Strong Strong Second Harmonic Generation Research Articles

A3Sc(SO4)3 (A = K, Rb, Cs): Three Deep‐Ultraviolet Transparent Alkali‐Scandium Sulfates with Strong Second‐Harmonic Generation Response and High Thermal Stability

AbstractNonlinear optical (NLO) crystals that have the capability of achieving tunable lasers through frequency conversion are of great importance in solid‐state lasers. Herein, three new deep‐UV transparent alkali‐scandium sulfates with strong second‐harmonic generation (SHG) response and high thermal stability, A3Sc(SO4)3 (A = K, Rb, Cs), are successfully obtained. It is interesting that the slightly distorted [ScO6] octahedrons are in an ordered/oriented arrangement, which is in favor of the generation of SHG response. Especially, K3Sc(SO4)3 not only exhibits a strong phase‐matching SHG response of 1.6 × KDP, a short deep‐ultraviolet (deep‐UV) cutoff edge of 190 nm, a wide transparent window covering from the deep‐UV region to mid IR, a high laser‐induced damage threshold of 165 MW cm−2 and easy growth of large single crystal, but also behaves thermal stable up to 900 °C under an air atmosphere. Crystal structures and theoretical calculations reveal that their strong SHG responses mainly stem from the slightly distorted [ScO6] octahedron. This study highlights A3Sc(SO4)3 (A = K, Rb, Cs) as promising deep‐UV transparent NLO crystals and offers a new approach for the pursuit of high‐performance NLO crystals.

Sr2HgGe2OS6: A Hg-Based Oxychalcogenide Infrared Nonlinear Optical Material Exhibiting Favorable Balance between a Large Band Gap and Strong Second Harmonic Generation Response.

Currently, oxychalcogenides with mixed-anion groups that integrate the property advantages of oxides (wide optical band gap) and chalcogenides [strong second harmonic generation (SHG) response] through chemical substitution engineering have attracted widespread interest and are considered to be important candidates for infrared (IR) nonlinear optical (NLO) materials. Herein, the first Hg-based oxychalcogenide Sr2HgGe2OS6 with mixed anion [GeOS3] units has been successfully synthesized through a spontaneous crystallization method, which exhibits a favorable balance between the strong SHG response (0.7 × AgGaS2) and large optical band gap (2.9 eV). In addition, Sr2HgGe2OS6 shows high laser-induced damage threshold (LIDT, 2.1 × AgGaS2) as well as phase-matching (PM) performance. Theoretical calculations indicate that the Sr2HgGe2OS6 encompasses large birefringence of 0.128@2090 nm (3.3 × AgGaS2) and its SHG density mainly comes from [HgS4] tetrahedra and [GeOS3] units. This work not only demonstrates that Sr2HgGe2OS6 is a promising IR NLO material but also provides new ideas for the exploration of Hg-based oxychalcogenide IR NLO materials.

A bifunctional primitive strategy induces enhancements of large second harmonic generation and wide UV transmittance in rare-earth borates containing [B5O10] groups.

Strong second-harmonic generation (SHG) and a short ultraviolet (UV) cutoff edge are two crucial yet often conflicting parameters that must be finely tuned in the exploration of nonlinear optical (NLO) materials. In this study, two new rare earth borate NLO crystals, K7BaSc2B15O30 (KBSBO) and Rb21Sr3.8Sc5.2B45O90 (RSSBO), were rationally designed through a bifunctional primitive strategy to achieve an optimized balance between favorable SHG efficiency and UV transparency. As anticipated, both KBSBO and RSSBO exhibit a wide UV transparency window below 190 nm. Notably, these tailored crystals display strong SHG responses, with RSSBO achieving a remarkable enhancement in SHG efficiency (2 × KDP), surpassing that of most deep-UV rare earth borates containing [B5O10] groups known to date. Theoretical calculations and structural analyses reveal that the impressive SHG activities primarily stem from the [B5O10] groups and [ScO6] polyhedra. These findings suggest promising potential for KBSBO and RSSBO crystals as beryllium-free deep UV NLO materials.

Mg(C3 O4 H2 )(H2 O)2 : A New Ultraviolet Nonlinear Optical Material Derived from KBe2 BO3 F2 with High Performance and Excellent Water-Resistance.

