Wide band gap selenide infrared nonlinear optical materials A^IIMg₆Ga₆Se₁₆ with strong SHG responses and high laser-induced damage thresholds

Abstract

Infrared (IR) nonlinear optical (NLO) materials with strong NLO response, wide band gap and high laser-induced damage threshold (LIDT) are highly expected in current laser technologies. Herein, by introducing double alkaline-earth metal (AEM) atoms, three wide band gap selenide IR NLO materials A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba) with excellent linear and NLO optical properties have been rationally designed and fabricated. A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba) are composed of unique [A^IISe₆] triangular prisms, [MgSe₆] octahedra and [GaSe₄] tetrahedra. The introduction of double AEMs effectively broadens the band gaps of selenide-based IR NLO materials. Among them, CaMg₆Ga₆Se₁₆, achieving the best balance between the second-harmonic generation response (∼1.5 × AgGaS₂), wide band gap (2.71 eV), high LIDT (∼9 × AgGaS₂), and moderate birefringence of 0.052 @ 1064 nm, is a promising NLO candidate for high power IR laser. Theoretical calculations indicate that the NLO responses and band gaps among the three compounds are mainly determined by the NLO-active [GaSe₄] units. The results enrich the chemical diversity of chalcogenides, and give some insight into the design of new functional materials based on the rare [A^IISe₆] prismatic units.

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Introduction

Nonlinear optical (NLO) crystals are an important class of optoelectronic functional materials, which play a critical role in developing new laser light sources by the frequency conversion technology.^1–8 In the ultraviolet (UV) and visible-near infrared (IR) regions, oxide-based NLO materials have achieved great progress,^9–13 and a series of NLO crystals with excellent performances like LiB₃O₅ (LBO), BaB₂O₄ (β-BBO), KBe₂BO₃F₂ (KBBF), KH₂PO₄ (KDP), and NH₄B₄O₆F have been developed in recent years,^14–18 while in the mid- and far-IR regions, the commercially available IR NLO materials like AgGaS₂ (AGS), AgGaSe₂ (AGSe) and ZnGeP₂ (ZGP) suffer from their intrinsic defects, like low laser-induced damage thresholds (LIDTs) in AGS and AGSe, strong two-photon absorption around ∼1 μm and narrow IR transparent range in ZGP.^19,20 Therefore, exploring new IR NLO materials with excellent performances has become an urgent need, but challenging due to the conflicts among NLO response, band gap, birefringence and IR transparent range in one material.²¹

For an excellent IR NLO material, the following requirements should be satisfied:²² (1) noncentrosymmetric (NCS) crystal structure; (2) a large NLO response (≥0.5 × AGS, preferably ≥1.0 × AGS); (3) a wide IR transparent range that covers the two important IR atmospheric windows of 3–5 and 8–12 μm; (4) a high LIDT, which is usually related to the band gap and thermal conductivity; (5) suitable birefringence for phase-matching (PM) behavior; (6) good crystal growth habits. To achieve the large NLO response and wide IR transparent range, metal selenide has been demonstrated as a competitive system, due to the large hyperpolarizability of NLO-active [MSe₄] (M = Ga, In, Ge, Sn, Zn, Cd, Hg, etc.) tetrahedral units, and red shift of the vibration peaks of M–Se compared to M–O, M–S or M–P bonding.^23,24 However, for most selenides, their band gaps are narrow due to the presence of outer electrons on the 3d, 4s, 4p orbits in the Se atom.^25,26

