Viewpoint: Inorganic Materials for UV and Deep-UV Nonlinear-Optical

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Viewpoint: Inorganic Materials for UV and Deep-UV NonlinearOptical Applications P. Shiv Halasyamani* and Weiguo Zhang Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204, United States ABSTRACT: New inorganic nonlinear-optical (NLO) materials are necessary for solid-state lasers to generate coherent radiation in the ultraviolet (UV) and deep-UV regions. The purpose of this Viewpoint is to describe the chemical and physical requirements for such materials, as well as the current state of the field. In addition, the “birefringence challenge” is described and a flowchart is presented for characterizing new second-harmonic-generation materials. Finally, several new materials are suggested that may have UV and/or deep-UV NLO applications.

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INTRODUCTION Inorganic materials capable of second-harmonic generation (SHG) have been used for years to generate coherent radiation at wavelengths where laser sources are not available.1 SHG, or frequency doubling, is a nonlinear-optical (NLO) phenomenon that is defined as the conversion of a specific wavelength of light to half of its original, λ → 1/2λ, or with frequency, ω → 2ω.2 SHG is used to generate coherent radiation from the ultraviolet (UV) to the IR region. This Viewpoint will focus on inorganic materials with NLO applications in the UV and deep-UV. Throughout this Viewpoint, the fundamental laser wavelength will be 1064 nm (Nd:YAG). Thus, UV, 266 nm, and deep-UV, 177.3 nm, radiation may be generated through cascaded frequency conversion, i.e., fourth-harmonic generation (FOHG), 1064 nm/4 = 266 nm, or sixth-harmonic generation (6thHG), 1064 nm/6 = 177.3 nm. In order to generate either FOHG or 6thHG, the material in question must exhibit the following:3 (i) crystallographic noncentrosymmetry (NCS), (ii) wide transparency: absorption edge ≤ 250 nm (Eg ≥ 5.8 eV); (iii) large SHG coefficient: dij > 0.39 pm/V [d36(KDP)]; (iv) moderate birefringence: Δn ∼ 0.07−0.10 at 1064 nm; (v) chemical stability and large laser damage threshold; (vi) “easy” growth of large high-quality single crystals. Each will be described in more detail. i. Crystallographic NCS. SHG is only possible in materials lacking a center of symmetry, i.e., materials found in NCS or acentric crystal classes.4 A Venn diagram of the relationships between the 21 NCS crystal classes is shown in Figure 1. The NCS crystal classes that have the correct symmetry for SHG are shown in the bottom oval. Interestingly, the symmetry requirement for SHG and piezoelectricity are the same and are described mathematically by the same third rank tensor, dijk. As seen in Figure 1, all of the NCS crystal classes exhibit the correct symmetry for SHG except for cubic crystal class 432. This symmetry requirement may be taken as a necessary, but not sufficient, condition for SHG. ii. Wide Transparency Range. For FOHG or 6thHG, a wide transparency range is necessary. Given the required absorption edge, i.e., λ ≤ 250 nm (Eg ≥ 5.8 eV), the material in question needs to be colorless. Thus, cations in these materials cannot exhibit any d−d or f−f transitions; i.e., most transition © XXXX American Chemical Society

Figure 1. Venn diagram showing the relationships between the NCS crystal classes and their associated physical properties.

metals and lanthanide cations cannot be incorporated. Also, many main-group cations have absorption edges above 250 nm. Even with these limitations, there are a host of cations and anionic groups that may be used including alkali and alkalineearth cations, borates, borate fluorides, borate phosphates, and carbonate fluorides. iii. Large SHG Coefficients. The SHG conversion efficiency is directly related to the magnitude of the SHG coefficients, dij; the larger the dij value, the larger the SHG conversion efficiency. In the UV and deep-UV regimes, dij > 0.39 pm/V is required. 0.39 pm/V is the d36 value for KH2PO4 (KDP),5 the standard reference used in the measurements. Individual dij coefficients may be measured using the Maker Fringe technique on parallel-plate single crystals that have been cut, indexed, and polished.6,7 A detailed description of the technique has recently been published.8 iv. Moderate Birefringence. One of the most important requirements for a viable UV or deep-UV NLO material is a Received: August 26, 2017

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DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Spectrum from 225 to 125 nm showing the absorption edges of UV and deep-UV NLO materials. Reproduced with permission from ref 3. Copyright 2016 American Chemical Society.

moderate birefringence, i.e., Δn = 0.070−0.10. A moderate birefringence is required for phase-matching, an optical direction in the material where the refractive indices are equal, i.e., where n(ω) = n(2ω),9 in other words, where the fundamental and second-harmonic waves have the same propagation speed. With this condition, all materials crystallizing in the cubic NCS classes, 23 and −43m, are excluded from phase-matching applications attributable to their isotropic nature, i.e., Δn = 0. This requirement and its ramifications will be discussed in greater detail. v. Chemical Stability and Large Laser Damage Threshold. This perhaps is a rather obvious requirement. To have NLO applications, the material must be air- and moisturestable and not require any toxic reagents for its synthesis. With respect to the laser damage threshold, the material should be able to withstand 5 GW/cm2 for a single nanosecond pulse at 1064 nm. vi. “Easy” Growth of Large High-Quality Single Crystals. Perhaps the greatest challenge for NLO materials is their large high-quality crystal growth. With respect to size, a minimum of 5 mm in all three dimensions is necessary, and with respect to high quality, a rocking curve measurement around a Bragg reflection should generate a peak with a fullwidth at half-maximum (fwhm) of less than 100 arcseconds (0.0278°), ideally less than 50 arcseconds (0.0139°). There are a myriad of ways to grow crystals including top-seeded solution growth, Czochralski, Bridgman, and floating zone.10−16 As such, no one method is suitable for all materials. In addition to growing the crystal, the crystal must be cut, indexed, and polished to measure its linear-optical and NLO properties. Current State of the Field. Fundamental to the advancement of optical technologies is the synthesis of new NLO materials. The spectrum from approximately 225 to 125 nm is depicted in Figure 2. Included in this figure are a few materials that have NLO applications in the UV and deep-UV regions. Also denoted are the energies for the fifth-, sixth-, and seventhharmonic generation from 1064 nm radiation (dotted green lines), as well as the ArF and F2 excimer wavelengths (dashed blue lines). The wavelengths for 6, 7, 8, and 9 eV (dashed red