Acquiring high-performance ultraviolet (UV) nonlinear optical (NLO) materials that simultaneously exhibit a strong second harmonic generation (SHG) coefficients, as short as possible SHG phase-matching (PM) wavelength and non-hygroscopic properties has consistently posed a significant challenge. Herein, through multicomponent modification of KBe2 BO3 F2 (KBBF), an excellent UV NLO crystal, Mg(C3 O4 H2 )(H2 O)2 , was successfully synthesized in malonic system. This material possesses a unique 2D NLO-favorable electroneutral [Mg(C3 O4 H2 )3 (H2 O)2 ]∞ layer, resulting in the rare coexistence of a strong SHG response of 3×KDP (@1064 nm) and short PM wavelength of 200 nm. More importantly, it exhibits exceptional water resistance, which is rare among ionic organic NLO crystals. Theoretical calculations revealed that its excellent water-resistant may be originated from its small available cavity volumes, which is similar to the famous LiB3 O5 (LBO). Therefore, excellent NLO properties and stability against air and moisture indicate it should be a promising UV NLO crystal.

Mg(C3O4H2)(H2O)2: A New Ultraviolet Nonlinear Optical Material Derived from KBe2BO3F2 with High Performance and Excellent Water‐Resistance

AbstractAcquiring high‐performance ultraviolet (UV) nonlinear optical (NLO) materials that simultaneously exhibit a strong second harmonic generation (SHG) coefficients, as short as possible SHG phase‐matching (PM) wavelength and non‐hygroscopic properties has consistently posed a significant challenge. Herein, through multicomponent modification of KBe2BO3F2 (KBBF), an excellent UV NLO crystal, Mg(C3O4H2)(H2O)2, was successfully synthesized in malonic system. This material possesses a unique 2D NLO‐favorable electroneutral [Mg(C3O4H2)3(H2O)2]∞ layer, resulting in the rare coexistence of a strong SHG response of 3×KDP (@1064 nm) and short PM wavelength of 200 nm. More importantly, it exhibits exceptional water resistance, which is rare among ionic organic NLO crystals. Theoretical calculations revealed that its excellent water‐resistant may be originated from its small available cavity volumes, which is similar to the famous LiB3O5 (LBO). Therefore, excellent NLO properties and stability against air and moisture indicate it should be a promising UV NLO crystal.

Ultrashort Phase-Matching Wavelength and Strong Second-Harmonic Generation in Deep-UV-Transparent Oxyfluorides by Covalency Reduction.

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a "de-covalency" band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO2 F4 ] and [TaF7 ]), affording the first tantalum-based deep-UV-transparent NLO materials, A5 Ta3 OF18 (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d0 -TM-centered oxides, in addition to strong SHG responses (2.8 × KH2 PO4 (KTOF) and 2.6 × KH2 PO4 (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d0 -TMs, but also push the shortest PM wavelength into the short-wavelength UV region.

Ultrashort Phase‐Matching Wavelength and Strong Second‐Harmonic Generation in Deep‐UV‐Transparent Oxyfluorides by Covalency Reduction

AbstractThe development of urgently‐needed ultraviolet (UV)/deep‐UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second‐harmonic generation (SHG) response as well as a short phase‐matching (PM) wavelength. We herein employ a “de‐covalency” band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition‐metal (TM) polyhedra ([TaO2F4] and [TaF7]), affording the first tantalum‐based deep‐UV‐transparent NLO materials, A5Ta3OF18 (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d0‐TM‐centered oxides, in addition to strong SHG responses (2.8 × KH2PO4 (KTOF) and 2.6 × KH2PO4 (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep‐UV NLO materials by exploiting the currently neglected d0‐TMs, but also push the shortest PM wavelength into the short‐wavelength UV region.

Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 : Promising Multifunctional Nonlinear Optical Materials Created by Partial Fluorination Strategy under Corrosion Resistant Supercritical Reactions.

