Before revision: Neglecting the square term and using the relation within the Coulomb gauge , we obtain Then using the Bloch function (j = v, c that mean valence band, conduction band) the transition probability can be obtained such that ( means wave vector of light) Power dissipation of the electromagnetic waves per unit time and unit volume gives rise to following equation From the relation between the electric field and the vector potential, , we may put And finally we can get the imaginary part of the dielectric constant and surely the absorption coefficient.
After revision: Neglecting the square term and using the relation within the Coulomb gauge , we obtain Then using the Bloch function (j = v, c that mean valence band, conduction band) the transition probability can be obtained such that Power dissipation of the electromagnetic waves per unit time and unit volume gives rise to following equation From the relation between the electric field and the vector potential, , we may put And finally we can get the imaginary part of the dielectric constant and surely the absorption coefficient.