Recent Advances in Infrared Nonlinear Optical Crystal

Abstract

The search and growth of nonlinear optical (NLO) crystals in the infrared (IR) area are significant and of high importance in the fields of NLO, signal communication, solid-state chemistry, and laser frequency conversion. Infrared NLO crystals have a wide IR transparent range, high laser damage threshold (LDT) value, and large NLO coefficients. This chapter presents the recent advances in IR-NLO crystals and especially emphasizes their crystal growth method, crystal structures, band gap value, LDT, and NLO properties. Based on its structural variety, it is categorized into chalcogenides, chalcohalides, oxides, halides, and oxyhalides. This chapter describes several kinds of IR-NLO crystals and their structural, band gap value, thermal, optical, LDT, and NLO properties and also describes the significance of these crystals in laser frequency conversion, optical parameter oscillator, and other optical applications.

1. Introduction

Nonlinear optical (NLO) crystals are important for frequency conversion and are widely used in different laser-oriented applications. High-efficiency NLO crystals need in high-efficiency laser methods, so it is essential to growing novel NLO crystals with good properties. In the past five decades, many valuable NLO crystals in the near-infrared (NIR), visible, and ultraviolet areas have been commercialized, such as LiNbO₃, LiB₃O₅ (LBO), β-BaB₂O₄ (β-BBO), KTiOPO₄ (KTP), and KH₂PO₄ (KDP). These crystals are commonly used in basic science and technology, such as laser generation, artificial nuclear fusion, and so on. However, due to the increasing practical or market necessities, only a few crystals can be successfully used in deep-UV(DUV) and mid/far-IR areas. Nevertheless, NLO crystals that can powerfully produce high-power mid-IR lasers in the spectral area of 2−25 μm are very rare. Up to now, several useful NLO crystals have been originated and used in DUV, UV-Vis, and near IR, but they cannot be implemented in mid-infrared spectra because they have two atmospheric transparent regions, 3–5 and 8–14 mm, owing to strong absorption [1, 2, 3, 4, 5].

Second-order nonlinear optical (NLO) crystals are significant for producing coherent energy in the IR region (3−20 μm). IR lasers have several vital applications in various devices, such as optical parametric oscillators (OPO), remote sensing, optical sensing, instrumental spectroscopy, industry, military, analytical devices optical imaging, laser guidance, telecommunication, medical diagnostics, and long-distance communications. Such instruments are used to identify various elements and precise vibrational spectra [6, 7, 8, 9]. Even after several years of deep research, only three unresolved NLO crystals have been commercially accessible in the mid and far-IR areas, namely, AgGaSe₂, AgGaS_2, and ZnGeP₂. However, some inadequacies still exist in these IR-NLO crystals, such as the inherent efficiency loss arising from dual photon absorption, low MIR cut-off edge, non-phase matchable behavior, and low laser damage threshold. Already commercial ones cannot meet the commercial conditions because of their inherent disadvantages. Hence, it is essential to find out new efficient MIR NLO crystals with more stable efficiency [3, 7, 8, 10].

The large size high-quality mid-IR-NLO crystals for laser device applications are grown by Bridgman-Stockbarger (BS) method and molecular beam epitaxy (MBE) growth method [5]. The IR-NLO crystal can be separated into five categories: chalcogenides, chalcohalides, oxides, oxyhalides, halides, and chalcohalides [4]. This chapter will emphasize second-order NLO inorganic crystals in the MIR region. We did not focus on the commercially accessible LiNbO₃, LiB₃O₅ (LBO), β-BaB₂O₄ (β-BBO), KH₂PO₄, and KTiOPO₄ NLO crystals, these crystals have absorption bands in this region. Instead, this chapter focus on the chalcogenides, chalcohalides, oxides, halides, and oxyhalides. These crystals are promising materials for MIR applications due to they have wide transmittance in the MIR region [11].

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]:

1. High second harmonic generation (SHG) responses

2. High laser damage threshold

3. Wide IR optical transparency range

4. Moderate birefringence

5. Good thermal stability

6. Good mechanical properties

7. Good facile growth of big size crystals

8. Splendid chemical stability

9. Crystal with non-centrosymmetric space group

10. Congruent-melting performance to enable single-crystal growth

11. Low absorption loss at suitable laser wavelengths

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, AgGaGeS_4, and Ba₂GaGeS_6, 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].

