  We report on a >100-fold enhancement of Raman responses from Bi2Se3 thin
films if laser photon energy switches from 2.33 eV (532 nm) to 1.58 eV (785
nm), which is due to direct optical coupling to Dirac surface states (SS) at
the resonance energy of ~1.5 eV (a thickness-independent enhancement) and due
to nonlinearly excited Dirac plasmon (a thickness-dependent enhancement). Owing
to the direct optical coupling, we observed an in-plane phonon mode of
hexagonally arranged Se-atoms associated with a continuous network of Dirac SS.
This mode revealed a Fano lineshape for films <15 nm thick, resulting from
quantum interference between surface phonon and Dirac plasmon states.
