There is an accepted concept based on the literature explains that the intraband transition in graphene is merely attributed to the THz radiation i.e. such transitions are only occured for a THz radiation. However, according to Kenichi L. Ishikawa (PHYSICAL REVIEW B 82, 201402R 2010), "The nonlinear optical response is not sufficiently described neither by pure intraband nor by pure interband dynamics but their interplay has to be taken into account"; can this mean that the intraband transitions are also triggered at the other frequencies like the visible range or not.

Appreciated in advance for answers.