lines) are also shown. The arrows indicate the absorption edge for each material, whereas the numbers in square brackets are the wavelength(s) where coherent radiation has been reported from a fundamental 1064 nm source; e.g., for CsLiB6O10, coherent radiation has been reported at 213 and 266 nm.17−19 As seen in Figure 2, there are only a handful of materials where coherent radiation has been reported in the UV and deep-UV regions. For 266 nm radiation, i.e., FOHG, 1064 nm/4 = 266 nm, four materials have been reported: YAl3(BO3)4 (YAB),20 Li2B4O7 (LB4),21,22 CsLiB6O10 (CLBO),17,23 and β-BaB2O4 (βBBO).24 Although YAB has a large SHG coefficient, d11 = 1.7 pm/V,25 the low transmittance in the 200−320 nm range prohibits its practical application.26 For LB4, high-quality crystals have been grown;22 however, the SHG coefficient is very small, i.e., d31 = 0.15 pm/V.27 Thus, the two remaining materials, CLBO and β-BBO, are routinely used to generate 266 nm coherent radiation in industry and academia. Highquality crystals of both are commercially available from a variety of sources. However, both CLBO and β-BBO have their drawbacks. CLBO is hygroscopic, whereas β-BBO has a large birefringence, resulting in walk-off issues that lead to a reduction in the SHG conversion efficiency. Simply put, both materials are at the end of their technological life. With 177.3 nm generation, the situation is even more restrictive. Only two materials, KBe2BO3F2 (KBBF) and RbBe2BO3F2, have been reported to generate coherent radiation at 177.3 nm through 6thHG.28,29 There are two major issues with both materials. First, toxic BeO must be used in the synthesis. Second, and more importantly, both materials exhibit a layered crystal structure with weak A+−F− interactions. Attributable to these weak interactions and layering tendencies, single crystals no larger than 4 mm along the optical axis have been grown.29,30 As such, criterion vi has not been met. There are a number of other materials shown in Figure 2 with very short absorption edges and strong SHG conversion efficiencies that, in principle, could be used for FOHG or 6thHG. However, the birefringence of these materials renders them unusable for UV or deep-UV NLO applications; i.e., phase-matching is not possible. This “birefringence challenge” is central to NLO applications. In B

DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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length range is from approximately 800 to 3000 nm (from 400 to 1500 nm). These wavelengths are read off the lower (upper) x axes in Figure 3a. In Figure 3b, the birefringence is now approximately 0.070. The phase-matching condition remains the same, i.e., n(ω) = n(2ω). As seen in Figure 3b, the fundamental (second-harmonic) phase-matching wavelength range is from approximately 400 to 4250 nm (from 200 to 2125 nm). Thus, increasing the birefringence has increased the phasematching wavelength range or has blue-shifted the SHG limit. The hypothetical material could frequency double to 800 nm with a birefringence of 0.027, whereas frequency doubling to 400 nm is possible with a birefringence of 0.070. Designing, synthesizing, and growing crystals of an NLO material with a suitable birefringence is forefront research and remains an ongoing challenge. Figure 4 shows a variety of SHG materials and their absorption edges and SHG limits. The SHG limit is the wavelength for which SHG is possible, i.e., an SHG limit of less than 266 nm means that the material can frequency-double 532 nm radiation. As seen in Figure 4, BPO431 has an absorption edge of 134 nm with d36 = 0.76 pm/V. However, its birefringence is 0.005 at 1064 nm.32 As such, phase-matching in the UV and deep-UV regions is not possible. In addition, Ba4B11O20F (BBOF)33 has an absorption edge of 190 nm but a birefringence of 0.0146 at 1064 nm.34 Thus, its SHG limit is 525 nm. It should not be inferred from the above discussion that the bigger the birefringence, the better. In fact, if the birefringence is too large, i.e., Δn ≫ 0.10, serious walk-off effects occur.35,36 Too large of a birefringence results in a decrease of the intensity of the second-harmonic beam. This is the issue with β-BBO. β-BBO has a birefringence of 0.113 at 1064 nm, and although its SHG coefficient is large (d22 = 2.20 pm/V), its SHG intensity is reduced, attributable to walk-off effects.37 Flowchart for Characterization of an SHG Material. Once an SHG material is synthesized and structurally characterized, what are the next steps? A flowchart of this is given in Figure 5. The first steps after an SHG material has been synthesized, assuming one has approximately 1 g of pure material, is to perform powder SHG (PSHG) measurements and diffusereflectance experiments. The PSHG measurements are done on sieved powders, usually from 20 to 200 μm, with the SHG intensity measured as a function of the particle size. A particle size versus SHG intensity measurement enables one to determine if the material is phase-matchable at the incident wavelength used (see Figure 6). Ideally, the PSHG intensity should be greater than that of KDP and the material should be phase-matchable. Also measured is the absorption edge through diffuse-reflectance experiments. For NLO applications in the UV (deep-UV) region, an absorption edge below 250 nm (200 nm) is required. If the material in question has a PSHG intensity greater than KDP, is phase-matchable, and exhibits an absorption edge below 250 nm, large crystal growth is warranted. As seen in Figure 5, crystal growth plays a central role with respect to NLO applications. A large, >5 mm, highquality crystal that has been cut, indexed, and polished is required to determine if the material has any NLO applications. After successful crystal growth, the crystal faces must be cut, indexed, and polished. Once this has been done, Maker Fringe, rocking curve, refractive index, and laser damage threshold measurements may be performed. The latter is destructive, so obviously the measurement should be done last. Maker Fringe measurements are performed in order to determine the dij