To enhance the band gap of chalcogenides, introducing an alkaline-earth metal (AEM) without d–d and f–f electron transitions into the crystal structures has been regarded as a feasible strategy.^27,28 Recently, by coupling [MgQ₆] (Q = S, Se) octahedral and [MQ₄] (M = Al, Ga) tetrahedral units, a series of Mg-containing compounds A^IB₃^IIC₃^IIIQ₈^VI (A^I = Li, Na, Ag; B^II = Mg; C^III = Al, Ga; Q^VI = S, Se) have been developed.²⁹ In this work, by introducing double AEM atoms, three wide band gap selenide IR NLO materials A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba) have been synthesized by the high temperature solid-state reactions in sealed quartz tubes. A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba) crystallize in the NCS P6̄ space group, and are built by [A^IISe₆] triangular prisms, [MgSe₆] octahedral and [GaSe₄] tetrahedral units. The compounds show balanced optical properties including strong second-harmonic generation (SHG) responses (1.5 × AGS for CaMg₆Ga₆Se₁₆, 1.0 × AGS for SrMg₆Ga₆Se₁₆, 1.1 × AGS for BaMg₆Ga₆Se₁₆), wide band gaps (in selenides) (2.69–2.71 eV), high LIDTs (8.0–9.0 × AGS), and moderate birefringences (0.044–0.052 @ 1064 nm). Density functional theory (DFT) calculations reveal that the SHG responses and band gaps of the compounds are mainly determined by the [GaSe₄] tetrahedral units.

Results and discussion

The single crystals of A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba) were synthesized by high temperature solid-state reactions at 900 °C, and were picked under an optical microscope for structural determinations by single crystal X-ray diffractions. The energy dispersive X-ray spectroscopy (EDS) spectra and elemental maps demonstrate the presence of Ca/Sr/Ba, Mg, Ga, Se in the structures (Fig. S1†). Meanwhile, the Raman spectra confirm the chemical bonding in the compounds (Fig. S2†). The calculated bond valence sums and global instability indexes (Tables S2, S6 and S10†) verify the reasonability of the crystal structures in the compounds. The crystal data and structure refinement information including atomic coordinates and equivalent isotropic displacement parameters, bond length and angle information are provided in Tables S1–S13.†

Since the three compounds show similar cell parameters and crystal structures, CaMg₆Ga₆Se₁₆ is utilized as an example to illustrate their structures here. CaMg₆Ga₆Se₁₆ crystallizes in the P6̄ space group with a = b = 17.5327(3) Å, c = 7.7603(2) Å. In the asymmetric unit of CaMg₆Ga₆Se₁₆, there are three Ca atoms, six Mg atoms, three Ga atoms and eleven Se atoms. The Ca atom and Mg atom are coordinated with Se atoms to form [CaSe₆] triangular prisms and [MgSe₆] octahedral units, while Ga atoms are bonded to four Se atoms to construct [GaSe₄] tetrahedra (Fig. 1a). [GaSe₄] tetrahedra are connected with each other by vertex-sharing to form the [GaSe₃] single-chains and [Ga₂Se₅] double-chains (Fig. 1a). [MgSe₆] octahedra are linked with each other by edge-sharing to form [MgSe₄] single-chains and [Mg₂Se₆] double-chains (Fig. 1b). The formed [GaSe₃] and [MgSe₄] single-chains are linked by vertex-sharing to build the windmill-like [Mg₃Ga₃Se₂₄] channels (Fig. 1c), which are further grouped with [Ga₂Se₅] and [Mg₂Se₆] double-chains to form the channel-like [Mg₉Ga₉Se₂₄] framework (Fig. 1d). The six-coordinated Ca ions are filled in the channels (Fig. 1e) to balance the charge, resulting in the final 3D structure of CaMg₆Ga₆Se₁₆ (Fig. 1f). It is worth noting that, different from CaMg₆Ga₆Se₁₆ and SrMg₆Ga₆Se₁₆, the asymmetric unit of BaMg₆Ga₆Se₁₆ contains two crystallographically independent Ba atoms at Wyckoff 1c, 1e positions, and two partially occupied Ba atoms with the site occupancy of 0.9 and 0.1 at Wyckoff 1a, 1b positions, respectively.

