  The polarization dependence of the double resonant Raman scattering (2D) band
in bilayer graphene (BLG) is studied as a function of the excitation laser
energy. It has been known that the complex shape of the 2D band of BLG can be
decomposed into four Lorentzian peaks with different Raman frequency shifts
attributable to four individual scattering paths in the energy-momentum space.
From our polarization dependence study, however, we reveal that each of the
four different peaks is actually doubly degenerate in its scattering channels,
i.e., two different scattering paths with similar Raman frequency shifts for
each peak. We find theoretically that one of these two paths, ignored for a
long time, has a small contribution to their scattering intensities but are
critical in understanding their polarization dependences. Because of this, the
maximum-to-minimum intensity ratios of the four peaks show a strong dependence
on the excitation energy, unlike the case of single-layer graphene (SLG). Our
findings thus reveal another interesting aspect of electron-phonon interactions
in graphitic systems.
