**2. Main conditions at NLO crystal select for changing coherent energy in the IR region**

The selection of high-quality crystals is mainly tough when designing new, appropriate NLO crystals for the IR region. It should be highlighted that the balance between SHG coefficients and energy gap is an important feature to attain noble optical functioning in a mid-IR-NLO crystal [3]. The mid-IR-NLO crystals are significant to develop high-power tunable laser output extending the two atmospheric bands (3–5 μm and 8–14 μm) [4]. The accessibility of bulk-size single crystals is vital for the production of NLO devices. It is a great task to grow novel mid-IR-NLO crystals with desirable properties for useful applications. The good mid-IR-NLO crystals should satisfy the following basic criteria [2, 3, 6, 10, 12, 13]:


*Recent Advances in Infrared Nonlinear Optical Crystal DOI: http://dx.doi.org/10.5772/intechopen.108173*


## **3. Chalcogenides**

Chalcogenides are new kind of material denoting the chalcogen elements (sulfide, selenide, and telluride) of the group VIA. The chalcogenides are designed by covalent bonding and originate in a variety of structures, mostly formatted in octahedral or trigonal geometry. Chalcogenides are useful in many fields, such as photocatalyst, thermoelectric, MIR-NLO, photovoltaic, sensor, fuel cell, and battery [6, 14, 15]. Chalcogenides are suitable crystals for MIR-NLO as they exhibit wide transparency in IR regions and can obtain large SHG responses in this region [16]. Normally, chalcogenides are capable materials for MIR-NLO devices owing to their many benefits, such as large optical nonlinearity, broad transparency range, and large birefringence. II-IV-V2 and I-III-VI2 chalcopyrites are now the leading functional MIR-NLO crystals in the market and laboratory. In the past two decades, more consideration has been given to discovering chalcogenides as MIR-NLO crystals for their structural diversity. Quaternary chalcogenide crystals own a high bandgap (Eg) and high LDT. But the small nonlinearity coefficients slowed down their use in high-power laser generation. Though such kinds of crystals have many MIR-NLO benefits, many of them also have some inadequacies. To meet the requirement of laser device manufacture, some advanced growth methods are adopted to produce high purity and high-quality big size crystals of the chalcogenide. But, quaternary chalcogenide crystals, such as Li2Ga2GeS6, LiGaGe2Se6, AgGaGeS4, and Ba2GaGeS6, are grown in bulk-sized crystals, which is desirable for optical devices [5, 6, 10, 16]. There are different kinds of chalcogenides, generally, alkali metal chalcogenides and transition metal chalcogenides (TMCs), which again can be categorized into binary, ternary, and quaternary chalcogenides. The chalcogenides have weaker interatomic bonds than the oxides, resulting in good optical transparencies in the IR regions. Meanwhile, the chalcogenides exhibit adjustable structure and optical properties [6, 14, 15, 17].

#### **3.1 Cataloguing of chalcogenides based on number of components**

Chalcogenides are classified based on their number of components, such as binary, ternary and quaternary structures, number of metals, number of chalcogen ions, and so on though, both ternary and quaternary elements of chalcogenides are systematically analyzed compared to binary chalcogenides [18, 19].

#### *3.1.1 Binary chalcogenides*

Binary chalcogenides containes two kinds of ions (metal ions and chalcogen anion). The CdS, CdSe, Ga2S3, GaS, In2S3, GaSe MnS, SnS, SnS2, ZnS, and ZnSe are an example of binary chalcogenides. For instance, CdS is one of the most considered chalcogenides. It has an energy gap value of 2.3 eV and is comparatively active under visible light. Its special size and structure-based optical and electronic properties are desirable for various kinds of applications. Owing to its numerous possible applications CDS chalcogenides are assumed to be the most significant materials [18, 19].
