Optical Properties of Single Crystals

*Senthilkumar Chandran and Srinivasan Manikam*

### **Abstract**

Nonlinear optical crystals plays important role in the field of photo electronics, optical communication, optical modulators, laser spectroscopy, frequency conversion and so on. Semi-organic crystals exhibit high NLO response, thermal stability, laser damage threshold, mechanical stability, wide optical window transmittance and structural diversity. Combinations of inorganic and organic molecules yield the semi-organic crystals. Based on its structural diversity it's classified into three categories. In this chapter explains various kinds of semi-organic crystals and their optical, thermal, mechanical, laser damage threshold value and NLO properties and also explains the importance of these crystals in the field of optoelectronics, frequency conversion and other optical applications.

**Keywords:** nonlinear optical materials, optoelectronics, semi-organic single crystals, laser damage thresholds, optical device

## **1. Introduction**

Over the last three decades, the discovery of new crystals for optical applications has been an emerging area of research. Nonlinear optical crystals are significant in science and modern technology because of their technological importance in the areas of optical communication, optical modulators, laser spectroscopy, frequency conversion, optical bi-stable devices, electro-optical device applications in photonics technology, optoelectronics, information processing, sensors, laser technology, frequency doubling and color displays. Optical applications depend upon various physical features, such as refractive index, birefringence, thermal stability and physicochemical behaviors. Materials with high second-order optical nonlinearity, high optical transmittance with low cut-off wavelength, high laser damage threshold value and easy growth with large dimensions are needed to understand many of these applications. The growth of the new kind of optical crystals with good physical and chemical properties are very important in optoelectronics, photonics laser processing and other applications. The search for high non-linear optical crystals for efficient signal processing has been stimulated by optoelectronics [1–8].

In the field of optoelectronics and photonics, nonlinear optical (NLO) materials are capable of generating the second harmonic frequency. In various device applications, nonlinear optical (NLO) crystals with high conversion efficiency for the second harmonic generation (SHG) and transparent in ultraviolet–visible regions are required. In the current technology world, there is a lot of competition for powerful nonlinear optical devices to satisfy the day-to-day requirements. Usually, organic materials show excellent nonlinear optical (NLO) characteristics. Due to this reason, 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].
