Salt-inclusion chalcogenides: an emerging class of IR nonlinear optical materials

Non-centrosymmetric metal chalcogenides such as AgGaS 2 and AgGaSe 2 are two of the commercial nonlinear optical (NLO) crystals widely used in the infrared (IR) region. Nevertheless, the inherent incompatibility between the wide energy gap ( E g ) and large second-harmonic generation (SHG) efficiency ( d ij ) hinders their high-power laser applications. Recently, the development of salt-inclusion chalcogenides with non-centrosymmetric structures has attracted more and more attention and interest owing to their intensive potential applications originating from their wide E g , strong d ij , ultrahigh laser-induced damage thresholds (LIDTs) and large IR transmission range. In this frontier paper, we review the recent progress of salt-inclusion chalcogenides (including 28 related compounds) as favourable candidates for IR-NLO materials, which can be divided into 3 types according to their chemical compositions and structural characteristics: (i) the [R a X b ][Ga n Q 2 n ] type and its derivatives, (ii) [NaBa 4 Cl][Ge 3 S 10 ] and its derivatives, and (iii) the [A 3 X][MB 12 (MQ 4 ) 3 ] type. The relationships between the non-centrosymmetric structures and NLO properties of these 3 types of compounds are summarized and briefly remarked. In addition, the present challenges of creating new IR-NLO salt-inclusion chalcogenides and future perspectives in this field are discussed. This frontier article discusses the recent progress and challenges of non-centrosymmetric (NCS) salt-inclusion chalcogenides (SICs) in the infrared nonlinear optical (IR NLO) field.