that contains the current-matrix elements J ν ν β β ∝ ∝ ∑ ∑ k ϕ ϕ ν ν ⋆ ⋆ ( k ) k ⋅ ⋅ k T H z ϕ ϕ β β ( k ) between two exciton states. The unit vector k T H z is determined by the direction of the THz field. This leads to dipole selection rules among exciton states, in full analog to the atomic dipole selection rules. Each allowed transition produces a resonance in S ν ν , λ λ ( ω ω ) and the resonance width is determined by a dephasing constant γ γ λ λ , ν ν ( ω ω ) that generally depends on exciton states involved and the THz frequency ω ω . The THz response also contains γ γ ( ω ω ) that stems from the decay constant of macroscopic THz currents.