3.1.2 Ternary chalcogenides

When selecting a crystal for laser energy conversion in the IR, it is essential to have an ideal mixture of various considerations like birefringence value not less than 0.03 and LDT value of around 100 Mcm⁻², energy gap value of more than 3.3 eV and the NLO coefficient should be more than 4 pm V⁻¹ [20]. The Li- and Ba-having chalcogenides meet these desires. The ZnGeP₂, AgGaSe₂, CdSiP_2, and AgGaTe₂ crystals have high NLO susceptibility, but their forbidden energy band is too low. The most commonly used nonlinear crystals for the MIR are AgGaS₂ AgGaSe_2, and ZnGeP₂. However, they all own serious disadvantages [6, 21, 22]. In recent times, consideration was given to chalcogenide crystals, such as alkali and alkali-earth metals (Li and Ba) (Table 1). These crystals permit one to resolve some difficulties in the MIR region. The birefringence value in Li-comprising crystals is significantly larger. LiBC₂ (B = Ga; C = S,Se) crystals can be applied for SHG applications wavelengths between 1.4 and 12 μm [23, 24]. Telluride crystals also have MIR properties, especially LiGaTe₂ has phase-matching in the entire transparency region. The SHG conversion efficiency of LiGaTe₂ is 10.6 μm, which is higher than that of AGSe [25]. To enhance the energy gap, Ag cation is to be substituted with alkali/alkaline earth metal (Li, Ba). Adding these metal, we can get LiBC₂ (B = In, Ga; C = S, Se, Te) and BaGa4C7 (C = S, Se) group crystals. These crystals own a high bandgap value [26, 27, 28]. A little mass of Li is the reason for high thermal conductivity and high vibrational frequencies. The thermal conductivity of Li mixtures is around five times higher than that of AGS (Se) [29] and four to eight times more than that of BaGa₄S₇ (Se) (Figure 1) [6, 28]. The laser damage threshold for LGS is 3.5 Jcm⁻², which is five times larger than LISe [30]. A similar result has been obtained when the Ag ion is substituted with Ba. BaGa₄S₇ crystal has high LDT and NLO susceptibility. Though, Ba cation slightly drops the band gap value. The point group is mm₂ and m for BaGa₄S₇ (Figure 1) and BaGa₄Se₇, respectively. Wide transparency regions of 0.35–12 and 0.47–15.0 μm, and energy gaps of 3.54 and 2.64 eV were found for both BaGa₄S₇ and BaGa₄Se₇ crystals, respectively. Both crystals have strong absorption peak at 15 μm. The BaGa₄Se₇ showed high nonlinear susceptibility of d₁₁ = 18.2 pm V⁻¹ [6, 31, 32].

Crystal	Point group	Transparency range (μm)	Band gap (ev)	Nonlinear coeff. (pm V^−1)	Laser damage threshold (MW cm^−2)
AgGaS2	42 m	0.47–13	2.7	d32 = 8, 1.06 d36 = 19, 1.06	34 at 10 ns, 1064 nm
ZnGeP2	42 m	0.74–12	2	d36=75, 9.6	100 at 10 ns, 2000 nm
AgGaSe2	42 m	0.76–18	1.8	d32 = 19.6,3.4 d36 = 39, 1.06	13 at 30 ns, 2000 nm
LiGaTe2	42 m	0.52–20	2.31	d36 = 43, 4.5	—
LiGaS2	mm2	0.32–12	4.15	d31 = 5.8, 2.3 d24 = 5.1, 2.3	240 at 14 ns, 1064 nm
LiGaSe2	mm2	0.37–14	3.57	d31 = 9.9,2.3 d24 = 7.7, 2.3	80 at 5.6 ns, 1064 nm
LiInS2	mm2	0.34–13.2	3.57	d31 = 7.25 d24 = 5.66, 2.3	40* 14 ns, 1064 nm
LiInSe2	mm2	0.46–14	2.86	d31 = 11.78 d24 = 8.17, 2.3	40* 10 ns, 1064 nm
BaGa4S7	mm2	0.35–13.7	3.54	d32 = 5.7, 2.3	250 at 14 ns, 1064 nm
BaGa4Se7	m	0.47–18	2.64	d11 = 18.2, 2.3 d13 = −0.6, 2.3	—

Table 1.

Point group, transparency range, band gap, and LDT value of ternary chalcogenides.

Figure 1.

Single crystals of BaGa₄S₇.

