Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical Materials: Expanding from Borates to Borate Fluorides to Fluorooxoborates Miriding Mutailipu,†,‡ Min Zhang,†,‡ Zhihua Yang,†,‡ and Shilie Pan*,†,‡

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 02/22/19. For personal use only.

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CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS, and Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

CONSPECTUS: Coherent light radiation down to the deepultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahighresolution photolithography to photochemical synthesis to highprecision microprocessing. Actually, it is hard to immediately obtain certain wavelengths, deep-ultraviolet coherent light in particular, from commercial laser sources. However, the direct second harmonic generation process governed in part by nonlinear optical crystals is a feasible and effective approach to generate deep-ultraviolet coherent light, which motivates chemists and materials scientists to find potential deep-ultraviolet nonlinear optical materials that can practically meet the scientific requirements. The research progress required to go from a new singlecrystal structure to final device applications involves many pivotal steps and is highly time-consuming and challenging, and therefore, it is necessary to commence systematic studies aimed at shortening the research cycle and accelerating the rational design of deep-ultraviolet nonlinear optical materials. In this Account, we choose borates as raw materials because they have ever-greater possibilities to form desired noncentrosymmetric structures, wide optical transparency windows, rich structural chemistry, and also large polarizabilities to guarantee the coexistence of large second-order nonlinear optical coefficients and suitable birefringence. Besides, the effects of fluorine atoms on the structural chemistry and optical properties of borates have been summarized and analyzed. On the basis of these favorable influences, three specific rational design strategies, including experimental and theoretical methods, have been proposed in order to shorten the investigational cycle of discovering the new expected compounds with high physicochemical performances required for practical applications. In this way, the progress of searching for candidates for the next generation of deep-ultraviolet nonlinear optical materials was accelerated from borates to borate fluorides to fluorooxoborates with three effective strategies: (1) expansion of the frontier from borates to borate fluorides with the introduction of fluorine to achieve enhanced optical performance; (2) computer-assisted design of new deep-ultraviolet nonlinear optical materials with a newly introduced systematic global structure optimization method; and (3) expansion of the frontier from borate fluorides to fluorooxoborates by proposed functionalized oxyfluoride [BOxF4−x](x+1)− (x = 1, 2, 3) chromophores to balance multiple criteria. The preliminary development of fluorooxoborates exhibiting high performance as a new fertile field to search for deepultraviolet nonlinear optical materials is highly encouraging and inspiring and can guide chemists and materials scientists with new directions and thoughts aimed at finding the next generation of practical deep-ultraviolet nonlinear optical materials.

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should be simultaneously satisfied:3,4,7,8 (1) crystallization into an effective acentric crystal class; (2) high transparency in the UV spectral range with short deep-UV cutoff edges (λcutoff < 200 nm); (3) large second-order NLO coefficients (dij > 0.39 pm/V); (4) a suitable birefringence (0.05−0.10 @ 1064 nm) to achieve the phase-matching conditions in the UV and deep-

INTRODUCTION

Nonlinear optical (NLO) materials are emerging as a significant branch of optoelectronic and photonic technologies, as they can produce coherent light radiation from the deepultraviolet (deep-UV, λ < 200 nm) to far-infrared spectral range.1−3 Therefore, the pursuit of novel NLO materials with balanced physicochemical properties has never stopped.4−6 In principle, the necessary requirements for a practical deep-UV NLO material are rather strict, and the following criteria © XXXX American Chemical Society

Received: December 19, 2018

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DOI: 10.1021/acs.accounts.8b00649 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

