Strong SHG Response via High Orientation of Tetrahedral Functional Motifs in Polyselenide A₂Ge₄Se₁₀ (A = Rb, Cs)

Abstract

and strong nonlinearity. However, these materials suffer from intrinsic drawbacks, such as low laser damage threshold and two-photon absorptions, which seriously limit their high-power applications. Therefore, new promising IR-NLO materials need to be discovered.Motivated by Chen's anionic group theory, [6] which states that NLO efficiency mainly originates from anionic groups, many NLO-active structure building units (NLO functional motifs) [7] have been developed over the past few decades, such as those with d 0 transition-metal centers (e.g., Ti 4+ , Zr 4+ , Nb 5+ , Ta 5+ , Mo 6+ , etc.) [8] or maingroup cations with stereoactive lone pairs (e.g., Pb 2+ , Sb 3+ , Te 4+ , I 5+ , etc.), [9] and distorted tetrahedral MQ 4 (M = Zn, Ga, Ge, In, Sn, etc.; Q = S, Se). [10] The flexible assembly of these units produces some IR-NLO materials, such as 3These discoveries highlight the guiding role of NLO functional motifs in the development of new IR-NLO materials.SHG may originate from static and induced dipole moments. Some well-known IR-NLO functional motifs, including the distorted tetrahedral MQ 4 that usually exists in many IR-NLO metal chalcogenides, [10] possess a static dipole moment. However, their overall contributions to the total SHG responses of some compounds are negligible because the tetrahedral units are not properly aligned, thereby canceling out to a large extent the static contributions along different directions and leaving only the laser-induced contributions. For instance, in AgGaS 2 , AgGaSe 2 , and ZnGeP 2 , the static SHG contributions from GaS 4 , GaSe 4, and GeP 4 tetrahedra are exactly zero because of their nonpolar symmetry. Therefore, to maximize the static NLO contributions to the SHG response, structures with NLO-active units which are aligned in an almost parallel manner must be designed.The induced contribution of an NLO material to the SHG response is dominated by charge transfers which are close to the Fermi level. The terminal chalcogen atoms and those atoms which form QQ bonds in metal chalcogenides usually have a much larger contribution than the bridged chalcogen atoms which are connected to two neighboring metal Nonlinear optical (NLO) materials with strong second harmonic generation (SHG) responses are technologically and scientifically important for tunable lasers. This work puts forward a new strategy for designing promising infrared nonlinear optical (IR-NLO) materials with strong SHG responses by strengthening both the static and induced contributions via the high orientation of NLO functional motifs. Two new polyselenides Rb 2 Ge 4 Se 10 and Cs 2 Ge 4 Se 10 are studied to verify the strategy, they have 2D infinite 2 ∞ ∞ [Ge 4 Se 10 ] 2− layers comprising highly oriented distorted GeSe 4 tetrahedra. Their large SHG signals (8.0 and 8.5 times that of commercial AgGaS 2 ) can be ascribed to both the static contribution enhancement from the high orientation of the local dipole moment of GeSe 4 tetrahedra and the induced contribution enhancement from terminal and SeSe bon...

LiBa4Ga5Q12 (Q = S, Se): Noncentrosymmetric Metal Chalcogenides with a Cesium Chloride Topological Structure Displaying a Remarkable Laser Damage Threshold

“…Recently, numerous experiments have shown that metal chalcogenides are abundant research resources for developing novel IR NLO materials due to their broad optical transparency and strong SHG responses derived from favorable covalent bonding characters. − Specifically, the main-group-metal elements (e.g., Ga, In, Si, Ge, Sn) are often four-coordinated with chalcogen atoms to adopt asymmetric tetrahedral configurations, which are regarded as the typical “NLO active units” and play a very important role in achieving both noncentrosymmetric structures and strong SHG responses. − In addition, enlarging the band gap is known as an effective strategy to overcome the most demanding problems of TPA and low LDT in IR NLO materials. Hence, the alkali or alkaline-earth metals without d–d or f–f electronic transitions have been taken into consideration in the tetrahedra-containing chalcogen system to improve the LDTs of NLO materials. − The strategy has been confirmed by recently reported metal chalcogenides with large LDT, such as LiGaS 2 (11× that of AGS), BaGa 4 S 7 (3× that of AGS), Li 2 ZnSiS 4 (10× that of AGS), Li 2 BaGeS 4 (11× that of AGS), Ba 6 Zn 7 Ga 2 S 16 (28× that of AGS), Rb 10 Zn 4 Sn 4 S 17 (5× that of AGS), (K 0.38 Ba 0.81 )­Ga 2 Se 4 (13× that of AGS), Na 4 MgSi 2 Se 6 (9× that of AGS), Na 2 BaGeS 4 (8× that of AGS), Li 2 MnGeS 4 (40× that of AGS), and Na 2 Hg 3 Si 2 S 8 (4.5× that of AGS). − …”

[ABa2Cl][Ga4S8] (A = Rb, Cs): Wide-Spectrum Nonlinear Optical Materials Obtained by Polycation-Substitution-Induced Nonlinear Optical (NLO)-Functional Motif Ordering

“…), − and tetrahedral blocks (MQ 4 ) n − (M = Zn, Ga, Ge, etc. ; Q = S, Se). − These acentric anionic motifs were frequently assembled into NCS structures with other cations. However, when incorporating strongly electropositive cations (alkali metal, alkaline-earth metal, or rare-earth metal), which usually form directionless ionic bonding interactions, acentric anionic motifs could be packed centrosymmetrically. , Another simple but effective strategy is transforming a parent centrosymmetric (CS) structure to an NCS one through structure modification methods, which shares the benefit of inheriting the NLO-functional motifs from the parent CS compounds in the target NCS ones.…”

Li[LiCs2Cl][Ga3S6]: A Nanoporous Framework of GaS4 Tetrahedra with Excellent Nonlinear Optical Performance

“…Among these units, versatile metal chalcogenides MQ 4 (M=Zn, Cd, Ga, In, and Ge; Q=S, Se), with a tetrahedral structure, are fundamentally NLO‐active functional motifs used to build IR NLO compounds . Strategies such as mixing different MQ 4 motifs, forming highly oriented tetrahedral arrays, and constructing supertetrahedral clusters and chains by modulating MQ 4 functional motifs, have been adopted to optimize SHG responses. Meanwhile, highly electropositive alkali or alkali‐earth metals were widely incorporated to widen band gaps, such as in BaGa 4 S 7 , Li 2 CdGeS 4 , and Na 2 BaGeS 4 …”