3.1.3 Quaternary chalcogenides

Quaternary chalcogenides have four kinds of ions together with a chalcogen anion. These kinds of materials have different applications, such as MIR-NLO, solar cell absorbers, photocatalysts, and so on. Quaternary materials adopt different kinds of elements, which permits comparatively complex structural, electronic, and optical properties [6, 33]. Using the quaternary crystals, the IR-NLO parameters can be enhanced with a high content of NLO-active parts [6]. A compact organization of the microscopic NLO-active parts increases high macroscopic NLO outcomes [6]. The birefringence value of AgGaSe₂ is 0.05 and this value for AgGaGe₃Se₈ is 0.11. The enhanced LDT value of AgGaGeS₄ shows that it is a potential alternative crystal to the generally used AgGaS₂ for IR-NLO applications. Li₂In₂SiSe₆, Li₂In₂GeSe₆, Li₂Ga₂GeS_6, and LiGaGe₂Se₆ are the Li-having quaternary chalcogenides crystals (Table 2) [6, 34, 35, 36, 37]. All these crystals have non-centrosymmetric crystal structures. Li₂Ga₂GeS₆ (Figure 2), LiGaGe₂Se₆ (Figure 3) crystals have orthorhombic crystal systems with space group Fdd₂ and Li₂In₂GeSe₆, Li₂In₂SiSe6 crystals own monoclinic crystal systems with space group Cc. Ba-having quaternary BaGa₂GeS₆, BaGa₂GeSe₆ structures, which are promising crystals for NLO applications [37]. The NLO susceptibilities of Li₂Ga₂GeS₆ are 16 pm/V, which is significantly higher than LiGaS₂ (5.8 pm/V). A similar result was noted for LiGaGe₂Se₆. BaGa₂GeS₆ and BaGa₂GeSe₆ crystals also have improved NLO parameters. The SHG experiments showed that both materials have phase-matched behavior. The calculated SHG coefficient is ∼2.1 and ∼3.5 times higher than that of AgGaS₂. The nonlinear susceptibilities are 26.3 pm/V and 43.7 pm/V for BaGa₂GeS₆ and BaGa₂GeSe₆, respectively [37]. The transparency region in Li₂Ga₂GeS₆ and LiGaGe₂Se₆ is 0.35–12 and 0.37–14 μm, respectively. For BaGa₂GeS₆ and BaGa₂GeSe_6, the transparency regions are 0.380–13.7 μm and 0.44–18 μm, respectively. Band gaps of BaGa₂GeS₆ and BaGa₂GeSe₆ are 3.26 and 2.81 eV, respectively. Li₂In₂GeSe₆ and Li₂In₂SiSe₆ crystals have the energy gap values of 2.30 and 3.61 eV, respectively [6].

Crystal	Point group	Transparency range (μm)	Band gap (ev)	Nonlinear coeff., (pm V^−1)	Optical damage threshold (MW cm^−2)
AgGaGeS4	mm2	0.42–12	2.8	d31 = 15, 1.06	50 at 15 ns, 1064 nm
AgGaGe3Se8	mm2	0.6–18	2.4	d31 = 33.4	–
Li2Ga2GeS6	mm2	0.35–14	2.51	deff = 16, 1.06	>50 at 15 ns, 1064 nm
LiGaGe2Se6	mm2	0.47–18	2.64	d15 = 18.6, 2.09	50 at 10 ns, 1064 nm
Li2In2GeS6	m	0.36	3.45	≈d36 = 12.6, 10.6	–
Li2In2GeSe6	m	0.54	2.30	≈d36 AGSe	–
Li2In2SiS6	m	0.34	3.61	≈d36 AGS	–
BaGa2GeS6	3	0.38–14	3.26	deff = 26.3, 2.09	–
BaGa2GeSe6	3	0.44–18	2.81	deff = 43.7, 2.09	–

Table 2.

Point group, transparency range, band gap, and LDT value of quaternary chalcogenides.

Figure 2.

Crystal of LiGaGe₂Se₆.

Figure 3.

Crystal structure of LiGaGe₂Se₆.

4. Chalcohalides

Chalcohalides could be considered potential materials for mid-IR applications. The chalcohalides contain a combination of sulfur and halogen. Chalcohalides have non-centrosymmetric coordinated surroundings that can stimulate mid-IR-NLO efficiency. Based on the chemical compounds present in the chalcohalides, it can be divided into [4]:

