Strong Second Harmonic Generation Response Research Articles

A2Pb3[H2N(CH2COO)2]3Cl5 (A = K, Rb): Porous Crystals Constructed on [Pb3Cl5O6] Clusters with Comprehensive Nonlinear Optical Performance

AbstractNonlinear optical (NLO) crystals that can extend the frequency range of coherent light have significant applications in laser technology, semiconductor manufacturing, and lithography. Herein, two interesting NLO crystals, A2Pb3[H2N(CH2COO)2]3Cl5 (A = K, Rb), are rationally obtained. Their porous structures are constructed on [Pb3Cl5O6] clusters that possess large hyperpolarizability and polarizability anisotropy, and are arranged in a parallel manner. Remarkably, A2Pb3[H2N(CH2COO)2]3Cl5 (A = K, Rb) exhibits comprehensive NLO performance, including strong phase‐matching second‐harmonic generation (SHG) response, the widest bandgap among lead oxychloride NLO crystals, larger laser damage threshold than 127 MW cm−2, suitable birefringence of 0.11@1064 nm, high thermal stability, non‐deliquescence, as well as facile crystal growth. Theoretical calculations and structural analysis demonstrate that their strong SHG response is predominantly originated from the [Pb3Cl5O6] cluster. This study will provide a unique method for the discovery of new interesting NLO crystals in future.

Rational Combination of π-Conjugated and Non-π-Conjugated Groups Achieving Strong Nonlinear Optical Response, Large Optical Anisotropy, and UV Light-Switchable Fluorescence.

Combining π-conjugated and non-π-conjugated groups is an important strategy for synthesizing new nonlinear optical (NLO) crystals. However, the second harmonic generation (SHG) response and optical anisotropy can be limited by improper spatial alignment of these functional groups in the crystal structure. In this work, it is revealed that non-π-conjugated [NH2 SO3 ] group acts as both hydrogen bond donor and acceptor, effectively regulating the 2D planar structure formed by π-conjugated [C4 N3 H6 ] groups. The resulting organic-inorganic hybrid crystal C4 N3 H6 SO3 NH2 exhibits a strong SHG response (2.5 × KDP), large optical anisotropy (0.233@546nm), and blue-violet and green fluorescence near 360 and 520nm, respectively. This work expands the methodology for creating new NLO crystals through organic-inorganic hybridization, while also showcasing the potential of C4 N3 H6 SO3 NH2 as a multifunctional optical material.

Record Second-Harmonic Generation and Birefringence in an Ultraviolet Antimonate by Bond Engineering.

Oxides have attracted considerable attention owing to their potential for nonlinear optical (NLO) applications. Although significant progress has been achieved in optimizing the structural characteristics of primitives (corresponding to the simplest constituent groups, namely, cations/anions/neutral molecules) comprising the crystalline oxides, the role of the primitives' interaction in determining the resultant functional structure and optical properties has long been underappreciated and remains unclear. In this study, we employ a π-conjugated organic primitive confinement strategy to manipulate the interactions between primitives in antimonates and thereby significantly enhance the optical nonlinearity. Chemical bonds and relatively weak H-bonding interactions promote the formation of cis- and trans-Sb(III)-based dimer configurations in (C5H5NO)(Sb2OF4) (4-HPYSOF) and (C5H7N2)(Sb2F7) (4-APSF), respectively, resulting in very different second-harmonic generation (SHG) efficiencies and birefringences. In particular, 4-HPYSOF displays an exceptionally strong SHG response (12 × KH2PO4 at 1064 nm) and a large birefringence (0.513 at 546 nm) for a Sb(III)-based NLO oxide as well as a UV cutoff edge. Structural analyses and theoretical studies indicate that polarized ionic bond interactions facilitate the favorable arrangement of both the inorganic and organic primitives, thereby significantly enhancing the optical nonlinearity in 4-HPYSOF. Our findings shed new light on the intricate correlations between the interactions of primitives, inorganic primitive configuration, and SHG properties, and, more broadly, our approach provides a new perspective in the development of advanced NLO materials through the interatomic bond engineering of oxides.

Molecular Crystals Constructed by Polar Molecular Cages: A Promising System for Exploring High-performance Infrared Nonlinear Optical Crystals.

