The primary goal of this review is to present a clear chemical perspective of borates in order to stimulate and facilitate the discovery of new borate-based optical materials. These materials, whic...

Borates: A Rich Source for Optical Materials.

ConspectusCoherent light radiation down to the deep-ultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahigh-resolution photolithography to photochemical synthesis to high-precision 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 single-crystal 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 sho...

Targeting the Next Generation of Deep-Ultraviolet Nonlinear Optical Materials: Expanding from Borates to Borate Fluorides to Fluorooxoborates

A novel noncentrosymmetric (NCS) polar fluoride sulfate, CsSbF2 SO4 , was obtained by ionothermal synthesis. A meticulously designed co-substitution approach was used to successfully replace the [TiO6 ]8- and [PO4 ]3- functional groups in KTiOPO4 (KTP) with [SbO4 F2 ]7- and [SO4 ]2- units, respectively. The structure of CsSbF2 SO4 features a pseudo-3D framework consisting of interconnected 1D [SbF2 O2 SO4 ]5- chains of corner-sharing [SbO4 F2 ]7- octahedra and [SO4 ]2- tetrahedra. The title compound exhibits a sharply enlarged band gap compared to its parent compound, KTP, benefitting from the introduction of F- ions and the displacement of Sb3+ cations. Second harmonic generation (SHG) measurements manifested that CsSbF2 SO4 is phase-matchable and revealed a strong SHG response of about 3.0 KH2 PO4 (KDP), which is the highest value reported for any metal sulfate reported to date. The reported fluoride sulfate is a promising near ultraviolet (UV) nonlinear optical (NLO) material.

CsSbF 2 SO 4 : An Excellent Ultraviolet Nonlinear Optical Sulfate with a KTiOPO 4 (KTP)-type Structure.

Advances in the generation of coherent lasers at wavelengths below 200 nm as well as the associated equipment and technologies have greatly spurred the development of nonlinear optical (NLO) materials that are capable of phase-matching in the deep-ultraviolet (deep-UV) spectral regions. However, few crystals can meet the technical demands, and studies to date have focused mainly on finding the next competitive candidates. Consequently, several crystals have emerged that hold great potential for the production of deep-UV harmonic coherent light, which benefit from newly developed systems with novel NLO-active units and chromophores. This Review provides a comprehensive overview of the developments of emergent candidates in the field of deep-UV NLO materials, discussing the recent efforts and active advances in the search for superior systems and in understanding their structure-property relationships. The intention is to provide a clear perspective on the current emergent deep-UV NLO candidates, and to accelerate the discovery of next candidates with high performances to meet the urgent demands.

Emergent Deep-Ultraviolet Nonlinear Optical Candidates.

To combine noncentrosymmetry, large second order nonlinear optical (NLO) susceptibility, deep UV transparency, and phase matchability simultaneously into a single structure is a great challenge. Especially, the relationship between the macrophase matchability and microstructure has been less investigated. Herein, we report a unique Ba2NaClP2O7 representing the first phase-matchable example of the fresnoite family, which exhibits excellent NLO properties, including short absorption edge (<176 nm, ca. 7.0 eV), strong second-harmonic generations at the incident lasers of 850–1110 nm (obv. 0.9–1.3 × KDP; cal. dave, 1.11 vs 0.96 pm/V of KDP), and large laser-induced damage threshold (1.4 × KDP). More interestingly, Ba2NaClP2O7 illustrates an unprecedented mechanism of phase matchability that is promoted by crystallographic disorder. The uniaxial symmetry breaking caused by the disorder of [P2O7]4– group results in a structural anisotropy that changes the band dispersions along k-paths of G–Y and G–X, resulting...

Ba2NaClP2O7: Unprecedented Phase Matchability Induced by Symmetry Breaking and Its Unique Fresnoite-Type Structure.

The search of new borates with improved functional properties has attracted considerable attention. Herein, a new polar fluorooxoborate, NaB4 O6 F (NBF) was prepared by high-temperature solid-state reaction. NBF belongs to the AB4 O6 F family (A=alkali metal or ammonium), a series of compounds that undergoes significant cation-dependent structural changes. NBF is of particular interest owing to the special cation position. Temperature-dependent ionic conductivity measurements show that NBF is a solid ionic conductor, and it has the lowest active energy of 32.5 kJ mol-1 of fluorooxoborates. NBF also shows a second-harmonic generation (SHG) response of 0.9×KH2 PO4 and 0.2×β-BaB2 O4 , at 1064 and 532 nm, respectively, and it has a short UV cutoff edge below 180 nm. Based on bond valence (BV) concepts, symmetry analysis, and the first principles calculation, the unique [B4 O6 F]∞ layer can be regarded as the "multifunctional unit", which is responsible for the observed properties of NBF.