other words, if criteria i−iii, v, and vi are met but the birefringence is too small, the material in question has no UV or deep-UV NLO applications. Exactly how the birefringence impacts the NLO properties is discussed next. Birefringence Challenge. A moderate birefringence, Δn = 0.070−0.10, is required for a viable UV or deep-UV NLO material. The birefringence of a material is the difference in the refractive indices at a specific wavelength. For uniaxial systems, hexagonal, tetragonal, and trigonal, the birefringence is the difference between the refractive indices of the ordinary and extraordinary beams, i.e., Δn = |no − ne|. For biaxial systems, orthorhombic, monoclinic, and triclinic, the birefringence is the difference between the refractive indices along the z and x axes, i.e., Δn = |nz − nx|. The magnitude of the birefringence directly impacts the phase-matching wavelength range. A discussion of phase-matching with respect to inorganic NLO materials has recently been published.9 Briefly, the most efficient SHG will occur when there is a propagation direction in the material where n(ω) = n(2ω), i.e., where the refractive index of the fundamental wave is equal to the refractive index of the second harmonic. A hypothetical example with a uniaxial system should prove illustrative. In Figure 3a, the birefringence at 1064 nm is approximately 0.027, i.e., Δn = ne(ω) − no(ω). The phasematching wavelength range (type I) is where n(ω) = n(2ω), where the red dashed line crosses the green solid line. In Figure 3a, the fundamental (second-harmonic) phase-matching wave-

Figure 3. Refractive indices of a hypothetical uniaxial material are given for n(ω) and n(2ω). The birefringence increases from part a to b, with an increase in the phase-matching wavelength range. C

DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Absorption edge and SHG limit for a variety of NLO materials. Note that a short absorption edge does not guarantee a short SHG limit. The materials in purple have been shown to generate 177.3 nm radiation (6thHG), whereas those in blue are routinely used to generate 266 nm radiation (FOHG).

Figure 6. PSHG measurements showing phase-matching (blue) and non-phase-matching (red) behavior. Figure 5. Flowchart depicting the characterization steps following the synthesis of an SHG material. Note that crystal growth is central to determining whether the material will have any NLO applications.

values, SHG coefficients, of the material. Ideally, the dij of the material should be larger than 0.39 pm/V (d36 of KDP). In principle, the same crystal used for Maker Fringe experiments can be used for rocking curve measurements. A schematic for the Maker Fringe experiments is shown in Figure 7. Briefly, an Nd:YAG laser is neededthe same laser that is used for PSHG measurements may be usedalong with a beam splitter and a rotation stage for the crystal. A recent example with BBOF should be illustrative. Large single crystals, 20 × 17 × 12 mm3, of BBOF were recently grown by the top-seeded solution growth method. For Maker Fringe experiments, cut and polished crystal “wafers” (5 mm × 5 mm × 2 mm) along specific crystallographic directions are needed. With BBOF, (100) and (010) wafers are needed (see Figure 8a). Also shown in Figure 8a are the specific dij coefficients that may be measured with these wafers. The magnitude of the SHG coefficients is determined relative to d36 of KDP. The Maker

Figure 7. Schematic of the Maker Fringe equipment. Reproduced with permission from ref 8. Copyright 2016 American Chemical Society.

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Figure 8. (a) Orientation of the crystal wafers for BBOF and KDP for Maker Fringe experiments and the (010) and (100) crystal wafers of BBOF. (b) Maker Fringe data for BBOF (d31, d32, and d33) and KDP (d36). Reproduced with Permission from ref 34. Copyright 2017 American Chemical Society.

Figure 9. Rocking curve (left) of BBOF using the crystal on the right. A fwhm of 0.017° is observed, indicating that the crystal is of high quality. Reproduced with Permission from ref 34. Copyright 2017 American Chemical Society.

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DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. (a) Measured refractive indices at different wavelengths, (b) refractive index dispersion curves (red) by fitting the Sellmeier equation, and (c) phase-matching range determined based on the Sellmeier equation.

