The same approach can be applied to compute absorption spectrum for fields that are in the terahertz (abbreviated as THz) range of electromagnetic radiation. Since the THz-photon energy lies within the meV range, it is mostly resonant with the many-body states, not the interband transitions that are typically in the eV range. Technically, the THz investigations are an extension of the ordinary SBEs and/or involve solving the dynamics of two-particle correlations explicitly. Like for the optical absorption and emission problem, one can diagonalize the homogeneous parts that emerge analytically with the help of the exciton eigenstates. Once the diagonalization is completed, one can then compute the THz absorption analytically.