manufacture of optical devices for practical laser output. Taking the star material KBBF as an example, the crystal structure was determined in 197024 and recorrected in 1996.25 Single crystals of KBBF were grown up to 3.7 mm in thickness in 2011, and an average power of 200 mW at 177.3 nm was output by a KBBF-prism-coupled device in 2015.26,27 Obviously, it took almost 45 years from the discovery of the new compound to practical generation of a deep-UV laser. There is no doubt that achieving large-scale development will take more time and face additional challenges. Against this background, it is necessary to commence studies aimed at shortening the research cycle and accelerating the rational design of UV and deep-UV NLO materials. Therefore, considering that the performance of an NLO material is dictated by the arrangement of atoms at the microscopic level, several structure-oriented design strategies are being put forward to accelerate the first step, for example, using a chemical strategy to effectively obtain the targeted compounds and making structural modifications to enhance the interlayer force between the layers in the KBBF family and its derivatives.4,13 This Account describes how the progress of searching for candidates for next generation deep-UV NLO materials was accelerated following the strategy of fluorine introduction to go from borates to borate fluorides to fluorooxoborates. On the basis of the favorable influences of fluorine atoms on the structural chemistry and optical properties of borates, three specific rational design strategies, including experimental and theoretical methods, are proposed in order to shorten the investigation cycle of discovering new expected compounds with high physicochemical performance.

UV spectral ranges; (5) a large laser damage threshold with favorable chemical stability; (6) easy growth of large-sized single crystals. Targeting the above necessary prerequisites, research on the potential applications of phosphates,9,10 borates,11−15 and carbonates16−18 has been accelerated to meet the scientific requirements as practical UV or even deep-UV NLO materials. Among them, borates can be regarded as great suitable candidates for deep-UV harmonic generation, as they have ever-greater possibilities to form noncentrosymmetric structures, wide optical transparency windows activated by the large electronegativity difference between the B and O atoms, and also large polarizabilities to guarantee the coexistence of suitable second-order NLO coefficients and birefringence.11−15,19 Thus, a great number of borate-based NLO materials have been continuously developed as front-line candidates, including β-BaB 2 O 4 (β-BBO), 20 LiB 3 O 5 , 21 CsB 3 O 5 , 22 CsLiB 6 O 10 , 23 KBe 2 BO 3 F 2 (KBBF), 24 and Sr2Be2B2O7 (SBBO).1 Even so, KBBF is the sole material that can practically generate deep-UV coherent laser light by a direct second harmonic generation (SHG) method, and the applications are limited by the strong layering tendency during its single-crystal growth process. Besides, carcinogenic BeO is indispensable for the synthesis and crystal growth of KBBF, so high demands for experimental security are put forward. Therefore, the exploration for new beryllium-free deep-UV NLO crystals is still greatly desirable to satisfy the current scientific and industrial demands. The research progress required to go from new crystal structures to final device applications is highly time-consuming and challenging and involves five pivotal steps (Figure 1): (1)

1. EXPANDING THE FRONTIER FROM BORATES TO BORATE FLUORIDES WITH THE INTRODUCTION OF FLUORINE TO ACHIEVE ENHANCED OPTICAL PERFORMANCE Borates are state-of-the-art candidates for deep-UV NLO materials, and the introduction of fluorine into the crystallographic structure to generate borate fluorides (borates with fluorine atoms exclusively connected with metal cations) will bring greater additional benefits from many crucial aspects: (1) Introducing fluorines can increase the probability that a compound will crystallize in a noncentrosymmetric space group, as the corresponding proportions of noncentrosymmetric structures are in total 15%, 36%, and 45% for the branches of inorganic crystals, borates, and borate fluorides, respectively.11,19 (2) The fluorine atom, with the highest electronegativity, can reduce the negative effective charges at terminal oxygens, resulting in a widened transparency range and blue-shifted cutoff edges. This will make the targeted crystals require higher energy for electronic excitation when they are irradiated by the laser. (3) Fluorine-centered distorted polyhedra have strong influences on the local structure environments, resulting in high SHG efficiency. From the aforementioned aspects, borate fluorides stand out among other borates, and thus, more active research was activated, as described below.

