Here, and c represent the valence and the conduction band states, respectively. ( , ) ( ) and ( , ), are the corresponding energy and eigenfunction of these states. ⃗ and ̂ are the momentum operator and polarization vector, respectively. The equation ( ) displays the connection between optical and electronic properties. Further, real part of dielectric fuction 1 ( ) is obtained by Kramer-Kronig transformation (KK) of 2 ( ) and is expressed as: