal growth. Our measurements reveal that K3Sr3Li2Al4B6O20F exhibits an absorption edge of 190 nm and relatively large powder SHG (PSHG) intensities of approximately 1.7KH2PO4 (KDP) and 0.3β-BBO at 1064 and 532 nm, respectively. As large crystals were grown, we were able to measure the refractive indices and determine the birefringence, 0.0574, at 1064 nm. On the basis of the birefringence, we were able to determine the type I (type II) PM limit for fundamental light as 448 nm (670 nm); i.e., the SHG limit is 224 nm (335 nm). This indicates that K3Sr3Li2Al4B6O20F is type I phase-matchable for direct FOHG at 266 nm from a 1064 nm laser. Materials with planar building units such as borates, carbonates, and nitrates may exhibit large SHG responses with a moderate birefringence and a short absorption edge.24−27 Using this idea, Chen et al. designed and synthesized KBBF. KBBF exhibits coplanar BO3 triangles resulting in a large SHG response, dij = 0.47 pm/V,28 and a moderate birefringence, Δn = 0.077 at 1064 nm.29 The terminal oxygen atoms of the BO3 triangles are bonded to the beryllium atoms, creating [Be2BO3F2]∞ layers that result in an absorption edge of 147 nm.30 Unfortunately, the weak interlayer K−F bonds along the optical axis (c axis) hinder large crystal growth. To the best of our knowledge, the largest KBBF crystal grown is less than 4 mm along the optical axis.31 To overcome this layering tendency, Sr2Be2B2O7 (SBBO)32 was designed and synthesized. With SBBO, [Be2B2O7]∞ double layers are observed that do impart some stability; however, toxic BeO is still required in the synthesis, and large crystals have yet to be reported.31 Recently, Zhao et al. reported K3Ba3Li2Al4B6O20F, with a short absorption

ABSTRACT: A beryllium-free deep-ultraviolet (DUV) nonlinear-optical (NLO) material, K3Sr3Al4Li2B6O20F, has been synthesized and characterized. Unlike KBe2BO3F2 (KBBF), the reported NLO material does not require the use of toxic BeO in the synthesis, and through the judicious selection of cations, strong interlayer interactions are observed that facilitate the crystal growth. K3Sr3Al4Li2B6O20F exhibits second-harmonic generation (SHG) at both 1064 and 532 nm with efficiencies of 1.7KH2PO4 and 0.3β-BaB2O4 and has an absorption edge of 190 nm. Because of the strong interlayer interactions, we were able to grow well-faceted large crystals, 8 × 8 × 5 mm3, through a top-seeded-solution-growth technique. With these crystals, we determined a birefringence of 0.0574 at 1064 nm and a type I phase-matching SHG limit of 224 nm.

D

eep-ultraviolet (DUV) coherent light is of relevance for complex optical technologies such as photolithography, high-resolution spectroscopy, and laser cooling.1−7 In fact, a solid-state laser at 177.3 nm could replace the current ArF excimer laser (193 nm) that is used in high-resolution photolithography and ophthalmological applications. An effective method for obtaining coherent radiation in the DUV and ultraviolet (UV) is through cascading second-harmonic generation (SHG), i.e., fourth harmonic −1064 nm/4 = 266 nm and sixth harmonic −1064 nm/6 = 177.3 nm. Over the last 30 years, a number of nonlinear-optical (NLO) materials have been discovered,8−20 including β-BaB 2O 4 (β-BBO) 8 CsLiB 3 O 6 (CLBO)12 and some newly discovered materials Ba3(ZnB5O10)PO4,16 Li4Sr(BO3)2,17 and Sr3[(BexB1−x)3B3O10][Be(O1−xFx)3] (x = 0.30).20 Of them, β-BBO and CLBO are currently used in solid-state lasers to generate coherent radiation in the UV, i.e., 266 nm. However, β-BBO has a large birefringence, 0.11, which results in walk-off effects reducing the SHG efficiency, whereas CLBO is hygroscopic. As such, new materials for fourthharmonic generation (FOHG) are needed. In the DUV, there is currently one material, KBe2BO3F2 (KBBF), capable of lasing at 177.3 nm.21 However, KBBF suffers from two serious drawbacks. First, toxic BeO must be used in the synthesis. Second, single crystals of the material layer along its optical axis and, as such, large single crystals have not been grown. Two important design components for synthesizing DUV NLO materials include using nontoxic elements and overcoming the layering tendency of © 2017 American Chemical Society