Polar Fluorooxoborate, NaB4O6F: A Promising Material for Ionic Conduction and Nonlinear Optics

Noncentrosymmetric (NCS) fluorooxoborates are one of the most attractive systems for discovering the new deep-ultraviolet (DUV) nonlinear optical (NLO) materials. Here, we report the formation, crystal structure, and optical properties of a new NCS alkaline earth fluorooxoborate: CaB5O7F3. It exhibits excellent NLO performances including large band gap (8.75 eV), large second harmonic generation intensity (2 × KH2PO4 and 0.4 × β-BaB2O4, under 1064 and 532 nm radiation, respectively), and moderate birefringence (0.07 @ 1064 nm) that enables frequency doubling down to 183 nm. It demonstrates that CaB5O7F3 is a promising DUV NLO material.

CaB5O7F3: A Beryllium-Free Alkaline-Earth Fluorooxoborate Exhibiting Excellent Nonlinear Optical Performances

Two new alkaline earth metal fluorooxoborates, CaB4 O6 F2 and SrB4 O6 F2 , have been synthesized and characterized. The title compounds are isostructural and crystallize in centrosymmetric space group P1‾ (No. 2). Structurally, the fundamental building blocks, B4 O8 F2 groups, are connected by B-O bonds to form [B4 O6 F2 ]∞ layers, which stack parallelly along the c axis. In addition, BaB4 O6 F2 , a known fluorooxoborate with different crystal structure, was also synthesized and characterized for comparison. The cation size influence on the structure of this MB4 O6 F2 (M=Ca, Sr, Ba) family is discussed in detail. The diffuse-reflectance spectroscopy measurements indicate that the cutoff edges of title compounds are below 190 nm. Moreover, first-principle calculation results demonstrate that MB4 O6 F2 (M=Ca, Sr, Ba) have large birefringence of 0.091, 0.089, and 0.085 at 1064 nm. Both experimental and theoretical studies suggest that MB4 O6 F2 (M=Ca, Sr, Ba) are promising birefringent materials for deep-UV applications.

Designing Deep-UV Birefringent Crystals by Cation Regulation.

A new fluorooxoborate, BaB8O12F2, was obtained in this study. It inherits the structural merits of α-BBO (large polarizability of the B3O6 groups) and the nearly parallel arrangement of [B4O6F]∞ layers in CBF. First-principles calculations show that BaB8O12F2 exhibits a large bandgap (6.85 eV) and possesses a large birefringence of 0.116 at 1064 nm.

BaB8O12F2: a promising deep-UV birefringent material

Exploration of new infrared (IR) nonlinear optical (NLO) materials is still in urgency owing to the indispensable roles in optoelectronic devices, resource exploration, and long-distance laser communication. The formidable challenge is to balance the contradiction between wide band gaps and large second harmonic generation (SHG) effects in IR NLO materials. In the present work, we proposed new kinds of NLO active units, d0 transition metal fluorooxo-functional groups for designing mid-IR NLO materials. By studying a series of d0 transition metal oxyfluorides (TMOFs), the influences of fluorooxo-functional groups with different d0 configuration cations on the band gap and SHG responses were explored. The results reveal that the fluorooxo-functional groups with different d0 configuration cations can enlarge band gaps in mid-IR NLO materials. The first-principles calculations demonstrate that the nine alkali/alkaline earth metals d0 TMOFs exhibit wide band gaps (all the band gaps > 3.0 eV), large birefringence Δn (> 0.07), and two W/Mo TMOFs also exhibit large SHG responses. Moreover, by comparing with other fluorooxo-functional groups, it is found that introducing fluorine into building units is an effective way to enhance optical performance. These d0 TMOFs with superior fluorooxo-functional groups represent a new exploration family of the mid-IR region, which sheds light on the design of mid-IR NLO materials possessing large band gap.

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Zhizhong Zhang

Papers

Polar Fluorooxoborate, NaB4O6F: A Promising Material for Ionic Conduction and Nonlinear Optics

CaB5O7F3: A Beryllium-Free Alkaline-Earth Fluorooxoborate Exhibiting Excellent Nonlinear Optical Performances

Designing Deep-UV Birefringent Crystals by Cation Regulation.

BaB8O12F2: a promising deep-UV birefringent material

Designing excellent mid-infrared nonlinear optical materials with fluorooxo-functional group of d0 transition metal oxyfluorides