Figure 1. Overview of the research progress from new crystal structures to final optical device applications.

discovery of new compounds that crystallize in noncentrosymmetric crystal classes, which requires innumerable exploratory experiments because of the uncertainty of crystal structure symmetry and the fact that in total only 15% of the reported inorganic crystal structures exhibit a noncentrosymmetric space group;19 (2) obtaining polycrystalline samples of targeted new compounds for primary optical performance characterizations to evaluate their potential as NLO materials, including powder SHG measurements, UV−vis−NIR diffusereflectance spectra, thermal behavior, etc.; (3) and (4) growth of first small-sized and then larger crack-free centimeter-sized single crystals of high quality to allow measurements of pivotal optical properties such as transmittance spectra, birefringence, NLO coefficients, laser damage thresholds, etc.; and (5)

1.1. Mono-Alkaline-Earth Metal Borate Fluorides: Ba4B11O20F and M3B6O11F2 (M = Sr, Ba, Pb)

Ba4B11O20F. Ba4B11O20F with a polar structure (Cmc21) was designed and synthesized.28 It features a three-dimensional (3D) B−O open framework (Figure 2a) with Ba−F−Ba infinite chains filled in the tunnels. The polyanionic structures B


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Figure 2. (a, b) Views of (a) the 3D framework and (b) the [B21O24]15− FBB and Ba−F−Ba chain, (c) photo of an as-grown single crystal, (d) ELF isosurface plot, (e) transmission spectrum, and (f) experimental and theoretical Maker fringes for d31 of Ba4B11O20F. Adapted from refs 28 and 29. Copyright 2013 and 2017, respectively, American Chemical Society.

of Ba4B11O20F are based on an unprecedented [B21O24]15− fundamental building block (FBB) (Figure 2b). These FBBs connect with neighboring ones to construct the whole B−O open framework structure. Better yet, centimeter-sized Ba4B11O20F single crystals with high optical homogeneity were grown under the optimal conditions for further evaluation of the optical performance (Figure 2c).29 As expected, Ba4B11O20F fully achieves the coexistence of a broad transmission range with a short deep-UV cutoff edge (190 nm) (Figure 2e) and large SHG coefficients (d31 = 1.57 pm/V, d32 = 0.27 pm/V, and d33 = 0.46 pm/V) (Figure 2f), and these two pivotal performance parameters can be comparable to those of the commercial UV NLO material β-BBO. Theoretical approaches revealed that the enhanced SHG intensities relative to those of β-BBO mainly originate from the fluorine-directed polar displacements of B and O atoms. Besides, the asymmetric localizations are clearly visualized by the electron localization function (ELF) diagrams (Figure 2d), which indicate that the discernible asymmetric localization of charge density about the

Ba sites (related to the F atoms) activated by acentric atomic distortions contributes to the high SHG intensity. Because the SHG coefficients of Ba4B11O20F are large enough and centimeter-sized Ba4B11O20F crystals can be grown, high laser power are expected to be output by the Ba4B11O20F-based NLO device. M3B6O11F2 (M = Sr, Ba, Pb). Three isostructural fabianitelike borate fluorides with different templating cations were obtained by using both high-temperature solution and hydrothermal methods.30,31 They feature 3D frameworks with M-based polyhedra filled in the tunnels (Figure 3a). The B(2,3) and B(1,4,5,6) atoms are three- and fourcoordinated with O atoms to give triangular [B(2,3)O3]3− and tetrahedral [B(1,4,5,6)O4]5− basic blocks, respectively, that further polymerize into the [B6O14]10− FBB (Figure 3b). The final 3D B−O polyanionic structure can be extended by corner-sharing connections. The F atoms serve as common vertexes for M atoms to form fluorine-centered FM3 units, which are further linked to construct 2D [F2M3]4+ infinite C


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Figure 3. (a, b) Views of (a) the 3D framework and (b) the [B6O14]10− FBB, (c) SHG intensities, and (d) partial density of states of the F atoms of the M3B6O11F2 series. Adapted from refs 30 and 32. Copyright 2012 and 2016, respectively, American Chemical Society.

Figure 4. (a, b) Views of the 3D framework, (c) refractive index dispersion curves, (d) transmission spectrum, (e) experimental and theoretical Maker fringes for d33, and (f) SHG density maps of Ba3Mg3(BO3)3F3 polymorphs. Adapted from ref 33.