In Figure 10a, the refractive indices for a uniaxial crystal are measured at several wavelengths, and a birefringence of 0.033 is determined at 1064 nm. In Figure 10b, the refractive index data are fit to the Sellmeier equation (solid and dashed red lines). Using this equation, we can calculate the refractive index at the appropriate wavelengths and determine the phase-matching range (Figure 10c). In Figure 10c, the refractive indices are calculated at second-harmonic wavelengths (solid and dashed green lines), i.e., wavelengths that are half with respect to the fundamental. These are the solid and dashed green lines. Where the dashed red line crosses the solid green line, i.e., where n(ω) = n(2ω), is the phase-matching wavelength range (black circle). Thus, from measuring the refractive indices at several wavelengths, one can ultimately determine the phase-matching wavelength range. New Materials for UV and deep-UV NLO Applications. With all of the above restrictions and requirements, what are the possible new materials for UV and deep-UV NLO applications? A few possibilities are given below. (a) AMgCO3F (A = K, Rb):40,41 These recently discovered materials have short absorption edges, 6.2 eV), and contain relatively benign elements. Both materials crystallize in the uniaxial, acentric, and nonpolar space group P6̅2m and exhibit three-dimensional structures consisting of corner-shared Mg(CO3)F2 polyhedra. The Mg2+ cations are bonded to the carbonate groups in the ab plane and along the c direction of a bridging fluoride. The “A” cation, K+ or Rb+, resides in the cavities formed by the Mg(CO3)F2 groups. (b) ABCO3F (A = K, Rb, Cs; B = Sr, Ca):42,43 With this family, KSrCO3F, RbSrCO3F, and KCaCO3F are isostructural and crystallize in the acentric space group P6̅m2. All of the CO32− groups are aligned in a parallel manner in the ab plane that results in a large PSHG intensity, i.e., >3.0 × KDP at 1064 nm. The other two materials in this family, RbCaCO3F and CsCaCO3F, are isostructural and crystallize an acentric space group of P6̅2m. The PSHG intensities for these materials are

Fringe data for BBOF are shown in the upper and lower left panels of Figure 8b, with the d36 Maker Fringe data shown in the lower right panel. Rocking curve measurements are necessary to assess the quality of the single crystal. The fwhm of a Bragg reflection should be less than 100 arcseconds (0.0278°); less than 50 arcseconds (0.0139°) is ideal. The rocking curve and crystal used for BBOF are shown in Figure 9. As seen, the fwhm of 0.017° (61.7 arcseconds) indicating the crystal is of high quality. A laser damage threshold measurement may also be performed at this time, but in practice it is best to leave this measurement until the crystal is no longer needed because the method is destructive. With this measurement, the laser power is increased until a mark appears on the crystal. As previously stated, the birefringence plays a critical role in determining the phase-matching wavelength range. The birefringence of the material is determined through refractive index measurements. Again, a cut, indexed, and polished crystal is required. The measurement may be done using the minimum deviation technique38 on a large crystal (>5 mm) that has been cut as a wedge or prism or through the prism coupling method on a smaller crystal (2−4 mm). Both methods produce the same result. Once the refractive indices have been measured, the data may be fit to the Sellmeier equation39 ni 2 = A +

B − Dλ 2 λ −C 2

where ni is the refractive index in question, i.e., no or ne for uniaxial or nx, ny, or nz for biaxial, λ is the wavelength in micrometers, and A, B, C, and D are the Sellmeier parameters. The refractive index must be measured at a minimum of five wavelengths to fit the Sellmeier equation. Once the Sellmeier equations are known, it is possible to plot the refractive index at the fundamental and second harmonic, n(ω) and n(2ω), as a function of λ. A uniaxial example is given in Figure 10a−c. F

DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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ammonium fluorooxoborate, NH4B4O6F, was synthesized and grown into large crystals very recently in Pan’s group.59 The experimental results indicate that NH4B4O6F has an absorption edge of 156 nm with a birefringence of 0.1171 at 1064 nm. On the basis of the fitted Sellmeier equation, the shortest SHG phase-matching wavelength is 158 nm.59 The experimental and calculated results strongly suggest that NH4B4O6F is an outstanding deep-UV NLO material. (g) RbBa2(P2O7)2 and RbBa2(PO3)5:60 These phosphates were reported by Zhao et al. in 2014. Both materials have short absorption edges, i.e., 167 nm (7.3 eV) for RbBa2(PO3)5 and at least 200 nm (6.2 eV) for RbBa2(P2O7)2. In addition, moderate PSHG responses at 1064 nm were reported. Large single crystals have yet to be grown; thus, birefringence, phasematching wavelengths, and SHG coefficients have not been reported. (h) AAlCO3F2 (A = alkali metal):61 A family of acentric alkali-metal aluminum carbonate fluorides has been proposed to be suitable for deep-UV NLO applications. Denoted as “hypothetically synthesized”, these materials are predicted to have very short absorption edges, i.e., 142−152 nm (8.73−8.16 eV). The authors also calculate a birefringence greater than 0.10 at 1064 nm that suggests that phase-matching in the UV is possible. It remains to be seen if any of these predicted materials can be synthesized.