Fig. 1 The crystal structure of CaMg₆Ga₆Se₁₆. (a) The coordination environment of Ca, Mg and Ga atoms, [GaSe₃] single-chain and [Ga₂Se₅] double-chain viewed from the b direction; (b) [MgSe₄] single-chain and [Mg₂Se₆] double-chain viewed from the a direction; (c) windmill-like [Mg₃Ga₃Se₂₄] geometries; (d) [Mg₉Ga₉Se₂₄] framework; (e) [CaSe₆] triangular prisms; (f) the 3D structure of CaMg₆Ga₆Se₁₆ viewed from the c direction.

Compared to the crystal structures of A^IB^II₃C^III₃Q^VI₈ family compounds, the title compounds can be regarded as the atomic aliovalent substitution of monovalent A^I (A^I = Li, Na, Ag) by divalent AEM A^II (A^II = Ca, Sr, Ba). The aliovalent substitution does not cause the change of space group, but induces a doubled cell parameter c in A^IIMg₆Ga₆Se₁₆ compared to A^IMg₃Ga₃Se₈, which can be attributed to the formed ordered vacancy in A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr), or the partially occupied Ba atoms in BaMg₆Ga₆Se₁₆. In addition, accompanying the fluctuation of A-site cations (Fig. 2a), the channel size fluctuates from 2.49–2.56 Å to 2.39–2.65 Å (Fig. 2b). Most of them are smaller than the A^I/A^II–Se bond lengths in the compounds, except in the case of AgMg₃Ga₃Se₈ (Fig. 2b). It is worth noting that Ag is three-coordinated with three Se atoms to form a planar [AgSe₃] unit in AgMg₃Ga₃Se₈, different from the six coordinated [Li/NaSe₆] triangular prism units in A^IMg₃Ga₃Se₈ (A^I = Li, Na) and [Ca/Sr/BaSe₆] triangular prism units in A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba).

Fig. 2 (a) The atomic modes of [Mg₃Ga₃Se₂₄] channels in A^IMg₃Ga₃Se₈ (A^I = Li, Na, Ag) and A^IIMg₆Ga₆Se₁₆ (A^II = Ca, Sr, Ba); (b) the statistical analyses of the channel size and A^I/A^II–Se band length in the six compounds.

To evaluate the optical properties, the pure-phase powder samples of the title compounds were prepared and characterized. The experimental XRD patterns are matched well with the theoretical results derived from cif files (Fig. 3a and S3†). To obtain experimental band gaps, UV-vis-NIR diffuse reflectance spectra were measured. Based on the Kubelka–Munk function,^30–32 the experimental band gaps of CaMg₆Ga₆Se₁₆, SrMg₆Ga₆Se₁₆ and BaMg₆Ga₆Se₁₆ were measured to be 2.71, 2.71 and 2.69 eV, respectively (Fig. 3b). The large band gaps inherently contribute to high resistances to laser damage.^33,34 The LIDTs of the three compounds were evaluated by the single-pulse LIDT method with AGS as the reference.^35–37 The measured results show that the LIDTs of CaMg₆Ga₆Se₁₆, SrMg₆Ga₆Se₁₆ and BaMg₆Ga₆Se₁₆ powder samples were ∼9.0, ∼9.0, and ∼8.0 × AGS, respectively.

Fig. 3 (a) The XRD patterns of CaMg₆Ga₆Se₁₆; (b) the experimental band gaps of A^IIMg₆Ga₆Se₁₆ (A = Ca, Sr, Ba); (c) SHG intensity vs. AGS with similar particle sizes; (d) typical selenide IR NLO candidates with PM behaviours, AGS used as the references.