Received: June 14, 2017 Published: July 25, 2017 8755

DOI: 10.1021/acs.inorgchem.7b01517 Inorg. Chem. 2017, 56, 8755−8758

Communication

Inorganic Chemistry edge (190 nm) and a large SHG response (1.5KH2PO4).33 Here, the SBBO stoichiometry is tripled, resulting in Sr6Be6B6O21. With Sr6Be6B6O21, Sr6 is replaced by K3Ba3, Be6 by Li2Al4, and O21 by O20F, resulting in K3Ba3Li2Al4B6O20F. In the Zhao et al. paper, “thick” crystals are reported (5 mm), purportedly along the optic axis, but the crystal faces were not indexed. Additionally, birefringence data and PM wavelength ranges were not given. As such, it is unclear whether K3Ba3Li2Al4B6O20F is phasematchable in the UV. K3Sr3Li2Al4B6O20F, is not isostructural with K3Ba3Li2Al4B6O20F and has a smaller interlayer distance, i.e., 4.426 versus 4.808 Å, resulting in an enhanced SHG response and facilitates single-crystal growth. Polycrystalline K3Sr3Li2Al4B6O20F was synthesized through conventional solid-state methods using stoichiometric amounts of reagents (see the Supporting Information). The phase purity was confirmed by powder X-ray diffraction (Figure S1). A crystal of K3Sr3Li2Al4B6O20F was obtained through a Li2O−SrF2−B2O3 flux. The EDX analysis confirms the existence of K, Sr, Al, and F, with a K:Sr:Al molar ratio of approximately 3:3:4 (Figure S2). The crystal structure was determined by single-crystal X-ray diffraction (Table S1). K3Sr3Li2Al4B6O20F crystallizes in the chiral acentric trigonal space group R32 (No. 155). The B3+ cations are coordinated to three oxygen atoms to form the BO3 triangles. The Al3+ and Li+ cations are bonded to four anions, and form AlO4 and LiO3F tetrahedra, respectively. The AlO4 and LiO3F tetrahedra connect with isolated BO3 triangles to create [Li2Al4B6O20F]∞ double layers that contain nine-coordinated K+ cations within the layer and eight-coordinated Sr2+ cations between the layers (Figure 1).

Figure 2. AlO4 and LiO3F tetrahedra linked to the BO3 groups through oxygen atoms in the ab plane, forming rings wherein the K+ cations reside.

Tables S2 and S3). The IR spectrum also confirms the presence of BO3 and AlO4 groups (Figure S4). Three structural features are evident in K3Sr3Li2Al4B6O20F. First, the BO3 groups exhibit a coplanar configuration that favors a large SHG response and moderate birefringence; second, the oxygen atoms on the BO3 groups are fully bonded; a short absorption edge is observed. Third, the remaining Al−O and Li− F bonds connect the [LiAl2B3O6]∞ layer to create the double layers that generate stability and facilitate crystal growth. Before the crystal growth and associated properties are delved into, a discussion on the PSHG efficiency and thermal and moisture stabilities is necessary. The PSHG efficiencies, as a function of the particle size, were measured at 1064 and 532 nm (Figure 3). The SHG intensities

Figure 3. PSHG data for K3Sr3Li2Al4B6O20F at 1064 nm (left) and 532 nm (right). Curves are drawn to guide the eye and are not a fit to the data.

increase with increasing particle sizes for both wavelengths, indicating that K3Sr3Li2Al4B6O20F is phase-matchable at both wavelengths, consistent with our PM calculations based on experimental refractive index data. The SHG intensities of K3Sr3Li2Al4B6O20F at 1064 and 532 nm are approximately 1.7KDP and 0.3β-BBO, respectively, in the 125−150 μm particle size range. The thermogravimetric and differential thermal analysis data indicate an endothermic peak around 812 °C (Figure S5). Above 812 °C, K3Sr3Li2Al4B6O20F decomposes to K2Al2B2O7 and other unknown phases (Figure S1). Additionally, a 0.1930 g crystal was submerged in water for 1 week. After 1 week, the crystal was removed from the water and reweighed. No weight loss was observed, and no degradation in the crystal was apparent (Figure S6). Large single crystals of K3Sr3Li2Al4B6O20F were grown by the top-seeded-solution-growth (TSSG) technique (Figure 4).38 One issue with KBBF and SBBO is the tendency to layer along the optic axis, i.e., the c axis, which hinders large crystal growth. As seen in Figure 4, the crystal dimension along the c axis is 5 mm, which suggests that the short double-layer distance in K3Sr3Li2Al4B6O20F favors large crystal growth.

Figure 1. Structural evolution from SBBO to K3Sr3Li2Al4B6O20F. Note that the double layers in K3Sr3Li2Al4B6O20F have been reinforced through Al−O and Li−F bonds.