1. Alkali/alkaline-earth metal chalcohalides

2. Adduct-type chalcohalides

3. Lewis acid adducts chalcohalides

4. Main group element clusters chalcohalides

5. Other group main metal chalcohalides

6. Transition metal chalcohalides

4.1 Alkali/alkaline-earth metal chalcohalides

The Ba₄Ge₃S₉Cl₂ chalcohalide has excellent mid-IR-NLO properties [38]. It has a space group P6₃. The bandgap energy of Ba₄Ge₃S₉Cl₂ was 2.91 eV. The SHG response of Ba₄Ge₃S₉Cl₂ was 2.4 times higher than that of AGS [38]. The bandgap of Ba₄Ge₃S₉Cl₂ is 2.67 eV, which was calculated by the DFT method. NLO coefficients are found to be d₁₅ = d₂₄ = 7.61 and d₃₃ = 13.81 pm/V. The bandgap of NaBa₄Ge₃S₁₀Cl was 3.49 eV and SHG efficiency was 0.3 times that of AgGaS₂ (AGS) [39]. The theoretically calculated bandgap of this crystal is 2.94 eV. Then, the NLO coefficients are calculated to be d₁₅ = d₂₄ = 3.89 and d₃₃ = 9.32 pm/V. The mid-IR-NLO crystals [A₃X] [Ga₃PS₈] (A = K, Rb; X = Cl, Br) were synthesized and reported by B.W. Liu et al. in the year 2016 [40]. Two Cl-crystals [K₃Cl][Ga₃PS₈] and [Rb₃Cl][Ga₃PS₈] are having isostructural with space group Pmn₂₁, while the other two Br-crystals [Rb₃Br][Ga₃PS₈] and [K₃Br][Ga₃PS₈] belong to Pm space group. These four compounds showed outstanding mid-IR-NLO behavior and the energy gaps of [K₃Cl][Ga₃PS₈], [Rb₃Cl][Ga₃PS₈], [K₃Br][Ga₃PS₈], and [Rb₃Br][Ga₃-PS₈] are 3.60, 3.65, 3.85, and 3.50 eV, respectively. All four compounds showed large SHG responses of 4.0, 5.0, 7.0, and 9.0 times that of AgGaS₂ (AGS) at 1064 nm [39]. Moreover, these four compounds have higher laser threshold damage (LDT) of 37, 35, 31, and 29 times than that of AGS (Table 3).

Crystal	Space group	Band gap (ev) (DFT)	SHG AgGaS2 (AGS) times	Nonlinear coefficients (pm/V)	Laser threshold damage (LDT) AgGaS2 (AGS)
Ba4Ge3S9Cl2	P63	2.67	2.4	d15 = d24 = 7.61 and d33 = 13.81	–
NaBa4Ge3S10Cl	—	3.49	0.3	are d15 = d24 = 3.89 and d33 = 9.32	–
[K3Cl][Ga3PS8]	Pmn21	3.60	4.0	–	37
[Rb3Cl][Ga3PS8]	Pmn21	3.65	5.0	–	35
[K3Br][Ga3PS8]	Pm	3.85,	7.0	–	35
[Rb3Br][Ga3PS8]	Pm	3.50	9.0	–	29

Table 3.

Space group, band gap, and LDT value of alkali/alkaline-earth metal chalcohalides.

4.2 Adduct-type chalcohalides

The (SbI₃) (S8)₃ and (SnI₄)-(S8)₂ are the adduct-type of chalcohalides. The (TI₃). (S₈)₃ (T = As, Sb) [4, 41, 42, 43, 44] adduct-type chalcohalides have isostructural with R₃m space group (Table 4). The (SbI₃). (S₈)₃ and (AsI₃). (S₈)₃ chalcohalides have reasonable energy gap value of 2.52 and 2.31 eV and their theoretical energy gaps are 2.69 and 2.21 eV, respectively. SHG responses of these two chalcohalides have 1.0 and 0.8 times that of AgGaS₂. They also have wide IR transparent wavelengths around 2.5–25.0 μm and 0.4–25.0 μm [41, 42]. The calculated NLO coefficients are d₁₅ = 9.21, d₂₂ = 9.22, d₃₃ = 6.91 pm/V for (SbI₃).(S₈)₃ and d₁₅ = 3.40, d₂₂ = 6.21, d₃₃ = 0.73 pm/V for (AsI₃).(S₈)₃. The chalcohalides (SnI₄). (S₈)₂ has space group Fdd₂ [44] and a wide IR window in the range of 2.5–25.0 μm. The energy gap of (SnI₄). (S₈)₂ is 2.17 eV. The SHG response of (SnI₄). (S₈)₂ is 0.5 times stronger than that of AgGaS₂ at 2.1 μm.

Crystal	Space group	Band gap (ev)	SHG AgGaS2(AGS) times	IR transparent ranges (μm)	NLO coefficients (pm/V)
(SbI3) (S8)3	R3m	2.52	1.0	2.5–25.0	d15 = 9.21, d22 = 9.22, d33 = 6.91
(AsI3). (S8)3	R3m	2.31 eV	0.8	0.4–25.0	d15 = 3.40, d22 = 6.21, d33 = 0.73
(SnI4). (S8)2	Fdd2	2.17 eV	0.5	2.5–25.0	–

Table 4.

Space group, band gap, SHG, and transparency range value of adduct-type chalcohalides.