slightly larger than that of KDP, attributable to some antiparallel orientation of the CO32− groups. Recently, we have grown large crystals of KSrCO3F through top-seeded solution growth methods.44 We measured a birefringence of 0.105 at 1064 nm, which results in a SHG limit of 200 nm. In addition, through Maker Fringe experiments, d22 = 0.50 pm/V was determined. All of the above suggest that KSrCO3F may be a viable material to replace CLBO or β-BBO. (c) BaMBO3F (M = Mg, Zn):45−47 These materials were reported by Li et al. in 2010. Both materials [BaMgBO3F (BaZnBO3F)] exhibit short absorption edges, 190 nm (223 nm), and moderate PSHG intensities, 0.60 (2.8) × KDP. The difference in the PSHG intensities may be attributable to the misalignment (alignment) of the BO3 groups. Although large crystals of both materials have been reported, the crystal quality was too poor to determine the refractive indices and, subsequently, the birefringence. Calculations indicate birefringence values of 0.058 (0.070), suggesting a possible SHG limit below 266 nm. (d) AZn2BO3X2 (A = K, Rb; X = Cl, Br):48,49 This family of materials was reported nearly simultaneously by Huang et al. and Yang et al. in 2016. The stoichiometry may be derived from KBBF by substituting “Zn2O3X2” for “Be2O3F2”. All four materials (KZn2BO3Cl2, RbZn2BO3Cl2, KZn2BO3Br2, and RbZn2BO3Br2) have absorption edges below 220 nm and exhibit PSHG intensities greater than 2.5 × KDP at 1064 nm. Additional PSHG measurements indicate that the materials are phase-matchable at 1064 and 532 nm. Because large single crystals have yet to be grown, the refractive indices, birefringences, and SHG limits have not been determined. (e) A3B3Li2Al4B6O20F (A = alkali metal; B = alkaline-earth metal): This seemingly complex stoichiometry may be understood if we start with KBBF. As stated earlier, KBBF suffers from two major drawbacks. First toxic BeO must be used in the synthesis, and second weak interlayer K+−F− interactions have prevented its large crystal growth. To address this layering issue, Sr2Be2B2O7 (SBBO) was discovered.50,51 With SBBO, [Be2B2O7]∞ double layers are observed that impart greater stability in comparison with KBBF. However, toxic BeO is still required in the synthesis. To achieve the A3B3Li2Al4B6O20F stoichiometry, we start with the SBBO formula and multiply it by 3, i.e., Sr2Be2B2O7 × 3 = “Sr6Be6B6O21”. Replacing “Sr6” with “A3B3”, “Be6” with “Li2Al4”, and “O21” with “O20F” results in A3B3Li2Al4B6O20F. We have synthesized and, importantly, grown large crystals, >5 mm in the optic axis direction, of both K3Sr3Li2Al4B6O20F52 and Rb3Ba3Li2Al4B6O20F.53 Both materials exhibit absorption edges below 200 nm and have a moderate birefringence. Both materials are potential candidates to replace CLBO and β-BBO to generate 266 nm radiation. (f) Fluorooxoborates: Other than borate halides, in which B3+ cations solely connect with O2− anions, fluorooxoborates contain (BO3F)4−, (BO2F2)3−, and/or (BOF3)2− groups. These groups may produce a suitable birefringence without layering and simultaneously exhibit a short absorption edge down into the deep-UV region.54 LiB6O9F, Li2B6O9F2, and Li2B3O4F3 were initially reported as ionic conductors by Jansen’s group.55−57 Recently, these materials were investigated as NLO materials.54,58 The calculated optical properties of Li2B6O9F are very attractive for deep-UV NLO applications, i.e., the birefringence is 0.07 at 1064 nm and the shortest phasematching wavelength is 192 nm.54 To date, large single crystals of Li2B6O9F have yet to be grown. Interestingly, a new

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OUTLOOK

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AUTHOR INFORMATION

New materials are urgently needed for NLO technologies in the UV and deep-UV regions. Specifically, materials capable of FOHG (1064 nm/4 = 266 nm) and 6thHG (1064/6 = 177.3 nm) are of the greatest need. The chemical and physical requirements for such materials are strict; however, with judicious selection of cations and anions, synthesizing such materials is possible. One of the greatest challenges after the material is synthesized is its large single-crystal growth. In the authors’ opinion, this represents the greatest bottleneck to advancing NLO technologies. Once large single crystals have been successfully grown, it is possible to determine the refractive indices and birefringences. After these data have been fit to the Sellmeier equation, the phase-matching wavelengthSHG limitmay be determined. All of this is predicated on the growth of large single crystals.

