Second-order nonlinear optical (NLO) material possesses frequency conversion effect in a laser source that has broad application prospects in modern optical science and technology. 

The leading infrared (IR) NLO materials, such as chalcopyrite AgGaS2 and AgGaSe2, exhibit considerable second-harmonic generation (SHG) responses; however, they encounter the drawback of low laser-induced damage thresholds (LIDTs) attributed to their narrow bandgaps. Therefore, developing new IR NLO crystals with excellent NLO performances, especially large SHG response and wide band gap concurrently, remain as research interests.

In a study published in Chemical Science, Prof. GUO Guocong and Prof. LIU Binwen from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, took inspiration from the dominant structure of salt-inclusion chalcogenides (SIC), and proposed an innovative approach to establish IR NLO materials with well-balanced properties.

The researchers concluded that metals containing d-orbitals exhibit substantial deformation and polarization, and were advantageous constituents for achieving significant NLO coefficients. However, chalcogenides with d-orbital components were associated with narrow bandgaps, rendering them unsuitable as high-power NLO crystals. Constructing SIC was an effective strategy to circumvent the challenges imposed by narrow bandgap.

Besides, the researchers integrated the [MS4] unit into the SIC family, resulting in the formation of two new isostructural SICs [K4Cl][MGa9S16] (M = Mn, Hg). Characterization of the optical properties demonstrated that both compounds held immense applications potential as IR NLO candidates.

Specifically, both crystals exhibited robust SHG responses (approximately 0.6~1.0 times of AgGaS2 @1910 nm) and remarkable optical bandgaps (3.02~3.41 eV). The bandgap of [K4Cl][HgGa9S16] stood out the widest among reported mercury-chalcogenide NLO crystals to date.

In addition, the researchers discovered that [K4Cl][MnGa9S16] showed remarkable photoluminescence with dual-emission centers (650 nm and 718 nm) at 77 K, accompanied by prolonged phosphorescence lifetimes of 0.94 and 1.35 ms, respectively.

The atom-resolved SHG contributions analysis revealed that [GaS4] and [MS4] tetrahedra, as NLO-active functional moieties, constituted the primary sources of large NLO coefficients. The strong ionicity of [K4Cl]3+ facilitated the occurrence of wide band gaps in [K4Cl][MGa9S16] (M = Mn, Hg).

This study offers novel insights for designing high-performance IR NLO and other optoelectronic functional materials.

Schematic diagram of respective dominant properties in two isostructural salt-inclusion chalcogenides. (Image by Prof. GUO's group)