4.3 Lewis acid adduct chalcohalides

Lewis acid adduct chalcohalides have sulfur-nitrogen rings with a variety of structures. These chalcohalides have moderate band gaps and IR-NLO properties (Table 5). A new chalcohalide (NSF)₄ [45] has a space group of P-421c. The calculated energy gaps are 4.57 eV (HSE06 method) and 3.58 eV (GGA method). For a high LDT, the large bandgap is more advantageous. The birefringence for (NSF)₄ is 0.220 at 1064 nm and the calculated NLO coefficient for (NSF)₄ is d₁₄ = 3.20 pm/V. Increased bandgap occurs due to the large electronegativity of F atoms in the compound. S₃N₅PF₂ crystal [46] has an R₃m space group. It has a wide bandgap of 3.49 eV (HSE06). The estimated birefringence is around 0.110 at 1064 nm and NLO coefficients are d₁₅ = 1.71, d₂₁ = 0.19 and d₃₃ = 3.69 pm/V. The d₃₃ coefficient is around 9.2 times stronger than that of KDP.

Crystal	Space group	Band gap (HSE06) (ev)	NLO coefficient (pm/V)
(NSF)4	P-421c	4.57	d14 = 3.20
S3N5PF2	R3m	3.49	d15 = 1.71, d21 = 0.19 and d33 = 3.69

Table 5.

Space group and band gap, and value of Lewis acid adduct chalcohalides.

4.4 Main group element clusters chalcohalides

The chalcohalides (Bi₄S₄)(AlCl₄)₄ belong to the space group I-4 (Figure 4) [47] and it has two classes of main group component clusters that are in AlCl₄ tetrahedron and Bi₄S₄ cube. The energy gap of (Bi₄S₄)(AlCl₄)₄ is calculated to be 3.59 eV. (Bi₄S₄)(AlCl₄)₄ is the largest main group element cluster chalcohalide, which has an NLO coefficient value of d₁₄ = 1.52 pm/V and it is 3.8 times stronger than that of KDP. The DFT method showed that the main group component cluster chalcohalides has insignificant NLO coefficients.

Figure 4.

Crystal structure of (Bi₄S₄)(AlCl₄)₄.

4.5 Other main groups of metal chalcohalides

Both In₅S₅Cl [48] and In₅S₅Br [49] have isostructural properties, which are having the space groups Pmn21. The In₅S₅Cl and In₅S₅Br materials have band gap of 1.76 and 1.84 eV, respectively (Table 6). The estimated NLO coefficients are found to be d₁₅ = 0.36, d₂₄ = 2.83, d₃₃ = 13.38 pm/V for In₅S₅Cl and d₁₅ = 2.07, d₂₄ = 2.21, d₃₃ = 7.38 pm/V for In₅S₅Br. When compared with the NLO coefficients of AGS, the main group metal chalcohalides In₅S₅Cl and In₅S₅Br have d₃₃=1.0 and 0.5 times than that of AGS. The (CS₃N₂Br)Br₃ main group metal chalcohalides [50] crystallizes in space group Pna₂₁. The estimated energy gap for (CS₃N₂Br)Br₃ is 2.21 eV (HSE0₆), and the NLO coefficients of (CS₃N₂Br)Br₃ are d₁₅ = 8.90, d₂₄ = 5.40, d₃₃ = 1.00 pm/V. From the NLO coefficient result, d₁₅ has a good NLO coefficient, which is about 0.6 times that of AGS.

Crystal	Space group	Band gap (HSE06) ev	NLO coefficient (pm/V)	NLO coefficient (AGS) times
In5S5Cl	Pmn21	1.76	d15 = 0.36, d24 = 2.83, d33 = 13.38	1.0
In5S5Br	Pmn21	1.84	d15 = 2.07, d24 = 2.21, d33 = 7.38	0.5
(CS3N2Br)Br3	Pna21	2.21	d15 = 8.90, d24 = 5.40, d33 = 1.00	0.6

Table 6.

Space group, band gap, and SHG value of other main groups of metal chalcohalides.

4.6 Transition metal chalcohalides

Asymmetric distribution of electron clouds is generally caused by polyhedron with the d₁₀ transition metals because of the dp orbital hybridization and group distortion, so it produces high SHG behavior. Due to the dp hybridization, the compound has a red-shifted absorption edge, this might reduce the bandgap of IR-NLO materials. To enhance the chalcogenides energy gap, halogen elements are introduced, that is, by combining cations with d10 configuration, which led to the equilibrium among energy gaps and SHG behavior in the IR-NLO crystals. So, they separated the asymmetric chalcohalides with a d₁₀ electronic configuration. The transition metal chalcohalide Ag₂HgSI₂ has the space group Cmc₂₁ [51]. The bandgap of Ag₂HgSI₂ is 2.65 eV and it is compared with AGS of (2.70 eV). The SHG of Ag₂HgSI₂ is 4.2 times stronger than that of KDP. The theoretical birefringence of Ag₂HgSI₂ is 0.210 at 1064 nm. The crystal (P4S₃)₃(CuCl)₇ belongs to the space group P₃₁c [52]. The bandgap of (P₄S₃)₃(CuCl)₇ is 2.77 eV (HSE0₆ method) and NLO coefficients of (P₄S₃)₃(CuCl)₇ are d₃₃ = 3.34 and d₁₅ = d₂₄ = 1.81 pm/V. In the NLO coefficients, the d₃₃ value is about 8.5 times that of KDP and has a birefringence value of 0.150 at 1064 nm.