The double layers consist of two [LiAl2B3O6]∞ single layers that are linked by oxygen and fluoride atoms. In the ab plane, with respect to connectivity, the AlO4 and LiO3F tetrahedra are linked to the BO3 groups through oxygen-forming rings wherein the K+ cations reside (Figure 2). The [Li2Al4B6O20F]∞ double layers of K3Sr3Li2Al4B6O20F are very similar to the layers found in K3Ba3Li2Al4B6O20F.33 However, in K3Sr3Li2Al4B6O20F, the [Li2Al4B6O20F]∞ double layers are connected by the smaller Sr2+ cations, which results in a 240° rotation of the adjancent double layers (Figure S3). This rotation results in a parallel arrangement of the Li−F−Li and Al−O−Al bonds in adjacent double layers and reduces the interlayer distance (Figure S3). These structural features of K3Sr3Li2Al4B6O20F will enhance the SHG response, stabilize the structure, and facilitate single-crystal growth. The reported bond lengths are consistent with those of previously reported compounds..34−36 Bond-valence-sum calculations37 are consistent with the reported oxidation states (see 8756

DOI: 10.1021/acs.inorgchem.7b01517 Inorg. Chem. 2017, 56, 8755−8758

Communication

Inorganic Chemistry

green dashed lines represent the calculated refractive indices for the fundamental and second-harmonic wavelengths, i.e., n(ω) and n(2ω), whereas the blue dashed line represents [no(ω) + ne(ω)]/2. Type I PM occurs when n(ω) = n(2ω) or, in Figure 6, when the red solid line crosses the green dashed line (black circle).41 The type I PM limit of K3Sr3Li2Al4B6O20F for fundamental (second-harmonic) radiation is 448 nm (224 nm). Type II PM occurs when [no(ω) + ne(ω)]/2 = ne(2ω), i.e., the blue circle in Figure 6. The type II PM limit for fundamental (second-harmonic) radiation is 670 nm (335 nm). Thus, the SHG limit for K3Sr3Li2Al4B6O20F is 224 nm, indicating that the material can achieve FOHG and produce 266 nm light from a 1064 nm laser. As such, K3Sr3Li2Al4B6O20F may be a viable replacement for β-BBO and CsLiB6O10. Addressing the layering tendency of NLO KBBF and SBBO, we have synthesized and grown large single crystals of K3Sr3Li2Al4B6O20F. The NLO-active material exhibits an absorption edge of 190 nm and is SHG-active at 1064 and 532 nm. Large single crystals were grown using the TSSG method. Optical measurements on these crystals revealed a moderate birefringence of 0.0574 at 1064 nm. Calculations indicate that a type I PM limit for fundamental (second-harmonic) radiation is 448 nm (224 nm), indicating that K3Sr3Li2Al4B6O20F is a viable material to generate 266 nm by FOHG from a 1064 nm laser.

Figure 4. Photograph and morphology of a 8 × 8 × 5 mm3 K3Sr3Li2Al4B6O20F crystal.

A crystal wafer for optical measurements was cut and polished. The transmission spectrum of K3Sr3Li2Al4B6O20F was collected on a 1-mm-thick crystal at room temperature (see the experimental section). An absorption edge of 190 nm was determined (Figure S7). We measured the refractive index at five wavelengths (450.2, 532, 636.5, 829.3, and 1062.2 nm) using the prism-coupling technique in order to determine the birefringence. A (001) crystal wafer was polished for the measurements (Figure 5). Because no > ne, K3Sr3Li2Al4B6O20F is a negative

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01517. Experimental and theoretical methods and additional tables and figures (PDF)

Figure 5. Refractive index data for K3Sr3Li2Al4B6O20F. The curves are fits given by the Sellmeier equation.

Accession Codes

CCDC 1556105 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

uniaxial crystal, with the birefringence (Δn = no − ne) ranging from 0.0574 to 0.0637. The Sellmeier equation can be fit to the measured refractive index data.40 The calculated and experimental refractive indices and Sellmeier coefficients are given in Tables S4 and S5. The calculated refractive index dispersion curves, based on the Sellmeier equation, are shown in Figure 6. The red solid and 39

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shilie Pan: 0000-0003-4521-4507 P. Shiv Halasyamani: 0000-0003-1787-1040 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (Grants U1303392, 51425206, and 91622107), the Western Light Foundation of CAS (Grant 2016-QNXZ-A-2), 973 Program of China (Grant 2014CB648400), and the Youth Innovation Promotion Association CAS (Grant 2015353). H.Y. and P.S.H. thank the Welch Foundation (Grant E-1457) and the NSF (Grant DMR-1503573) for support.

Figure 6. Refractive index dispersion curves for fundamental and second-harmonic light. The type I and II PM limits for second-harmonic light are 224 and 335 nm, respectively. 8757

DOI: 10.1021/acs.inorgchem.7b01517 Inorg. Chem. 2017, 56, 8755−8758

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01517 Inorg. Chem. 2017, 56, 8755−8758