Wire and extended ladder model predict THz oscillations in DNA monomers, dimers and trimers

We call \textit{monomer} a B-DNA base pair and study, analytically and numerically, electron or hole oscillations in \textit{monomers}, \textit{dimers} and \textit{trimers}. We employ two Tight Binding (TB) approaches: (I) at the base-pair level, using the on-site energies of the base pairs and the hopping parameters between successive base pairs i.e. \textit{a wire model}, and (II) at the single-base level, using the on-site energies of the bases and the hopping parameters between neighbouring bases, specifically between (a) two successive bases in the same strand, (b) complementary bases that define a base pair, and (c) diagonally located bases of successive base pairs, i.e. \textit{an extended ladder model} since it also includes the diagonal hoppings (c). For \textit{monomers}, with TB II, we predict periodic carrier oscillations with frequency $f \approx$ 50-550 THz. For \textit{dimers}, with TB I, we predict periodic carrier oscillations with $f \approx$ 0.25-100 THz. For \textit{trimers made of identical monomers}, with TB I, we predict periodic carrier oscillations with $f \approx$ 0.5-33 THz. In other cases, either with TB I or TB II, the oscillations may be not strictly periodic, but Fourier analysis shows similar frequency content. For dimers and trimers, TB I and TB II are successfully compared giving complementary aspects of the oscillations.