Corresponding Author

*E-mail: [email protected]. ORCID

P. Shiv Halasyamani: 0000-0003-1787-1040 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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(2) Franken, P. A.; Hill, A. E.; Peters, C. W.; Weinreich, G. Generation of Optical Harmonics. Phys. Rev. Lett. 1961, 7, 118−120. (3) Tran, T. T.; Yu, H.; Rondinelli, J. M.; Poeppelmeier, K. R.; Halasyamani, P. S. Deep Ultraviolet Nonlinear Optical Materials. Chem. Mater. 2016, 28, 5238−5258. (4) Nye, J. F. Physical Properties of Crystals; Oxford Science Publications: Oxford, U.K., 1957. (5) Roberts, D. A. Simplified characterization of uniaxial and biaxial nonlinear optical crystals: a plea for standardization of nomenclature and conventions. IEEE J. Quantum Electron. 1992, 28, 2057−2074. (6) Maker, P. D.; Terhune, R. W.; Nisenoff, M.; Savage, C. M. Effects of Dispersion and Focusing on the Production of Optical Harmonics. Phys. Rev. Lett. 1962, 8, 21−22. (7) Jerphagnon, J.; Kurtz, S. K. Maker Fringes: A Detailed Comparison of Theory and Experiment for Isotropic and Uniaxial Crystals. J. Appl. Phys. 1970, 41, 1667−1681. (8) Zhang, W.; Yu, H.; Cantwell, J.; Wu, H.; Poeppelmeier, K. R.; Halasyamani, P. S. LiNa5Mo9O30: Crystal Growth, Linear, and Nonlinear Optical Properties. Chem. Mater. 2016, 28, 4483−4491. (9) Zhang, W.; Yu, H.; Wu, H.; Halasyamani, P. S. Phase-Matching in Nonlinear Optical Compounds: A Materials Perspective. Chem. Mater. 2017, 29, 2655−2668. (10) Zhang, W.; Halasyamani, P. S. Top-seeded solution crystal growth of noncentrosymmetric and polar Zn2TeMoO7 (ZTM). J. Solid State Chem. 2016, 236, 32−38. (11) Moorthy, S.; Kumar, F.; Balakumar, S.; Subramanian, C.; Ramasamy, P. Top seeded solution growth of KTiOPO4 (KTP) single crystals and their characterisation. Mater. Sci. Eng., B 1999, 60, 88−94. (12) Yao, S.; Wang, J.; Liu, H.; Hu, X.; Zhang, H.; Cheng, X.; Ling, Z. Growth, optical and thermal properties of near-stoichiometric LiNbO3 single crystal. J. Alloys Compd. 2008, 455, 501−505. (13) Wang, S.; Gao, Z.; Zhang, X.; Zhang, X.; Li, C.; Dong, C.; Lu, Q.; Zhao, M.; Tao, X. Crystal Growth and Effects of Annealing on Optical and Electrical Properties of Mid-Infrared Single Crystal LiInS2. Cryst. Growth Des. 2014, 14, 5957−5961. (14) Anandha Babu, G.; Subramaniyan, R.; Karunagaran, N.; Perumal Ramasamy, R.; Ramasamy, P.; Ganesamoorthy, S.; Gupta, P. K. Growth improvement of AgGaSe2 single crystal using the vertical Bridgman technique with steady ampoule rotation and its characterization. J. Cryst. Growth 2012, 338, 42−46. (15) Pan, S.; Smit, J. P.; Lanier, C. H.; Marvel, M. R.; Marks, L. D.; Poeppelmeier, K. R. Optical Floating Zone Growth of β-BaB2O4 from a LiBa2B5O10-Based Solvent. Cryst. Growth Des. 2007, 7, 1561−1564. (16) Zhang, J.; Zheng, H.; Ren, Y.; Mitchell, J. F. High-Pressure Floating-Zone Growth of Perovskite Nickelate LaNiO3 Single Crystals. Cryst. Growth Des. 2017, 17, 2730−2735. (17) Yoshimura, M.; Kamimura, T.; Murase, K.; Mori, Y.; Yoshida, H.; Nakatsuka, M.; Sasaki, T. Bulk Laser Damage in CsLiB6O10 Crystal and Its Dependence on Crystal Structure. Jpn. J. Appl. Phys. 1999, 38, L129. (18) Sasaki, T.; Mori, Y.; Yoshimura, M. Progress in the growth of a CsLiB6O10 crystal and its application to ultraviolet light generation. Opt. Mater. 2003, 23, 343−351. (19) Mori, Y.; Kuroda, I.; Nakajima, S.; Sasaki, T.; Nakai, S. Nonlinear optical properties of cesium lithium borate. Jpn. J. Appl. Phys. 1995, 34, L296−L298. (20) Liu, Q.; Yan, X.; Gong, M.; Liu, H.; Zhang, G.; Ye, N. Highpower 266 nm ultraviolet generation in yttrium aluminum borate. Opt. Lett. 2011, 36, 2653−2655. (21) Krogh-Moe, J. The Crystal Structure of Lithium Diborate, Li2O· 2B2O3. Acta Crystallogr. 1962, 15, 190−193. (22) Kwon, T. Y.; Ju, J. J.; Cha, J. W.; Kim, J. N.; Yun, S. I. Characteristics of Critically Phase-Matched Second-Harmonic Generation of a Li2B4O7 Crystal Grown by the Czochralski Method. Mater. Lett. 1994, 20, 211−215. (23) Ryu, G.; Yoon, C. S.; Han, T. P. J.; Gallagher, H. G. Growth and characterisation of CsLiB6O10 (CLBO) crystals. J. Cryst. Growth 1998, 191, 492−500.

Weiguo Zhang earned his B.S. (2004) in Applied Chemistry from Shandong University. Weiguo continued his research at the State Key Laboratory of Crystal Materials as a Ph.D. student at Shandong University in Professor Tao’s group, earning his Ph.D. in 2009. He joined Professor Halasyamani’s group at the University of Houston to conduct single-crystal growth and characterization in July 2009. He has successfully grown many NLO crystals such as BaTeMo2O9 (BTM), Na2TeW2O9 (NTW), Zn2TeMoO7 (ZTM), LiFeP2O7, LiCrP2O7, K3V5O14, LiNa5Mo9O30, Ba2Zn(BO3)2, and KSrCO3F. He is currently a research associate in Professor Halasyamani’s laboratory and a member of the American Association for Crystal Growth.

