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• 1. INTRODUCTION

  Infrared (IR) nonlinear optical (NLO) crystals with its irreplaceable ability of extending existing wavelengths of IR laser sources to the desirable ranges play an important role in laser technology^[1]. Owing to the intrinsic deficiencies of commercially attainable IR NLO crystals (AgGaS₂, AgGaSe₂, ZnGeP₂, etc.), design and exploratory synthesis of novel IR NLO materials with superior overall performance have attracted growing attention. Admittedly, it is a great challenge to find a practically usable IR NLO material because the following criteria need to be met simultaneously: wide IR transparent range, strong NLO response, suitable birefrin-gence, high laser-induced damage threshold (LDT), good physicochemical stability and benign single-crystal growth habit. Thus, many kinds of compounds are generally unsuitable for mid-far IR NLO applications, such as oxides with strong lattice vibration absorption in mid-far IR region, halides with small nonlinearity, and organic crystals with low LDT. Explorations over the past decades have shown that metal chalcogenide is one of the most promising systems for the design and exploratory synthesis of IR NLO materials because of their inherently wide optical transparency, large polarizability, and rich structural diversity which greatly increase the possibility for achieving a favorable balance among the above criteria^[2]. In recent years, many excellent NLO chalcogenides have been discovered and plenty of useful strategies have been proposed, which provides much inspiration and guidance for the follow-up research^[3–7]. In this perspective, we focus on some important design and exploratory synthesis strategies and reviewed the according progress on advanced IR NLO materials. Furthermore, we conclude the future prospects of this research area.

  2. PROGRESS

  Metal chalcogenide is a broad stage for design and exploratory synthesis of IR NLO materials with excellent overall performance. And, many strategies have been summarized to give some illuminations and get rid of the aimless explorations like looking for a needle in a haystack. Among them, the following three strategies are proved repeatedly to be feasible and effective (Fig. 1).

  Figure 1

  

  Figure 1.  (a) Heterogeneous hybridization of various types of NLO-active units. (b) Element substitution, taking AM₄^IIIQ₇ (A = Ba, Pb, Sn; M^III = Al, Ga, Q = S, Se) family as an example. (c) Molecular construction based on the classic template, taking CuZnPS₄ as an example. (d) Excellent overall performance of designed crystal compared with benchmark AgGaS₂

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  (1) Heterogeneous hybridization of various types of NLO-active units. In general, typical NLO active units in chalcogenides encompass MQ₄ tetrahedra centered by main group elements, trigonal planar units such as BS₃, AgSe₃, HgSe₃; distorted polyhedra centered by stereochemically active lone pairs (SALP) cation; distorted polyhedra centered by second-order Jahn-Teller (SOJT) effect d⁰ or d¹⁰ cation and rare-earth cations centered polyhedra, etc. The combination of more than one kind of the above units tends to trigger novel covalent architectures and strengthen the NLO response via synergistic effect. For example, Sr₅ZnGa₆S₁₅^[8], Li₂CdGeS₄^[9] and Cd₄GeS₆^[10] are constructed by various tetrahedral units. Experiments as well as theoretical investigations demonstrate that they strike a good balance between strong SHG response and large band gaps (implying desirable NLO performance and good application prospects). Moreover, various combination modes of NLO active units have dominated the recent-developed NLO chalcogenides. Ba₆In₆Zn₄Se₁₉ incor-porates InSe₃ trigonal planar units and In/ZnSe₄ tetrahedra^[11], Ba₂₃Ga₈Sb₂S₃₈ is comprised of SbS₃ and GaS₄ units^[12], PbGa₂GeSe₆ is constructed by PbSe₄, GaSe₄, and Ga/GeSe₄ groups^[13], etc. Intriguingly, many of these compounds exhibit strong nonlinearity. The potential of constructing novel NLO chalcogenides by hybridization different active groups is far from fully developed owing to the unlimited possibilities of various linkage modes. Besides, the microscopic mechanism of the synergistic effect among these active units is not fully understood. Thus, the further experimental and theoretical investigations are still needed.

