Recent Advances in Infrared Nonlinear Optical Crystal

*Senthil Kumar Chandran, Chinnakannu Elavarasi, Srinivasan Manikam and John James Gnanapragasam*

### **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.

**Keywords:** crystal growth, laser-induced damage thresholds, infrared crystal, transmittance, optical parameter oscillator

#### **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 highpower 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 [1–5].

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 longdistance communications. Such instruments are used to identify various elements and precise vibrational spectra [6–9]. 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, AgGaS2, 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].
