band of frequencies is from 0.3-3 THz (band number 12, Deci millimetric waves) and from 3-30TH (band number 13, Centimillimetric waves). In recent years, high bandwidth frequencies have been required to have a better wireless communication which led to fifth generation (5G) networks. This demand now leads to sixth gener- ation (6G) using terahertz waves with frequency range from 100 GHz to 10 THz. The wavelength range of THz waves is about 30 um-3 um which provides extensive band- width resulting in terabits per link with 0.1 bit/sec/Hz being feasible [Elayan, Amin, Shubair, & Alouini (2018)]. So, it became an interesting and challenging field for the researchers to work in this THz-gap. The existence of the THz-gap with the THz being similar to the heatwave, has been noted by two researchers in 1897, H. Rubens and E. F. Nichols. After that, many works have been published in this electromagnetic region under the name of 'far-infrared' or 'sub-millimeter' waves [Blaney (1975); Lamarre, Desert, & Kirchner (1995)]. However, people had no idea about trusted sources and detectors in this region. For the past three decades, it has been gradually understood that THz radiation can contribute to many potential applications [Arnone et al. (1999)]. After the discovery of graphene and carbon nanotubes, researchers have shown that these can also be a good alternative for the generation of THz radiation. A. Geim and K. Novoselov [Geim and Novoselov (2010); Liu et al. (2014)] got the Nobel prize in physics for the use of graphene in different applications including THz generation. There are many challenges in scientific and engineering areas and some of them are also due to the terahertz channels. The THz waves are similar to the RF waves in many aspects, such as directivity, diffraction, and some of the antenna properties. However, the reflectivity, absorption, and transmission through material, especially in the atmosphere, are quite distinct in nature. The terahertz band has been restricted to some applications due to the la