Polar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter-molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure-house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4 Sn ] (n=3-9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, α-P4 S5 , which demonstrated exceptional IR optical properties, including a strong SHG response (1.1×AGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41-14.7 μm). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high-performance IR NLO materials.

Molecular Crystals Constructed by Polar Molecular Cages: A Promising System for Exploring High‐performance Infrared Nonlinear Optical Crystals

AbstractPolar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter‐molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure‐house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4Sn] (n=3–9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, α‐P4S5, which demonstrated exceptional IR optical properties, including a strong SHG response (1.1×AGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41–14.7 μm). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high‐performance IR NLO materials.

Giant Optical Anisotropy in a Covalent Molybdenum Tellurite via Oxyanion Polymerization.

Large birefringence is a crucial but hard-to-achieve optical parameter that is a necessity for birefringent crystals in practical applications involving modulation of the polarization of light in modern opto-electronic areas. Herein, an oxyanion polymerization strategy that involves the combination of two different types of second-order Jahn-Teller distorted units is employed to realize giant anisotropy in a covalent molybdenum tellurite. Mo(H2O)Te2O7 (MTO) exhibits a record birefringence value for an inorganic UV-transparent oxide crystalline material of 0.528 @ 546nm, which is also significantly larger than those of all commercial birefringent crystals. MTO has a UV absorption edge of 366nm and displays a strong powder second-harmonic generation response of 5.4 times that of KH2PO4. The dominant roles of the condensed polytellurite oxyanions [Te8O20]8- in combination with the [MoO6]6- polyhedra in achieving the giant birefringence in MTO are clarified by structural analysis and first-principles calculations. The results suggest that polymerization of polarizability-anisotropic oxyanions may unlock the promise of birefringent crystals with exceptional birefringence.

In Situ Chiral Template Approach to Synthesize Homochiral Lead Iodides for Second‐Harmonic Generation

AbstractHomochiral halide perovskites have gained increasing attention because of their fascinating optoelectronic properties and prospective applications in laser technologies. However, the limited choice of chiral organic templates severely restricts their structural diversity and second‐harmonic generation (SHG) effects. Here, we present an in situ chiral template approach for the synthesis of one‐dimensional (1D) homochiral lead iodides. A chiral imine (L‐ipp) template was generated in situ by reacting L‐proline (L‐pro) and acetone under ambient conditions. Notably, L‐ipp can cooperate with L‐pro to direct the formation of a homochiral lead iodide with dual chiral templates, which is unprecedented in crystalline metal halides. The homochiral lead iodide containing both L‐ipp and L‐pro shows a strong SHG response of 8.0 times that of KH2PO4 (8.0×KDP). The SHG efficiency is one of the largest values reported to date for any homochiral lead halides under 1064 nm laser irradiation. A comparative study shows that homochiral 1D lead iodides containing either L‐ipp or L‐pro exhibit relatively weak SHG responses (≤1.0×KDP). This work demonstrates the advantage of using two different chiral templates over a single chiral template in enhancing the SHG responses of halide materials.

In Situ Chiral Template Approach to Synthesize Homochiral Lead Iodides for Second-Harmonic Generation.

Homochiral halide perovskites have gained increasing attention because of their fascinating optoelectronic properties and prospective applications in laser technologies. However, the limited choice of chiral organic templates severely restricts their structural diversity and second-harmonic generation (SHG) effects. Here, we present an in situ chiral template approach for the synthesis of one-dimensional (1D) homochiral lead iodides. A chiral imine (L-ipp) template was generated in situ by reacting L-proline (L-pro) and acetone under ambient conditions. Notably, L-ipp can cooperate with L-pro to direct the formation of a homochiral lead iodide with dual chiral templates, which is unprecedented in crystalline metal halides. The homochiral lead iodide containing both L-ipp and L-pro shows a strong SHG response of 8.0 times that of KH2 PO4 (8.0×KDP). The SHG efficiency is one of the largest values reported to date for any homochiral lead halides under 1064 nm laser irradiation. A comparative study shows that homochiral 1D lead iodides containing either L-ipp or L-pro exhibit relatively weak SHG responses (≤1.0×KDP). This work demonstrates the advantage of using two different chiral templates over a single chiral template in enhancing the SHG responses of halide materials.