Beyond the band gap and LIDT, SHG response is another important parameter for IR NLO materials. The SHG responses of the title compounds were evaluated by the Kurtz and Perry method with a 2.09 μm Q-switched laser.^38–41 As shown in Fig. 3c, the SHG intensity of CaMg₆Ga₆Se₁₆, SrMg₆Ga₆Se₁₆ and BaMg₆Ga₆Se₁₆ was tested to be ∼1.5, ∼1.0, and ∼1.1 × AGS, respectively, showing that the A-site AEM has an influence on the SHG responses. It can be attributed to the changes of distortion degree and hyperpolarizability of nonlinear-active [GaSe₄] units induced by the different A^II–Se chemical bonding (the bond length d_Ca–Se = 3.0609–3.0640 Å, d_Sr–Se = 3.1520–3.1590 Å and d_Ba–Se = 3.2450–3.2612 Å) in the structures. Specifically, the hyperpolarizabilities of [Ga(1–3)Se₄] in CaMg₆Ga₆Se₁₆ were calculated to 283.24, 640.08, and 665.56, while 253.14, 615.32, and 625.81 for SrMg₆Ga₆Se₁₆, and 190.51, 575.27, and 591.85 for BaMg₆Ga₆Se₁₆, thus resulting in a larger SHG response in CaMg₆Ga₆Se₁₆. Moreover, the hyperpolarizabilities of [Ga(1–3)Se₄] in NaMg₃Ga₃Se₈ were calculated to be 278.91, 627.83, and 659.72, smaller than the ones in CaMg₆Ga₆Se₁₆. The results indicate that the alkali metal (AM) and AEM can exert an influence on the SHG response in an indirect way by modifying the geometry of NLO tetrahedral units in the A^IB^II₃C^III₃Q^VI₈ and A^IIB^II₆C^III₆Q^VI₁₆ families. Notably, the experimental results imply that CaMg₆Ga₆Se₁₆ achieves a good balance between wide band gap and large SHG response, which is comparable with the recently reported typical selenide IR NLO candidates like β-BaGa₂Se₄ (0.6 × AGS, 2.49 eV),²⁵ DCL-MgGa₂Se₄ (0.9 × AGS, 2.96 eV),²⁶ AgIn₅Se₈ (0.7 × AGS, 1.34 eV),⁴² Pb₄SeBr₆ (1.3 × AGS, 2.62 eV),⁴³ KSmP₂Se₆ (1.08 × AGS, 1.92 eV)⁴⁴ and Na₂Ga₃In₃Se₁₀ (1.5 × AGS, 2.47 eV)⁴⁵ (Fig. 3d).

To understand the structure–performance relationship, the band structures, total and partial density of states (TDOS/PDOS) were computed by DFT calculations.^46–48 As shown in Fig. 4a, S4a and c,† the title compounds could be direct band gap compounds with calculated GGA band gaps of 1.851 eV (CaMg₆Ga₆Se₁₆), 1.866 eV (SrMg₆Ga₆Se₁₆) and 1.853 eV (BaMg₆Ga₆Se₁₆). Considering the underestimation of band gaps in the standard DFT calculations with GGA because of the discontinuity of the exchange–correlation energy functional,^49,50 the HSE06 band gaps of the compounds were calculated to be 2.568 eV (CaMg₆Ga₆Se₁₆), 2.564 eV for (SrMg₆Ga₆Se₁₆) and 2.556 eV (BaMg₆Ga₆Se₁₆) (Table S14†), which are close to the experimental results in Fig. 3b. From the results of TDOS and PDOS (Fig. 4b, S4b and d†), it can be seen that the tops of the valence band maximum (VBM) near the Fermi level are determined by Se-4p orbitals, and the bottoms of the conduction band minimum (CBM) are mainly occupied by Se-4s4p and Ga-4s orbitals, indicating that the band gaps of the three compounds are mainly determined by Ga–Se bonding in [GaSe₄] tetrahedral units. Moreover, the calculated birefringence values are 0.052 for CaMg₆Ga₆Se₁₆, 0.048 for SrMg₆Ga₆Se₁₆, and 0.044 for BaMg₆Ga₆Se₁₆ at 1064 nm (Fig. 4c), matched with the PM behaviours in Fig. 3c. Meanwhile, the bonding electron density difference (Δρ^b) and contributions (Δω) indicate that [MgSe₆] and [GaSe₄] units make the main contribution to the optical anisotropy in the compounds (Fig. 4d and S5†).⁵¹