P. Shiv Halasyamani earned his B.S. in Chemistry from the University of Chicago (1992) and his Ph.D. under the guidance of Prof. Kenneth R. Poeppelmeier at Northwestern University in 1996. From 1997 to 1999, he was a postdoctoral associate and Junior Research Fellow at Oxford University in the laboratory of Prof. Dermot O’Hare. He started as an Assistant Professor in the Department of Chemistry, University of Houston, in 1999. He is currently a full Professor of Chemistry at the University of Houston. Prof. Halasyamani’s research interests involve the design, synthesis, crystal growth, characterization, and structure−property relationships in new functional inorganic materials.

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ACKNOWLEDGMENTS P.S.H. and W.Z. thank the Welch Foundation (Grant E-1457) and the NSF (Grant DMR-1503573) for support.

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REFERENCES

(1) Chen, C.; Sasaki, T.; Li, R. K.; Wu, Y.; Lin, Z.; Mori, Y.; Hu, Z.; Wang, J.; Uda, S.; Yoshimura, M.; Kaneda, Y. Nonlinear Optical Borate Crystals; Wiley-VCH: Weinheim, Germany, 2012. H

DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(47) Xia, M.; Li, R. K. Growth, structure and optical properties of nonlinear optical crystal BaZnBO3F. J. Solid State Chem. 2016, 233, 58−61. (48) Huang, Q.; Liu, L.; Wang, X.; Li, R.; Chen, C. Beryllium-Free KBBF Family of Nonlinear-Optical Crystals: AZn2BO3X2 (A = Na, K, Rb; X = Cl, Br). Inorg. Chem. 2016, 55, 12496−12499. (49) Yang, G.; Gong, P.; Lin, Z.; Ye, N. AZn2BO3X2(A = K, Rb, NH4; X = Cl, Br): New Members of KBBF Family Exhibiting Large SHG Response and the Enhancement of Layer Interaction by Modified Structures. Chem. Mater. 2016, 28, 9122−9131. (50) Chen, C.; Wang, Y.; Wu, B.; Wu, K.; Zeng, W.; Yu, L. Design and Synthesis of an Ultraviolet-Transparent Nonlinear Optical Crystal Sr2Be2B2O7. Nature 1995, 373, 322−324. (51) Kolis, J. W.; McMillen, C. D.; Franco, T. Hydrothermal Synthesis of the Deep-UV NLO Material Sr2Be2B2O7. MRS Online Proc. Libr. 2004, 848, FF1.1. (52) Wu, H.; Yu, H.; Pan, S.; Halasyamani, P. S. A Deep-Ultraviolet NLO Material K3Sr3Li2Al4B6O20F: Addressing the structural instability problem in KBe2BO3F2. Inorg. Chem. 2017, 56, 8755−8758. (53) Yu, H.; Young, J.; Wu, H.; Zhang, W.; Rondinelli, J. M.; Halasyamani, P. S. The Next Generation of Nonlinear Optical Material: Rb3Ba3Li2Al4B6O20F - Synthesis, Characterization, and Crystal Growth. Adv. Opt. Mater. 2017, In Press. (54) Zhang, B.; Shi, G.; Yang, Z.; Zhang, F.; Pan, S. Fluorooxoborates: Beryllium-Free Deep-Ultraviolet Nonlinear Optical Materials without Layered Growth. Angew. Chem., Int. Ed. 2017, 56, 3916−3919. (55) Cakmak, G.; Nuss, J.; Jansen, M. LiB6O9F, the First Lithium Fluorooxoborate - Crystal Structure and Ionic Conductivity. Z. Anorg. Allg. Chem. 2009, 635, 631−636. (56) Pilz, T.; Jansen, M. Li2B6O9F2, a New Acentric Fluorooxoborate. Z. Anorg. Allg. Chem. 2011, 637, 2148−2152. (57) Pilz, T.; Nuss, H.; Jansen, M. Li2B3O4F3, a new lithium-rich fluorooxoborate. J. Solid State Chem. 2012, 186, 104−108. (58) Andriyevsky, B.; Pilz, T.; Yeon, J.; Halasyamani, P. S.; Doll, K.; Jansen, M. DFT-based ab initio study of dielectric and optical properties of bulk Li2B3O4F3 and Li2B6O9F2. J. Phys. Chem. Solids 2013, 74, 616−623. (59) Shi, G.; Wang, Y.; Zhang, F.; Zhang, B.; Yang, Z.; Hou, X.; Pan, S.; Poeppelmeier, K. R. Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F. J. Am. Chem. Soc. 2017, 139, 10645− 10648. (60) Zhao, S.; Gong, P.; Luo, S.; Bai, L.; Lin, Z.; Ji, C.; Chen, T.; Hong, M.; Luo, J. Deep-ultraviolet transparent phosphates RbBa2(PO3)5 and Rb2Ba3(P2O7)2 show nonlinear optical activity from condensation of [PO4]3‑ units. J. Am. Chem. Soc. 2014, 136, 8560−8563. (61) Kang, L.; Luo, S.; Huang, H.; Ye, N.; Lin, Z.; Qin, J.; Chen, C. Prospects for Fluoride Carbonate Nonlinear Optical Crystals in the UV and Deep-UV Regions. J. Phys. Chem. C 2013, 117, 25684−25692.