ACu4AsS4 (A = K, Rb): Two infrared nonlinear optical crystals with strong second-harmonic generation responses originating from multiple chromophores

Infrared (IR) nonlinear optical (NLO) crystals are of great significance in modern laser technology, and metal chalcogenides are receiving increasing interest as promising IR NLO candidates. To explore new IR NLO materials with enhanced properties, two quaternary chalcogenides with multiple chromophores, KCu4AsS4 and RbCu4AsS4, were synthesized using the solvothermal method. KCu4AsS4 and RbCu4AsS4 feature 2D [Cu6As2S6]∞ anionic layers with multiple chromophores, and exhibit strong phase-matched second-harmonic generation (SHG) responses (1.8–2.0 × AgGaS2), wide band gaps, suitable birefringence (0.083 and 0.074 @ 2050 nm) and wide infrared transparency. The theoretical calculation results suggest that the strong SHG responses are ascribed to the 2D [Cu6As2S6]∞ anionic layers.

K4ZnV5O15Cl: A Congruent‐Melting Mid‐Infrared Nonlinear Optical Crystal Combined with Strong Second‐Harmonic Generation and High Laser‐Induced Damage Threshold

AbstractIt is extremely challenging to accomplish second‐harmonic generation (SHG) in the whole mid‐infrared (MIR) region for oxide crystals due to their short IR absorption cutoff wavelengths. Herein, a new MIR nonlinear optical (NLO) crystal of K4ZnV5O15Cl (KZVC) is designed and synthesized by the aliovalent substitution of [VIIIO5] with [ZnIIO4Cl] based on the K2(VO)(V2O7) matrix. KZVC features an ordered 2D [ZnV5O15Cl]∞ layered structure comprising [ZnO4Cl] / [VO5] square pyramids and [VO4] tetrahedron as functional units. Importantly, KZVC shows excellently balanced performances of strong SHG responses (12 × KDP @ 1064 nm, 1.1 × AgGaS2 @ 2.05 µm), broad IR transparency range (0.52‒10 µm), congruent‐melting behavior, and highest laser‐induced damage threshold (LIDT) (60 × AgGaS2 @ 1064 nm) among the reported vanadates. Most remarkably, KZVC represents the first example in the oxide‐based IR NLO crystals that can achieve phase matching in a wide wavelength range extending from 1.8 to 10 µm, fully covering the important MIR atmospheric windows (3‒5 µm).

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.

Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln2 Ae3 MIV 3 S12 Family.

Structural dissymmetry and strong second-harmonic generation (SHG) responses are key conditions for nonlinear optical (NLO) crystals, and targeted combinatorial screening of suitable anionic groups has become extremely effective. Herein, optimal combination of flexible SnSn (n=5, 6) groups and highly electropositive cations (lanthanides (Ln3+ ) and alkaline earth (Ae2+ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln2 Sr3 Sn3 S12 (Pmc21 ) and Ln2 Ca3 Sn3 S12 (P-62m); whereas 17 rigid GeS4 or SiS4 tetrahedra-constructed Ln2 Ae3 Ge3 S12 and Ln2 Ae3 Si3 S12 crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc21 ) in the Ln2 Ae3 MIV 3 S12 family indicates that chemical substitution of the tetrahedral GeS4 /SiS4 units with SnSn breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn(4+) S5 unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4×AgGaS2 and Δn=0.008) of La2 Ca3 Sn3 S12 to the strong phase-matching (PM) SHG effect (3.0×AgGaS2 and Δn=0.086) of La2 Sr3 Sn3 S12 . Therefore, Sn(4+) S5 is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnSn building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.

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.

Sr(NH2SO3)(NO3)·H2O: An Ultraviolet Nonlinear Optical Material Exhibiting Strong Second-Harmonic Generation Response and Sufficient Birefringence.

Second-harmonic generation (SHG) response and birefringence stand as fundamental optical properties for nonlinear optical (NLO) materials. Up to now, engineering a strong SHG response and substantial birefringence in the sulfamate system has proven to be exceedingly challenging. A novel noncentrosymmetric compound, Sr(NH2SO3)(NO3)·H2O, has been meticulously designed by introducing the (NO3)- group characterized by significant polarizability anisotropy and hyperpolarizability into the polar non-π-conjugated (NH2SO3)- system. This compound exhibits a robust SHG response (5.2 × KDP), ample birefringence (0.077 at 546.1 nm), and a short ultraviolet (UV) absorption edge (290 nm). The linear and nonlinear optical properties can be attributed to the (NO3)- group and the (NH2SO3)- group. This study presents an effective approach that contributes to the design of more composite anionic UV NLO materials with excellent and balanced optical properties.