Fig. 4 (a) The band structure and (b) total and partial density of states of CaMg₆Ga₆Se₁₆; (c) the calculated birefringences of the title compounds; (d) the contributions (Δω) of [A^IISe₆] triangular prisms, [MgSe₆] octahedra and [GaSe₄] tetrahedra in A^IIMg₆Ga₆Se₁₆ calculated by the response electron distribution anisotropy (REDA) method; (e and f) the SHG density maps of occupied (e) and unoccupied (f) orbitals in the VE process of CaMg₆Ga₆Se₁₆.

To detect the origin of the NLO effect, the NLO coefficients and SHG density maps of the title compounds were investigated by DFT calculations.^52–56 As shown in Table S14,† the calculated NLO coefficients are d₁₁ = −11.05 and d₂₂ = 3.92 pm V⁻¹ for CaMg₆Ga₆Se₁₆, d₁₁ = −10.67 and d₂₂ = 3.93 pm V⁻¹ for SrMg₆Ga₆Se₁₆, d₁₁ = −10.58 and d₂₂ = 3.71 pm V⁻¹ for BaMg₆Ga₆Se₁₆, which match the experimental results in Fig. 3c. Meanwhile, the SHG density maps of occupied and unoccupied orbitals in the Virtual-Electron (VE) process which occupied 93.74% in CaMg₆Ga₆Se₁₆, 95.25% in SrMg₆Ga₆Se₁₆, and 95.82% in BaMg₆Ga₆Se₁₆ (the Virtual-Hole (VH) process is neglected here because it accounts for a small quantity of contributions) indicate that the NLO responses of A^IIMg₆Ga₆Se₁₆ (A = Ca, Sr, Ba) also mainly originate from the [GaSe₄] tetrahedral units (Fig. 4e, f and S6†), matched with the anionic group theory promoted by Chen et al.^57,58

Conclusions

In summary, by introducing double AEM atoms into the structure, three wide band gap selenides CaMg₆Ga₆Se₁₆, SrMg₆Ga₆Se₁₆ and BaMg₆Ga₆Se₁₆ have been developed successfully. The compounds show strong phase-matching SHG responses (1.0–1.5 × AGS), wide band gaps (2.69–2.71 eV), high LIDTs (8.0–9.0 × AGS) and suitable birefringences (0.044–0.052 @ 1064 nm). Among them, CaMg₆Ga₆Se₁₆, achieving the best balance among strong SHG response (∼1.5 × AGS), wide band gap (∼2.71 eV) and high LIDT (∼9.0 × AGS), is a promising IR NLO candidate for high power IR laser. The theoretical calculations reveal that the SHG responses and band gaps of the series of double AEM compounds originate from or are determined by the NLO-active [GaSe₄] units. The results indicate that grouping 6-coordinated AEM units (triangular prisms and octahedra) and 4-coordinated NLO-active tetrahedra is a feasible way for the design of new IR NLO materials with excellent properties.

Data availability

Data available in the ESI† includes experimental sections, crystallographic data, EDS and Raman spectra, XRD patterns, band structures, the calculated bonding electron density differences and SHG density maps of the title compounds.

Author contributions

Linan Wang: investigation, methodology, writing – original draft; Dongdong Chu: DFT calculation; Zhihua Yang: software, validation; Junjie Li and Shilie Pan: conceptualization, supervision, funding acquisition, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the High-level Talent Project of Xinjiang Uygur Autonomous Region (2020000039), National Natural Science Foundation of China (22335007, 52002398, 61835014, 51972336), Xinjiang Key Laboratory of Electronic Information Materials and Devices (2017D04029), Xinjiang Tianchi Talents Project (2023000043), and Xinjiang Tianshan Talent Program (2022TSYCTD0005).

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2311758–2311760. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc00334a

‡ L. A. Wang and D. D. Chu contributed equally to this work.

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