It has historically been exceedingly challenging to create physically and chemically stable lanthanide compounds with strong second harmonic generation (SHG) due to their strong preference to central symmetry. In this work, five new non-centrosymmetric lanthanide selenites, namely, Ln2 F2 (OH2 )(MoO3 )2 (SeO3 )2 (Ln = Sm, Eu, Gd, Tb and Dy), are achieved by partial fluorination of the lanthanide oxygen polyhedron. An HF corrosion resistant supercritical hydrothermal method is developed, which is a facile and universal method for HF corrosion and high-temperature high-pressure environment. The title compounds displayed a novel 3D framework composed of 1D molybdenum selenite chains bridged by Ln2 F2 O12 (OH2 ) dimers. Their powder SHG responses showed a large difference, ranging from 1.0 to 9.0 × KH2 PO4 (KDP) at 1064nm. The half-filled Gd compound exhibited very strong SHG efficiency of up to 1.2 × KTP (KTiOPO4 ) at 2050nm. Compounds Tb and Gd are the first lanthanide selenites with SHG intensity reaching KTP level, which is very rare in this system. Furthermore, these compounds can also possess excellent physicochemical stability and strong luminescence emission, indicating that they are promising multifunctional nonlinear optical materials. This work offered an effective way for design and synthesis of multifunctional and high-performant nonlinear optical materials.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3 )2 and Y3 F(SeO3 )4.

It is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y3 F(SeO3 )4 , has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO3 )2 . The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO3 )2 has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non-centrosymmetric Y3 F(SeO3 )4 can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut-off edge (204 nm) and high thermal stability (690 °C). So, Y3 F(SeO3 )4 is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

The First UV Nonlinear Optical Selenite Material: Fluorination Control in CaYF(SeO3)2 and Y3F(SeO3)4

AbstractIt is a great challenge to develop UV nonlinear optical (NLO) material due to the demanding conditions of strong second harmonic generation (SHG) intensity and wide band gap. The first ultraviolet NLO selenite material, Y3F(SeO3)4, has been obtained by control of the fluorine content in a centrosymmetric CaYF(SeO3)2. The two new compounds represent similar 3D structures composed of 3D yttrium open frameworks strengthened by selenite groups. CaYF(SeO3)2 has a large birefringence (0.138@532 nm and 0.127@1064 nm) and a wide optical band gap (5.06 eV). The non‐centrosymmetric Y3F(SeO3)4 can exhibit strong SHG intensity (5.5×KDP@1064 nm), wide band gap (5.03 eV), short UV cut‐off edge (204 nm) and high thermal stability (690 °C). So, Y3F(SeO3)4 is a new UV NLO material with excellent comprehensive properties. Our work shows that it is an effective method to develop new UV NLO selenite material by fluorination control of the centrosymmetric compounds.

Noncentrosymmetric Lanthanide-Based MOF Materials Exhibiting Strong SHG Activity and NIR Luminescence of Er3+: Application in Nonlinear Optical Thermometry

Optically active luminescent materials based on lanthanide ions attract significant attention due to their unique spectroscopic properties, nonlinear optical activity, and the possibility of application as contactless sensors. Lanthanide metal-organic frameworks (Ln-MOFs) that exhibit strong second-harmonic generation (SHG) and are optically active in the NIR region are unexpectedly underrepresented. Moreover, such Ln-MOFs require ligands that are chiral and/or need multistep synthetic procedures. Here, we show that the NIR pulsed laser irradiation of the noncentrosymmetric, isostructural Ln-MOF materials (MOF-Er3+ (1) and codoped MOF-Yb3+/Er3+ (2)) that are constructed from simple, achiral organic substrates in a one-step procedure results in strong and tunable SHG activity. The SHG signals could be easily collected, exciting the materials in a broad NIR spectral range, from ≈800 to 1500 nm, resulting in the intense color of emission, observed in the entire visible spectral region. Moreover, upon excitation in the range of ≈900 to 1025 nm, the materials also exhibit the NIR luminescence of Er3+ ions, centered at ≈1550 nm. The use of a 975 nm pulse excitation allows simultaneous observations of the conventional NIR emission of Er3+ and the SHG signal, altogether tuned by the composition of the Ln-MOF materials. Taking the benefits of different thermal responses of the mentioned effects, we have developed a nonlinear optical thermometer based on lanthanide-MOF materials. In this system, the SHG signal decreases with temperature, whereas the NIR emission band of Er3+ slightly broadens, allowing ratiometric (Er3+ NIR 1550 nm/SHG 488 nm) temperature monitoring. Our study provides a groundwork for the rational design of readily available and self-monitoring NLO-active Ln-MOFs with the desired optical and electronic properties.