  (2) Element substitution. Many chalcogenide frameworks are flexible enough for a variety of element substitution (especially, the substitution with elements of the same family), while maintain the chemical stability. For instance, the alkali metal substitution in ANb₂PSe₁₀ (A = K, Rb, and Cs) family^[14]; the rare earth metal substitution in RE₄GaSbS₉ (RE = Pr, Nd, Sm, Gd–Ho and Y) family^[15]; the alkali earth metal substitution, II B group element substitution, IV A group element substitution and chalcogen substitution in AEM^IIM^IV-Q₄ (AE = Sr, Ba; M^II = Zn, Cd, Hg; M^IV = Si, Ge, Sn; Q = S, Se) family^[16]. Sometimes, elements with similar radii or coordination environments can also be substituted for each other, such as Li, Ag, Cu(I) element substitution in ACu₃PS₆ (A = Li, Ag, Cu) family^[7], Mg, Mn, Fe, Zn, Cd, Hg element substitution in A₂-M^II-M₃^IV-Q₈ (A = K, Rb, Cs; M^II = Mg, Mn, Zn, Cd, Hg; M^IV = Ge, Sn; Q = S, Se, Te) family^[17]. The feasibility and effectiveness of these substitutions largely lessen the pain of trial and error experiments and accelerate the discovery of novel NLO chalcogenides.

  More importantly, different element substitutions offer possibility for NLO performance regulation. In the periodic table, electron clouds of elements in the same group become more diffuse from top to bottom, indicating an increasing tendency of polarizability in similar crystal structure when exposed to external photoelectric field. And, such substitution can have inverse impact on the band gap. For example, BaGa₂SiSe₆, BaGa₂GeSe₆ and BaGa₂SnSe₆ are isostructural, and they show increasing SHG responses and birefringence but decreasing band gaps, due to the substitution from Si to Ge to Sn^[18]. More significantly, when the electronegative elements are substituted from S to Se to Te, the nonlinearity of chalcogenide increases dramatically, and the infrared transparency range can be significantly redshifted. However, the band gap and LDT will generally decrease. According to the anionic group theory^[19], the substitution of metal cations has little impact on the NLO response if the crystal structure maintains unchanged, because compounds' nonlinearity originates mainly from the anionic groups. However, some cations with large ionic radii can also make a nonnegligible contribution to NLO response in some cases, which may lead to some interesting phenomena. For instance, SHG responses of AZrPSe₆ (A = Cs, Rb, K) family show an obvious decreasing polarizability trend from Cs to K compounds. And, Rb⁺, Cs⁺ doping can also enhance KZrPSe₆'s SHG response, which verifies that the larger and more polarizable alkali metal atoms in this family contribute more to SHG responses^[20]. Except for the gradual change in NLO response and band gap, abrupt changes may sometimes occur during element substitution. For instance, AX₄^IIX₅^IIIQ₁₂ (A = K, Rb, Cs; X^II = Zn, Cd, Hg; X^III = In, Ga; Q = S, Se, Te) family demonstrates a surprising change from non-phase-matching to phase matching when X^II position is substituted by Hg atom because of Hg's high polarizability^[21].

  The element substitution can also induce crystal structure transformation. For example, in [(A₃X)(Ga₃PS₈)] (A = K, Rb; X = Cl, Br) family, [(A₃Cl)(Ga₃PS₈)] crystalizes in the or- thorhombic space group Pmn2₁ but [(A₃Br)(Ga₃PS₈)] belongs to the monoclinic space group Pm, owing to the different stacking fashions of cationic [A₃X]²⁺ and anionic [Ga₃PS₁₀]⁶⁻ layers induced by halogen substitution^[22]. In A₂-M^II-M^IV-Q₄ (A = Li, Na, K, Cu, Ag; M^II = Sr, Ba, Eu, Pb; M^IV = Si, Ge, Sn; Q = S, Se) family, six types of crystal structures come into existence on account of various substitution^[18]. The sub-stitution-induced structure transformation also provides an opportunity for NLO performance optimization. The rela-tionship between the stability/transition of crystal structure and properties of the substituted element is worthy of further investigations.