5. Oxide crystal

Normally, oxide-based materials have a high laser damage threshold and many of them have limited IR transparency within the range of 6 μm. Here, we would like to explain the new oxide-based NLO materials, which are very important for the development of IR transparent regions beyond 6 μm. Several new NLO oxide crystals were identified, which have high IR cutoff wavelengths that are up to 6 μm. For instance β-BaTeMo₂O₉ [53], MnTeMoO₆ [54], Cs₂TeW₃O₁₂ [55], V₂Te₂O₉ [56], Li₃VO₄ [57] , M₂LiVO₄ (M = Rb, Cs) [58], Li₂K₄TiOGe₄O₁₂ [59], and Rb₄Li₂TiOGe₄O₁₂ [60]. Moreover, for the oxide-based NLO material, IR transparent range is a difficult factor to attain. Pb₁₇O₈Cl₁₈ was new IR-NLO material discovered by Pan and Poeppelmeier et al. [61] in 2015. Pb₁₇O₈Cl₁₈ single crystal was synthesized by modified spontaneous crystallization in an open model. Tao’s et al. [62] reported a new LiNbO₃-type NLO crystal. Li₂ZrTeO₆ crystal (size: 16 × 15 × 12 mm³) is grown by top-seeded solution growth (TSSG) method. To maintain the structural qualities of LiNbO_3, Zr⁴⁺and Te⁶⁺ were substituted for Nb⁵⁺ to form Li₂ZrTeO₆ and it crystallized in the trigonal crystal system with space group R3 (Figure 5). LiNbO₃ belongs to the space group, R₃c, which has a close structural feature with Li₂ZrTeO₆ [63]. La₃SnGa₅O₁₄ is a new IR-NLO crystal, it belongs to the langasite family and has a space group P321 [64]. Polycrystalline La₃SnGa₅O₁₄ was produced using a solid-state reaction and a single crystal were grown by Czochralski method. The single crystal of Pb₁₇O₈Cl₁₈ has a bandgap of 3.44 eV and it has high IR transparency (13.9 μm). La₃SnGa₅O₁₄ has a wide energy gap value of 4.60 eV and transparency of 10 μm.

Figure 5.

Crystal structures of Li₂ZrTeO₆.

The SHG responses of Pb₁₇O₈Cl₁₈ showed a response at 2090 nm and 1064 nm, which is phase-matchable, and it is two times stronger than that of AgGaS₂ and four times higher than that of KDP. Li₂ZrTeO₆ showed a huge powder SHG behavior at 1064 nm, which is 2.5 times higher than that of KDP [63]. For Li₂ZrTeO₆ and LiNbO_3, there is a variation in the SHG responses, which is closely correlated, and it is due to the different sizes of octahedral distortions in their crystal structure. The SHG efficiency of La₃SnGa₅O₁₄ is 0.4 times that of AgGaS₂ and when compared to AgGaS₂ (13 μm) it has wide IR transparency. Pb₁₇O₈Cl₁₈ has an LDT of 12.8 MW/cm² (Table 7). These characteristics reveal that Pb₁₇O₈Cl₁₈ is one of the good mid-IR-NLO crystals for the next generation. Li₂ZrTeO₆ has an outstanding optical performance and a very high LDT greater than 1.3 GW/cm²and it was more than 22 times that of LiNbO₃. It has a higher IR transparent range that is up to 7.4 μm. La₃SnGa₅O₁₄,which has the highest LDT of 846 MW/cm², is an alternative NLO crystal in the mid-IR region. It is transparent beyond 10 μm. The langasite family provided valuable information to design a new NLO crystal in the IR area.

Crystal	Space group	Bandgap (eV)	IR range	SHG	LDT (× AGS, MW/cm^2)
Li2ZrTeO6	R3	4.06	7.4 μm	2.5 times (KDP)	> 1300 MW/cm^2
La3SnGa5O14	P321	4.60 eV	10 μm	0.4 times that of AgGaS2	28 × AGS, 846 MW/cm2
Pb17O8Cl18	Fmm2	3.44 eV	13.9 μm	2 times (AgGaS2) and 4 times (KDP)	12.8 × AGS (on powder)

Table 7.