Deep-ultraviolet nonlinear-optical crystal BaNa2[PO3(OH)]2 with large birefringence and strong second-harmonic-generation response

Deep-ultraviolet (DUV, λ ​< ​200 ​nm) nonlinear-optical (NLO) crystals play a pivotal role in modern laser science and technology. Metal phosphates have been considered as fascinating candidates for DUV NLO crystal material. However, how to generate both strong second-harmonic-generation (SHG) response and large birefringence for promising DUV NLO metal phosphates is a long-standing issue. Herein, a new DUV NLO metal phosphate, BaNa2[PO3(OH)]2, was successfully synthesized, which features a two-dimensional [NaPO3(OH)]∞ alveolate layer alternately connected by highly asymmetric [PO3(OH)] groups and NaO5 triangular bipyramids. BaNa2[PO3(OH)]2 can realize a good balance of DUV nonlinear properties covering short UV absorption edge less than 190 ​nm, strong SHG response with the magnitude of ∼1.2 ​× ​benchmark KH2PO4 (KDP), and the largest birefringence with the experimental value of 0.064@590 ​± ​3 ​nm in reported alkaline/alkaline-earth metal phosphates, producing the shortest phase-matching output down to the wavelength of 164 ​nm. These research results show that BaNa2[PO3(OH)]2 crystal is a potential DUV NLO crystal material.

Perfectly Encoding π‐Conjugated Anions in the RE5(C3N3O3)(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

AbstractNonlinear optical (NLO) crystal, which simultaneously exhibits strong second‐harmonic‐generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE5(C3N3O3)(OH)12 (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a “three birds with one stone” strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5–4.2× KH2PO4), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π‐conjugated (C3N3O3)3− anions. These findings provide high‐performance short‐wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

Perfectly Encoding π-Conjugated Anions in the RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence.

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH2 PO4 ), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C3 N3 O3 )3- anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

Design of a new ultraviolet nonlinear optical material KNO3SO3NH3 exhibiting an unexpected strong second harmonic generation response

Based on the co-crystal engineering strategy, a new polar non-centrosymmetric compound, KNO3SO3NH3, was meticulously designed by introducing KNO3 into the centrosymmetric SO3NH3 crystal. It has an unexpected strong second-harmonic generation (SHG) response (10 × KDP), which is a new record among the inorganic nonlinear optical (NLO) materials without NLO-active cations. Theoretical calculations showed that the enhanced SHG response should be attributed to the effect of hydrogen bond. Meanwhile, it has the shortest absorption edge of 216 nm among the known materials with strong SHG responses (>8 × KDP). Besides, it possesses a large birefringence of 0.096@546 nm. Our results showed that KNO3SO3NH3 should be an excellent UV NLO crystal.

Heteroanionic Melilite Oxysulfide: A Promising Infrared Nonlinear Optical Candidate with a Strong Second-Harmonic Generation Response, Sufficient Birefringence, and Wide Bandgap.

The achievement of balanced performance with a strong second-harmonic generation (SHG) response, proper birefringence, and wide band gap concurrently is a crucial but challenging task in infrared nonlinear optical (IR-NLO) crystals. Here, we theoretically confirmed that the heteroanionic oxysulfide tetrahedron would produce improved polarizability anisotropy and quadratic hyperpolarizability compared with the monoanionic oxide or sulfide tetrahedra. When this anion-mixing strategy was applied, melilite oxysulfide with the four representative members A2GeGa2OS6 (A = Ca, Sr) and Sr2MGe2OS6 (M = Zn, Cd) was successfully discovered as a promising IR-NLO material system. Remarkably, compared with the monoanionic melilite oxides, these compounds exhibited unbiased performances of proper birefringences (0.106-0.143), strong SHG responses (>10× melilite oxide Ba2CdGe2O7, 1.3-2.1× AgGaS2 (AGS) @1570 nm) with phase-matchable ability, wide band gaps (2.95-3.15 eV), and large laser-induced damage thresholds (LIDTs, 5.6-13.4× AGS). The high structure tolerance of melilite offers a great possibility to improve the SHG response via tuning the orbital composition and hybridization near band edges. This work provides an effective approach for the design of high-performance IR-NLO materials.