(24) Chen, C. T.; Wu, B. C.; Jiang, A. D.; You, G. M. A new type of ultraviolet SHG crystal: β-BaB2O4. Sci. Sin. B 1985, 28, 235. (25) Rytz, D.; Gross, A.; Vernay, S.; Wesemann, V. YAl3(BO3)4: A Novel NLO Crystal for Frequency Conversion to UV Wavelengths. Proc. SPIE 2008, 6998, 699814. (26) Yu, X.; Yue, Y.; Yao, J.; Hu, Z.-g. YAl3(BO3)4: Crystal growth and characterization. J. Cryst. Growth 2010, 312, 3029−3033. (27) Furusawa, S.; Chikagawa, O.; Tange, S.; Ishidate, T.; Orihara, H.; Ishibashi, Y.; Miwa, K., 2nd Harmonic Generation in Li2B4O7. J. Phys. Soc. Jpn. 1991, 60, 2691−2693. (28) Zhang, X.; Wang, L.; Wang, X.; Wang, G.; Zhu, Y.; Chen, C. High-power sixth-harmonic generation of an Nd:YAG laser with KBe2BO3F2 prism-coupled devices. Opt. Commun. 2012, 285, 4519− 4522. (29) Chen, C.; Luo, S.; Wang, X.; Wang, G.; Wen, X.; Wu, H.; Zhang, X.; Xu, Z. Deep UV nonlinear optical crystal RbBe2(BO3)F2. J. Opt. Soc. Am. B 2009, 26, 1519−1525. (30) Chen, C. T.; Wang, G. L.; Wang, X. Y.; Xu, Z. Y. Deep-UV nonlinear optical crystal KBe2BO3F2discovery, growth, optical properties and applications. Appl. Phys. B: Lasers Opt. 2009, 97, 9−25. (31) Li, Z.; Wu, Y.; Fu, P.; Pan, S.; Chen, C. Flux growth of BPO4 crystals. J. Cryst. Growth 2004, 270, 486−490. (32) Zhang, X.; Wang, L.; Zhang, S.; Wang, G.; Zhao, S.; Zhu, Y.; Wu, Y.; Chen, C. Optical properties of the vacuum-ultraviolet nonlinear optical crystal-BPO4. J. Opt. Soc. Am. B 2011, 28, 2236− 2239. (33) Wu, H.; Yu, H.; Yang, Z.; Hou, X.; Su, X.; Pan, S.; Poeppelmeier, K. R.; Rondinelli, J. M. Designing a Deep-Ultraviolet Nonlinear Optical Material with a Large Second Harmonic Generation Response. J. Am. Chem. Soc. 2013, 135, 4215−4218. (34) Wu, H.; Yu, H.; Zhang, W.; Cantwell, J.; Poeppelmeier, K. R.; Pan, S.; Halasyamani, P. S. Top-Seeded Solution Crystal Growth, Linear and Nonlinear Optical Properties of Ba4B11O20F (BBOF). Cryst. Growth Des. 2017, 17, 1404−1410. (35) Boyd, R. Nonlinear Optics, 3rd ed.; Academic Press: New York, 2008. (36) New, G. Introduction to Nonlinear Optics; Cambridge University Press: Cambridge, U.K., 2011. (37) Mori, Y.; Yap, Y. K.; Kamimura, T.; Yoshimura, M.; Sasaki, T. Recent development of nonlinear optical borate crystals for UV generation. Opt. Mater. 2002, 19, 1−5. (38) Leviton, D. B.; Madison, T. J.; Petrone Iii, P. Simple refractometers for index measurements by minimum-deviation method from far ultraviolet to near infrared. Proc. SPIE 1998, 3425, 148−159. (39) Born, M. W. E. Principles of Optics, 5th ed.; Pergamon Press: Oxford, U.K., 1975. (40) Tran, T. T.; He, J.; Rondinelli, J. M.; Halasyamani, P. S. RbMgCO3F - A New Deep-Ultraviolet Nonlinear Optical Material. J. Am. Chem. Soc. 2015, 137, 10504−10507. (41) Tran, T. T.; Young, J.; Rondinelli, J. M.; Halasyamani, P. S. Mixed-Metal Carbonates Fluorides as Deep-Ultraviolet Nonlinear Optical Materials. J. Am. Chem. Soc. 2017, 139, 1285−1295. (42) Zou, G.; Ye, N.; Huang, L.; Lin, X. Alkaline-Alkaline Earth Fluoride Carbonate Crystals ABCO3F (A = K, Rb, Cs; B = Ca, Sr, Ba) as Nonlinear Optical Materials. J. Am. Chem. Soc. 2011, 133, 20001− 20007. (43) Kang, L.; Luo, S.; Huang, H.; Ye, N.; Lin, Z.; Qin, J.; Chen, C. Prospects for Fluoride Carbonate Nonlinear Optical Crystals in the UV and Deep-UV Regions. J. Phys. Chem. C 2013, 117, 25684−25692. (44) Zhang, W.; Halasyamani, P. S. Crystal Growth and Optical Properties of a UV Nonlinear Optical Material KSrCO 3 F. CrystEngComm 2017, 19, 4742−4748. (45) Zhao, J.; Xia, M.; Li, R. K. Flux growth of a potential nonlinear optical crystal BaMgBO3F. J. Cryst. Growth 2011, 318, 971−973. (46) Li, R. K.; Chen, P. Cation Coordination Control of Anionic Group Alignment to Maximize SHG Effects in the BaMBO3F (M = Zn, Mg) Series. Inorg. Chem. 2010, 49, 1561−1565. I

DOI: 10.1021/acs.inorgchem.7b02184 Inorg. Chem. XXXX, XXX, XXX−XXX