  Interaction of very low-frequency primordial(relic) gravitational waves(GWs)
to cosmic microwave background(CMB) can generate B-mode polarization. Here, for
the first time we point out that the electromagnetic(EM) response to
high-frequency GWs(HFGWs) would produce quasi-B-mode distribution of the
perturbative photon fluxes, and study the duality and high complementarity
between such two B-modes. Based on this quasi-B-mode in HFGWs, it is shown that
the distinguishing and observing of HFGWs from the braneworld would be quite
possible due to their large amplitude, higher frequency and very different
physical behaviors between the perturbative photon fluxes and background
photons, and the measurement of relic HFGWs may also be possible though face to
enormous challenge.