D


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Figure 5. (a) View of the 3D framework, (b) photo of an as-grown single crystal, (c) refractive index dispersion curves, and (d, e) SHG intensities of K3Sr3Li2Al4B6O20F. Adapted from ref 34. Copyright 2017 American Chemical Society.

layers with the same orientation. Thus, the fluorine-containing layers have a favorable impact on the distribution of polymerization of the anionic structure in the three analogues. Thus, the M3B6O11F2 series possess large SHG efficiencies that are approximately 2.5, 3.0, and 4.0 times that of KDP (Figure 3c). For Sr- and Ba-based phases, they have a broad transmission range with short deep-UV cutoff edges lower than 190 nm, which can also achieve the coexistence of two pivotal performance parameters. Theoretical analyses revealed that the fluorine experiences an asymmetric electron distribution in M-centered polyhedra and that the 2p states of F atoms are located near the Fermi level, which decreases the energy of the valence band to produce a larger band gap, because of the larger electronegativity of the F atoms compared with the O atoms (Figure 3d).32 The large SHG densities of fluorine are observed in the SHG density maps of M3B6O11F2, further verifying its important roles for SHG enhancement.

In both polymorphs, the B atoms are of only one triangular threefold-coordinated [BO3]3− type, and they are almost in a coplanar arrangement. The Mg atoms are sixfold-coordinated to form distorted [MgO4F2]8− octahedra. The [BO3]3− basic blocks are located in and out of the [Mg3O9F6]18− triangular prisms to construct 2∞[Mg3O2F3(BO3)2] infinite layers, and the axial F−Mg−F bonds unite the neighboring single layers to form the multilayered 3D structure. Besides, centimeter-sized single crystals of Pna21-Ba3Mg3(BO3)3F3 were grown by the top-seeded solution growth method to evaluate the optical performance. The results reveal that it possess a wide transmission spectral range (184−3780 nm), a high laser damage threshold (6.2 GW/cm2), a large SHG response (about 1.8 times that of KDP), and favorable anisotropic thermal expansion and chemical stability, indicating the potential to produce UV light by the direct second and third harmonic generation method (Figure 4c−e). The interatomic interactions within the [BO3]3− triangles and fluorinecontaining distorted [MgO4F2]8− octahedra can be observed in the ELF diagrams. The π orbitals in the planar [BO3]3− triangles and 2p orbitals of the F atoms in the distorted MgO4F2 octahedra are synergistically responsible for the large SHG response, also further verifying the important role of fluorine atoms in the SHG enhancement (Figure 4f).

1.2. Mixed Alkaline-Earth Metal Borate Fluorides: Ba3Mg3(BO3)3F3 Polymorphs

Two new beryllium-free borate fluorides with the same chemical formula, Ba3Mg3(BO3)3F3, were prepared through the chemical cosubstitution design strategy (Figure 4a,b).33 The two polymorphs crystallize into the asymmetric space groups Pna21 and P6̅2m, respectively, and present similar 3D layered crystal configurations that are built up from 2 ∞[Mg3O2F3(BO3)2] layers and stacked by the Mg−F bonds. E


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Accounts of Chemical Research 1.3. Mixed Alkali and Alkaline-Earth Metal Borate Fluoride: K3Sr3Li2Al4B6O20F

materials, which needs a suitable descriptor to characterize the multiple criteria properties to realize rapid screening in a huge database; and (3) systematic global structure optimization in order to search for the global minimum on a potential energy surface for an experimentally undiscovered crystal structure. This method has been widely used in high-pressure-driven structure searching, but no crystal structure related to NLO materials has been predicted. Herein we first introduce the systematic global structure optimization method to search for deep-UV NLO materials in the quaternary beryllium borate system.