Space group, band gap, transparency range, and SHG value of oxide crystal.

6. Halides

A halide has a binary form in which one region is a halogen and the other region is a component or radical that is less electronegative/electropositive than the halogen [65]. For preparing NLO halides, the physical, chemical, and crystallographic aspects are important. In halides, hyperpolarizabilities were used to find out the NLO susceptibilities. Moreover, these crystals have high LDT and good mechanical properties. HgBr₂ crystal was grown using a lowering temperature technique with a size of about 15 × 15 × 1.5 mm³ [66]. Tl₃PbBr₅, Tl₄PbI₆, Tl₄HgI_6, and Tl₃PbI₅ were grown by the vertical Bridgman method [5]. The 3:1:5 ratio showed orthorhombic symmetry and the 4:1:6 ratio indicated tetragonal symmetry. In this method, they maintained the temperature gradients in the range from 20 K/cm to 30 K/cm and cooling rates were in the range from 5 to 10 K/h, which showed a growth of 1 cm/day and 3–5 cm/day. Tl₃PbCl₅ [67] and Tl₃PbBr₅ [68] single crystals were grown by Bridgman-Stockbarger technique. Tl₃PbI₅ is colorless and Tl₃PbBr₅ (Figure 6) crystal is a yellow color and transparent. Tl₄HgI₆ crystal is grown using Bridgman-Stockbarger method [69, 70] and they melt consistently at 396°C. The crystal was red and when the iodine concentration increases in the stoichiometric ratio, the crystal becomes changed to black. It belongs to the point symmetry group C₄v. BaMgF₄ crystal was grown using the Czochralski technique [71]. It belongs to the pyroelectric fluoride group BaMF₄ (M = Mg, Co, Ni, Zn) and it has space group Cmc₂₁. SrAlF₅ crystal belongs to the class of uniaxial ferroelectric [72] and is grown using Czochralski technique [71]. Tl₃PbCl₅ and Tl₃PbBr₅ are nonhygroscopic. In Tl₃PbBr_5, the phase transition is observed at ~237°C, and for Tl₃PbCl_5, phase transition is noted at 171°C. Tl₂HgI₄ compound has a melting point temperature of 318°C.

Figure 6.

Crystal of Tl₃PbBr₅.

HgBr₂ crystal showed good phase matchable SHG efficiency, which is 10 times greater than that of KDP and has transparency between 2.5 and 25 mm. It covers the whole mid-IR range. Tl₃PbCl₅ and Tl₃PbBr₅ have transparency of 0.5–20 mm and 0.65–24 mm, respectively. The Tl₄HgI₆ crystal is optically positive and transparent vfrom 1.2 to 40 mm. BaMgF₄ is a ferroelectric fluoride due to its wide transparency between 125 nm and 13 mm, and it can be used for UV and mid-IR optical applications [71, 72]. In the UV region, the shortest band is noted at 368 nm, which represents the potential behavior of BaMgF₄ as a nonlinear material. It can be used for the production of all solid-state lasers and mid-IR wavelength areas. BaMgF₄ and SrAlF₅ crystals are promising crystals for solid-state lasers. The LDT value of the HgBr₂ crystal is 0.3 GW/cm².

7. Oxyhalides

To develop high efficiency in mid-IR-NLO crystals, it is important to have an oxide-based system with an expandable IR transparent range. Many NLO oxides have a range from 3 to 5 μm atmospheric windows. When compared with the AgGaS₂ (13 μm), the oxide-based materials, such as La₃SnGa₅O₁₄ and Pb₁₇O₈Cl_18, have wide IR transparency between 12 and 13.9 μm. In this lead oxyhalide, NLO crystal Pb₁₇O₈Cl₁₈ plays an outstanding overall property. To obtain high LDT, halide and oxide-based crystals with high bandgaps are used. Nowadays, the mixture of heavy metal lone pair cation, Pb²⁺, and mixed oxyhalides are focused on IR applications. Xinglong Chen et al. discovered the lead mixed oxyhalides, such as Pb₁₃O₆C_l9Br₅, Pb₁₃O₆C_l7Br_7, and Pb₁₃O₆C_l4Br₁₀ [73], which have broad IR transparency up to 14 μm, high SHG behavior (0.6–0.9 × AgGaS₂) and wide bandgaps from 3.05 to 3.21 eV. Pb₁₃O₆C_l9Br₅ single crystal has the size of 2.9 × 1.3 × 0.5 cm³, which was grown using the top-seeded solution growth (TSSG) method. It has a wide transparent range from 0.384 to 14.0 μm and a high LDT value (14.6 × AgGaS₂). Many crystals, such as APbCO₃F (A = Rb, Cs), Pb₂BO₃Cl, Cs₃VO(O₂)₂CO₃, Bi₃TeBO_9, and BiFSeO₃ [74, 75, 76, 77, 78], have been reported with outstanding properties and it is a very good material for visible/near-IR nonlinear optical applications. Pb₁₃O₆C_l4Br₁₀, Pb₁₃O₆C_l7Br₇, and Pb₁₃O₆C_l9Br₅ have orthorhombic crystal structures with space group Fmm2 (Table 8). These three crystals are isomorphic.

