**2. Selection criteria for NLO materials**

In reality, there is no possibility to obtain the perfect nonlinear crystal. The applicability of a specific crystal depends on the nonlinear optical method used, the desired device features and the pump laser. In one application, unique and important material properties may not be relevant in another application. For example, a material with a large angular bandwidth requires the efficient doubling of very high power lasers with poor beam quality. A crystal, which has a lower nonlinearity but permits noncritical phase match angle, will work better than one, which would be more nonlinear, but is critically phase-matched. On the other side, one with a large nonlinearity would be the ideal material for the doubling of femtosecond light signal, even if a very thin crystal can be used to prevent the dispersive expansion of the second harmonic output signals. Nonlinear frequency converters are frequently employed with a capable non-tunable laser source. Kurtz and Perry powder SHG technique was introduced in 1960. In this method, the fine powdered material is irradiated with laser and scattered light is collected and studied for its harmonic capacity with the help of proper filters. This is a rapid and qualitative analyzing method for second-order NLO effect and this technique is suitable for inorganic, organic, semi-organic and new materials. To understand the nonlinear optical effect, the appropriate medium is essential. A non-centrosymmetric crystal, which shows the following characteristics, is required for nonlinear device fabrications (**Table 1**):

**129**

*Optical Properties of Single Crystals*

*DOI: http://dx.doi.org/10.5772/intechopen.95607*

**S.no Laser conditions Crystal parameters**

 Bandwidth Spectral acceptance Divergence Acceptance angle NLO method Kind of phase- Matching Repetition rate Surface damage threshold

1 Environment Temperature, Moisture acceptance 2 Beam size Crystal dimension, spatial walk-off

**3. NLO crystal of current interest**

*Parameters for selecting a nonlinear optical crystal.*

**Table 1.**

characteristics [5–12].

**4. Semi organic single crystals**

semi-organic crystals exist [1–3, 8]:

3.Organometallic compounds

2.Metal–Organic coordination complexes

1.Organic–Inorganic salts

Significant developments in new technologies are responsible for the development of new crystals of superior quality. The high-speed and significant amount of optical parallelism would ultimately lead to optoelectronics devices which were a wide range of optical functions will be implemented. The growth of photonic technology, however, depends mainly on the progress made in developing new optical material with improved performance. Crystals with the nonlinear optical response (NLO) are expected to play an important role in facilitating optoelectronic and photonic developments. In optical and electro-optical applications, several NLO crystals have been found out as potential candidates. With the development of many devices using solid-state laser sources, nonlinear optical crystals have received special revolution. For the manufacture of electro-optic modulators that converts an electrical signal into an optical signal and transmission on a fiber optic cable, NLO crystals are very important. Currently, such devices are made with inorganic NLO materials. In this view, further new other crystals should be developed. Recently researcher is showing their keen interest in the development of organic–inorganic salts which show good optoelectronics

Hybrid inorganic–organic structure materials constitute a crucial class of materials that have been extensively analyzed in the last few decades owing to their possible applications in the dielectric, optical luminescence, magnetic, and electronic properties. Because of their tremendous chemical and structural diversity and many technologically applicable properties, these are the fastest-growing fields in materials science. Hybrid inorganic–organic structural materials characterize new generations of crystalline solid-state materials which are formed from metal ions and organic linkers. It is suggested that the semi-organic crystals would have the properties of both inorganic and organic materials. There are three types of



**Table 1.**

*Optoelectronics*

**2. Selection criteria for NLO materials**

1.Good optical quality with large dimensions

4.High laser damage threshold value

6.Low absorption cut-off wavelength

5.Large birefringence

8.Easy to device fabrication

2.Wide transmittance with low cut-off wavelength

3.High thermal-mechanical and chemical stability

7.High second-order nonlinear optical coefficient

it becomes important to grow a more and more new organic-based single crystal. Most of the scientists have been focused their research on organic compounds over the past decades as it shows high nonlinear coefficients compared to inorganic materials. But apart from their nonlinearity, the organic molecules are attached with weak van der Walls and hydrogen bonds with π conjugated electrons that make the organic materials are soft, poor physico-chemical stability, low mechanical strength and difficult to polish. Further, these materials have strong absorption in the UV region. On the other hand, the inorganic materials have high laser damage threshold, high melting point and high mechanical strength, but these materials possess moderate NLO behavior. Compare to organic and inorganic materials, the semiorganic materials show combining the properties of both organic and inorganic materials. In this view, semi-organic materials must be analyzed [4–6, 9–12].

In reality, there is no possibility to obtain the perfect nonlinear crystal. The applicability of a specific crystal depends on the nonlinear optical method used, the desired device features and the pump laser. In one application, unique and important material properties may not be relevant in another application. For example, a material with a large angular bandwidth requires the efficient doubling of very high power lasers with poor beam quality. A crystal, which has a lower nonlinearity but permits noncritical phase match angle, will work better than one, which would be more nonlinear, but is critically phase-matched. On the other side, one with a large nonlinearity would be the ideal material for the doubling of femtosecond light signal, even if a very thin crystal can be used to prevent the dispersive expansion of the second harmonic output signals. Nonlinear frequency converters are frequently employed with a capable non-tunable laser source. Kurtz and Perry powder SHG technique was introduced in 1960. In this method, the fine powdered material is irradiated with laser and scattered light is collected and studied for its harmonic capacity with the help of proper filters. This is a rapid and qualitative analyzing method for second-order NLO effect and this technique is suitable for inorganic, organic, semi-organic and new materials. To understand the nonlinear optical effect, the appropriate medium is essential. A non-centrosymmetric crystal, which shows the following characteristics, is required for nonlinear device fabrications

**128**

(**Table 1**):

*Parameters for selecting a nonlinear optical crystal.*
