In contrast to optical and photoluminescence spectroscopy, THz absorption can directly measure the presence of exciton populations in full analogy to atomic spectroscopy. For example, the presence of a pronounced 1 s -to-2 p resonance in THz absorption uniquely identifies the presence of excitons as detected experimentally in Ref. As a major difference to atomic spectroscopy, semiconductor resonances contain a strong excitation-induced dephasing that produces much broader resonances than in atomic spectroscopy. In fact, one typically can resolve only one 1 s -to-2 p resonance because the dephasing constant γ γ ν ν , λ λ is broader than energetic spacing of n- p and (n+1)- p states making 1 s -to-n- p and 1 s -to-(n+1) p resonances merge into one asymmetric tail.