Nonlinear Optical Benzoindolium-Based Single Crystals: H-Contacts Induced Noncentrosymmetric Alignment Approaching Efficient Terahertz-Wave Generation

Ionic-type nonlinear optical (NLO) crystals, which belong to noncentrosymmetric (NCS) space groups, have been the core device materials in the field of laser technology, especially in the field of terahertz-wave generation. In this study, three iodide benzoindolium-based crystals, namely, MBI (C24H24NOI, Cc), m-OHBI (C23H22NOI, P21), and p-OHBI (C23H22NOI, P21/c), with different substitution sites of oxygen-containing donor groups have been successfully synthesized and grown. Acentric MBI and m-OHBI crystals exhibit transparent edges below 639 nm, good environmental stabilities, and a strong second-harmonic generation (SHG) response close to typical OH1 (d33 = 120 pm/V at 1.9 μm), and the oxygen-containing donor groups are analyzed to be effective for inducing noncentrosymmetric alignment of the functional building units. Notably, for the first time, a widely tunable terahertz wave has been realized through an MBI prototype device by the difference frequency generation method. This work provides a feasible strategy for a new NLO crystal design to achieve strong SHG intensity and efficient terahertz-wave generation.

High‐Performance Sulfate Optical Materials Exhibiting Giant Second Harmonic Generation and Large Birefringence

AbstractDiscovering novel sulfate optical materials with strong second‐harmonic generation (SHG) and large birefringence is confronted by a great challenge attributed to the intrinsically weak polarizability and optical anisotropy of tetrahedral SO4 groups. Herein, two superior‐performing sulfate optical materials, namely, noncentrosymmetric Hg3O2SO4 and centrosymmetric CsHgClSO4 ⋅ H2O, have been successfully synthesized through the introduction of a highly polarizable d10 metal cation, Hg2+. The unique component layers in the reported compounds, [Hg3O2SO4]∞ layers in Hg3O2SO4 and [HgClSO4(H2O)] layers in CsHgClSO4 ⋅ H2O, induce enlarged birefringence in each sulfate. Remarkably, Hg3O2SO4 exhibits a very large SHG response (14 times that of KH2PO4), which is the strongest efficiency among all the reported nonlinear optical sulfates. Detailed theoretical calculations confirm that the employment of highly polarizable Hg2+ is an effective strategy to design superior optical materials with large birefringence and strong SHG response.

Rational design via dual-site aliovalent substitution leads to an outstanding IR nonlinear optical material with well-balanced comprehensive properties.

The acquisition of a non-centrosymmetric (NCS) structure and achieving a nice trade-off between a large energy gap (E g > 3.5 eV) and a strong second-harmonic generation (SHG) response (d eff > 1.0 × benchmark AgGaS2) are two formidable challenges in the design and development of infrared nonlinear optical (IR-NLO) candidates. In this work, a new quaternary NCS sulfide, SrCdSiS4, has been rationally designed using the centrosymmetric SrGa2S4 as the template via a dual-site aliovalent substitution strategy. SrCdSiS4 crystallizes in the orthorhombic space group Ama2 (no. 40) and features a unique two-dimensional [CdSiS4]2- layer constructed from corner- and edge-sharing [CdS4] and [SiS4] basic building units (BBUs). Remarkably, SrCdSiS4 displays superior IR-NLO comprehensive performances, and this is the first report on an alkaline-earth metal-based IR-NLO material that breaks through the incompatibility between a large E g (>3.5 eV) and a strong phase-matching d eff (>1.0 × AgGaS2). In-depth mechanism explorations strongly demonstrate that the synergistic effect of distorted tetrahedral [CdS4] and [SiS4] BBUs is the main origin of the strong SHG effect and large birefringence. This work not only provides a high-performance IR-NLO candidate, but also offers a feasible chemical design strategy for constructing NCS structures.

A Lanthanum Ammonium Sulfate Double Salt with a Strong SHG Response and Wide Deep‐UV Transparency

AbstractThe targeted synthesis of deep‐ultraviolet (deep‐UV) nonlinear optical (NLO) materials, especially those with non‐π‐conjugated sulfates, has experienced considerable difficulties due to the need to reconcile the oft‐competing requirements for deep‐UV transparency and strong second‐harmonic generation (SHG). We report herein the designed synthesis of the first rare‐earth metal‐based deep‐UV sulfate La(NH4)(SO4)2 by a double‐salt strategy involving introduction of complementary cations, together with optical studies that reveal a short‐wavelength deep‐UV absorption edge (below 190 nm) and the strongest SHG response among deep‐UV NLO sulfates (2.4×KDP). Theoretical calculations and crystal structure analysis suggest that the excellent balance between SHG response and deep‐UV transparency can be attributed to a synergistic interaction of the hetero‐cations La3+ and [NH4]+, which optimize alignment of the [SO4] tetrahedra and highly polarizable [LaO8] polyhedra.