Materials Perspective on Organic NLO Single Crystals for Efficient Generation and Detection of Terahertz Waves

There has been incessant quest for materials suitable for nonlinear optical and, under certain conditions, for Terahertz (THz) applications. In recent years extensive research is being carried out in the field of generation and detection of Terahertz (THz) waves. These waves are in great demand due to their applications in variety of areas such as basic and applied physics, biological, medical, communications and security. This THz region remained elusive in the electromagnetic spectrum due to lack of suitable and efficient sources and detectors and this gap is referred as “THz Gap”. Although there are presently a range of methods available for the generation and detection of THz waves, only few of them, such as Photoconductive (PC), free electron laser (FEL), Optical Rectification (OR), Difference Frequency Generation (DFG) are found to be efficient and they can facilitate to narrow down the “Gap” in the electromagnetic spectrum. The OR and DFG methods depend mainly on the availability of highly efficient nonlinear optical (NLO) methods. The present chapter focuses on the basic mechanism utilizing functional materials, i.e. molecular crystals, through which the THz waves can be generated and detected. Moreover, selected NLO crystalline materials such as stilbene derivatives, nitroanilines etc. and their role in realizing the generation and detection of THz waves will be discussed in a detailed manner. NLO and EO coefficients of inorganic semiconductor alloys are considerably lower than their organic counterparts such as 4-N, N-dimethylamino-4′-N′-methyl stilbazoliumtosylate (DAST), N-benzyl-2-methyl-4-nitroaniline (BNA), (4-N,N-dimethylamino-4′-N′-methyl-stilbazolium 2,4,6- rimethylbenzenesulfonate) (DSTMS), 2-(3-(4- hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene) malononitrile (OH1) etc. The present chapter deals with the materials perspective on select group of organic NLO materials suitable for generation and detection of THz waves.