Crystal	Space group	SHG (AgGaS2)	LDT (AgGaS2)
Pb13O6Cl4Br10	Fmm2	0.6	3.0
Pb13O6Cl7Br7	Fmm2	0.8	3.2
Pb13O6Cl9Br5	Fmm2	0.9	4.0

Table 8.

Space group and SHG value of oxyhalides.

By using Czochralski and flux method, oxide-based crystals can be obtained. The Pb₁₃O₆C_l4Br₁₀, Pb₁₃O₆C_l7Br₇, and Pb₁₃O₆C_l9Br₅ crystals were grown by flux method and the self-flux method was adopted for PbCl₂-PbBr₂. The crystals obtained by this method were optically transparent and they showed a good growth rate. Pb₁₃O₆Cl₉Br₅ was used to grow large-size crystals using the TSSG technique. After many technical optimizations, two big size crystals (Dimensions up to 2.9 × 1.3 × 0.5 and 3.7 × 0.4 × 0.7 cm³) were grown using the [001] and [100] oriented seeds. They have good transparency and good growth speed. Therefore, to get a better quality crystal the growth parameters (cooling rate, rotation speed, and temperature gradient) are very important in the crystallization process. DSC curves of Pb₁₃O₆Cl₄Br₁₀, Pb₁₃O₆Cl₇Br_7, and Pb₁₃O₆Cl₉Br₅ show that each of them has one endothermic peak at 501°C, 504°C, and 508°C, respectively, which belongs to the melting point and these crystals have two exothermic peaks, which indicate the decomposing of the compounds. Due to the volatility of the halide materials, there is no weight loss before 490°C in the TGA curves. From the DSC and TGA, it was concluded that these compounds have high thermal stability up to 490°C. Pb₁₃O₆C_l9Br₅, Pb₁₃O₆Cl₇Br_7, and Pb₁₃O₆C_l4Br₁₀ have high reflectance wavelengths in the region between 500 and 2500 nm. When compared with AgGaS₂ (2.67 eV, 0.53–13 μm) and ZnGeP₂ (1.68 eV, 0.74–12 μm), the crystals of Pb₁₃O₆C_l4Br₁₀, Pb₁₃O₆C_l7Br₇, and Pb₁₃O₆C_l9Br₅ own greater energy gaps and good transparency in IR region [79, 80].

In the application of high-power laser systems, the NLO crystal with laser-induced damage is one of the biggest problems. The crystals which have wider bandgaps are subjected to higher LDTs. Polycrystalline samples are used for the laser-induced damage threshold evaluation, and the polycrystalline material of AgGaS₂ was used as reference material. The Pb₁₃O₆C_l4Br₁₀, Pb₁₃O₆C_l7Br_7, and Pb₁₃O₆C_l9Br₅ have large LDTs values, which are 3.0, 3.2, and 4.0 times higher than that of AgGaS₂. The Pb₁₃O₆C_l9Br₅ crystal has LDT of 439 MW/cm^2, this is 14.6 times higher than that of the AgGaS₂ crystal (30 MW/cm²). The second harmonic generation intensity is 0.5 times higher than that of AgGaS₂. The SHG value of Pb₁₃O₆C_l4Br₁₀, Pb₁₃O₆C_l7Br_7, and Pb₁₃O₆C_l9Br₅ are found to be 0.6, 0.8, and 0.9, respectively. From this, we concluded that SHG responses of each material have all phase-matchable when it is under the 2090 nm wavelength. Due to the increase in particle size, it showed a positive movement in SHG response.

8. Conclusion

In summary, significant progress has been attained in the search for new, favorable IR nonlinear crystals, such as chalcogenides, chalcohalides, oxides, halides, and oxyhalides, for producing coherent energy in the MIR. For a good NLO crystal, the crystals must have good physical and chemical properties, such as wide transparency range, LDT, chemical stability, birefringence, and nonlinear susceptibility. Nevertheless, for practical applications, the crystals should have high qualities with promising properties. For promising optical applications, accurate optical properties should be measured. For MIR applications, high bandgap value, LDT, and NLO susceptibility optical transparency are important. Various IR crystals fulfill the basic conditions of IR-NLO applications. The significant behavior of these crystals recommends that this crystal can be used in different optical applications.