  (3) Molecular construction based on the classic template. The classic IR NLO materials (AgGaS₂, AgGaSe₂, ZnGeP₂, LiGaS₂, HgGa₂S₄, etc) have their own structural merits and exhibit excellent NLO performance but suffer from some intrinsic limitations simultaneously. Thus, researchers attempt to rationally design and synthesize novel IR NLO crystals with improved properties on the basis of employing these state-of-the-art materials as a parent model. For instance, Zhou et al., taking AgGaS₂ as the template, utilized partial substitution of Ag with Li to push up the bottom of the conduction band and flatten the top of the valence band, resulting in an obviously enlarged bandgap of 3.40 eV in Li_0.60Ag_0.4GaS₂^[23]. Besides, the partial incorporation of Li in AgGaS₂ brings down the sulfur-dislocation in the lattice, which makes the as-synthesized Li_0.60Ag_0.4GaS₂ exhibit enhanced SHG response. Yao's group successfully synthe-sized CuZnPS₄ which inherits the structural merits of AgGaS₂ and exhibits an enlarged energy gap originated from the cosubstitution of Ag with lighter Cu/Zn. Moreover, the designed CuZnPS₄ shows reinforced SHG response (3 × AgGaS₂) owing to the introduction of cation vacancy de-fects^[24]. Mao's group introduced the strong covalent PS₄ units into template AgGaS₂, thus synthesizing AgGa₂PS₆ crystal^[25]. AgGa₂PS₆ demonstrates improved LDT (5.1 × AgGaS₂) while remains similar SHG response to AgGaS₂. The lattice energy as well as thermal expansions analyses indicates that the incorporation of PS₄ group accounts for the improvement of NLO performance in template AgGaS₂. Researchers take advantages of the structural merits of these templates to construct novel crystals and improve the existing performance deficiency, greatly reducing the blindness of exploratory synthesis and increasing the success rate. First principles calculations are very effective tools for structural design and prediction. They have been widely applied in NLO oxides design and many KBe₂BO₃F₂ (KBBF)-like/Sr₂Be₂O₇ (SBBO)-like crystals have been discovered with the help of first principles calculations. However, the successful NLO chalcogenides prediction on the basis of first principles calculations is rare. It is of great significance to develop theoretical calculations suitable for chalcogenides.

  3. CONCLUSION

  In conclusion, to meet the growing market demands for applicable IR NLO materials, great efforts have been made to overcome the intrinsic drawbacks of traditional chalcopyrite NLO crystals and develop novel IR NLO crystals with superior performance during the past decades. Commendably, many useful strategies and design ideas have been proposed, which rendered the discovery of plenty of excellent NLO chalcogenides. Among them, the following three are widely used and proved to be effective: heterogeneous hybridization of various types of NLO-active units, element substitution and molecular construction based on the classic template. However, there are still some topics to be thoroughly investigated, such as the microscopic mechanism of the synergistic effect among various NLO-active units and the relationship between the stability/transition of crystal structure and properties of the substituted element. Besides, it is of great significance to develop theoretical calculations suitable for chalcogenides.

  Further, investigations of most of these newly developed IR NLO crystals are stuck in the preliminary characterization of micron crystals. Large size crystal growth, detailed physical property characterizations based on single crystals, and IR laser output experiments can be the next move of effort direction.

  
  
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  Figure 1  (a) Heterogeneous hybridization of various types of NLO-active units. (b) Element substitution, taking AM₄^IIIQ₇ (A = Ba, Pb, Sn; M^III = Al, Ga, Q = S, Se) family as an example. (c) Molecular construction based on the classic template, taking CuZnPS₄ as an example. (d) Excellent overall performance of designed crystal compared with benchmark AgGaS₂

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