A potassium lead 2-methylmalonate-iodide nonlinear optical crystal with strong second-harmonic generation response and large birefringence

Nonlinear optical (NLO) crystals play a significant role in optoelectronic industry. Herein, a new NLO crystal, K(H2O)[KI][PbI(C4H4O4)], has been rationally obtained by tuning halide perovskite materials with 2-methylmalonate group. K(H2O)[KI][PbI(C4H4O4)] features that the highly distorted [PbO4I2] polyhedron with large hyperpolarizability and prominent polarizability anisotropy is served as a bifuntional unit. Remarkably, the distorted [PbO4I2] polyhedrons are aligned in an oriented arrangement that leads to the effective superposition of their microscopic second-order nonlinear susceptibilities and the enhancement of optical anisotropy. Notably, K(H2O)[KI][PbI(C4H4O4)] exhibits a strong phase-matching SHG response of 5.6 × KDP and a large birefringence (Δnobv. = 0.24@546 nm). Theoretical calculations elucidate that the strong SHG response and the large birefringence are predominantly attributed to the directional arrangement of the highly distorted [PbO4I2] polyhedrons. Our discovery provides an effective path for the rational exploration of high-performance NLO crystals.

Double alkaline earth metals sulfide SrMgGeS4 with high laser-induced damage threshold and strong second-harmonic generation

The exploration of anti-laser damage wide band gap infrared (IR) nonlinear optical (NLO) materials is urgently needed but challenging due to the contradiction between wide band gap and large NLO coefficient. Herein, by introducing [MQ4] (Q = S, Se) tetrahedral units into the AIIBIICIVD4VI family, three double alkaline earth metals chalcogenides SrMgGeS4, BaMgSiSe4 and SrMgSiSe4 with unprecedented [MgGeS6] or [MgSiSe6] dimers are rationally designed and fabricated. The compounds display an interesting structural change from centrosymmetric (CS) I41/acd (BaMgSiSe4, the first CS compound in this family) to non-centrosymmetric Ama2 (SrMgSiSe4) and Fdd2 (SrMgGeS4). Importantly, SrMgGeS4 shows a wide band gap (3.58 eV), a high laser-induced damage threshold (∼6 × AGS), a moderate birefringence (∼0.11@1064 nm), and a strong second-harmonic generation (SHG) response (∼1.1 × AGS), achieving a good balance between wide band gap (Eg ≥ 3.5 eV) and large SHG response (≥1.0 × AGS) for an excellent IR NLO material. The results enrich the chemical and structural diversity of chalcogenides and give insights into the design of high-performance IR NLO materials.

AllHgMIVS4 (All = Sr, Ba, MIV = Si, Ge): A Series of Materials with Large Second Harmonic Generation Response and Wide Band Gaps

AbstractWide band gaps and large second harmonic generation (SHG) response are two crucial yet contradictory parameters for nonlinear optical (NLO) crystals. Herein, through exploring the influence of anion groups on structural crystallization dimension and space group, a series of new NLO Hg‐based chalcogenides AIIHgMIVS4 (AII = Sr, Ba, MIV = Si, Ge) with wide band gaps and large SHG responses have been successfully designed and synthesized. The structures of these compounds consist of 2D [HgMS4]2− (M = Si and Ge) layers composed of distorted (HgS4 and MS4) tetrahedra arranged alternately, and the layers are separated by eight‐coordinated A2+ (A = Sr and Ba) cations. Their band gaps and SHG responses meet the basic requirements (SHG > 1 × AgGaS2; Eg > 2.33 eV). Especially, SrHgSiS4 and SrHgGeS4 exhibit both large SHG effects (2 and 2.4 × AgGaS2) and wide band gaps (3.06 and 2.97 eV), respectively, indicating they are promising IR NLO crystals. This work opens a new avenue for new NLO material design to achieve the balance between strong SHG response and large band gaps.