A mixed metal borate fluoride, K3Sr3Li2Al4B6O20F, was designed and prepared through the chemical cosubstitution design strategy by substituting the Be atoms of SBBO with Li and Al atoms (Figure 5a). 34,35 The framework of K3Sr3Li2Al4B6O20F is composed of 2∞[Li2Al4B6O20F] double layers with K-based polyhedra located within the double layers and Sr-based polyhedra residing in the interlayers. The B atoms are of only one threefold-coordinated [BO3]3− type and in a favorable nearly coplanar arrangement. The fourfoldcoordinated [AlO4]5− and [LiO3F]6− tetrahedra are connected by isolated [BO3]3− triangles to construct 2∞[LiAl2B3O6] single layers. The axial Al−O/F bonds act as bridges to create the 2 ∞[Li2Al4B6O20F] double layers, which are then stacked up to form the final 3D structure. The SHG signal of K3Sr3Li2Al4B6O20F was found to be about 1.7 and 0.3 times as large as those of the KDP and β-BBO references under irradiation at λω = 1064 nm and λω = 532 nm, respectively (Figure 5d,e). Blocklike single crystals of K3Sr3Li2Al4B6O20F with dimensions of up to 8 mm × 8 mm × 5 mm were grown by the top-seeded solution growth method (Figure 5b). K3Sr3Li2Al4B6O20F possesses a short deep-UV cutoff edge (190 nm) and suitable birefringence (0.0574 @ 1064 nm), and the shortest type-I phase-matching wavelength for harmonic radiation was calculated to be 224 nm, indicating the potential to generate 266 nm light by direct fourth harmonic generation (FHG) of a Nd:YAG laser (λω = 1064 nm) (Figure 5c). Although all of the above compounds can fully achieve the coexistence of a broad transmission range with deep-UV cutoff edges (0.39 pm/V), their birefringences are not large enough to make the shortest phase-matching wavelengths shift to the deep-UV spectral range. Therefore, more research should be focused on enhancement of the birefringence while simultaneously avoiding negative influences on the SHG coefficients and transmission range.

Be2BO3F

Be2BO3F is a successful case that was predicted by the computer-aided swarm structure searching technique. Several new undiscovered (P6̅2c, P6322, Ama2, and R3c) and known (R3̅c) phases were screened out.36 Among them, the P6̅2c (γBe2BO3F) phase was highlighted as a candidate of deep-UV NLO material. The B and Be atoms present threefoldcoordinated [BO3]3− and fourfold-coordinated [BeO3F]5− models, respectively, and these two basics blocks link alternately to generate infinite 2∞[BeBO3] single layers (Figure 2 6a). These ∞ [BeBO3] single layers are stacked in an −AABBAA− sequence and further interlinked together through the Be−F bonds. Compared with KBBF, γ-Be2BO3F has superior NLO properties in several pivotal aspects from a theoretical point of view: (1) γ-Be2BO3F exhibits a large NLO coefficient of 0.70 pm/V, which is about 1.5 times larger than

2. COMPUTER-ASSISTED DESIGN OF NEW DEEP-UV NONLINEAR OPTICAL MATERIALS WITH A NEWLY INTRODUCED SYSTEMATIC GLOBAL STRUCTURE OPTIMIZATION METHOD Actually, the discovery of the aforementioned borate fluorides and even other reported NLO materials followed the traditional inductive paradigm, in which experimental synthesis and characterizations provide information about the existing compounds with assigned molecular formula and then computational methods are employed to understand the structure−property relationships. Although this is a perfect combination between synthetic chemistry and computational chemistry, the natural mode aiming to shorten the investigation cycle of discovering new expected compounds exhibiting high performance should be recognized. Accordingly, several theoretical approaches to search for and predict novel candidates for NLO applications have been proposed and applied: (1) structural regulation based on the known crystal structure by substituting one or more atoms, chromophores, vacancies, or mixed anions to predict a new functionalized crystal structure and then giving theoretical performance characterizations; (2) high-throughput computational NLO materials design through searching and analyzing enormous known crystal structures to screen out potential

Figure 6. (a) View of the 3D framework and (b) the phase-matching capabilities of γ-Be2BO3F. Adapted from ref 36. Copyright 2018 American Chemical Society. F


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three critical parameters (birefringence, NLO coefficients, and UV cutoff edge), reflected by the polarizability anisotropy, hyperpolarizability, and band gap, respectively. In order to further confirm this supposition, the band gap, birefringence, and SHG tensors of three noncentrosymmetric lithium fluorooxoborates were investigated.38−40 Among them, Li2B6O9F2 was highlighted, as it exhibits superior performance compared with the other two lithium fluorooxoborates and the shortest type-I phase-matching wavelength (192 nm) is down to the deep-UV region. Thus, we synthesized Li2B6O9F2 polycrystalline samples for experimental measurements, which showed that Li2B6O9F2 exhibits a short deep-UV cutoff edge (

Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical

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