  We report a photoluminescence observation of robust excitonic polarons due to
strong coupling of exciton and longitudinal optical (LO) phonon as well as
Fano-type interference in high quality ZnO crystal. At low enough temperatures,
the strong coupling of excitons and LO phonons leads to not only traditional
Stokes lines (SLs) but also up to second-order anti-Stokes lines (ASLs) besides
the zero-phonon line (ZPL). The SLs and ASLs are found to be not mirror
symmetric with respect to the ZPL, strongly suggesting that they are from
different coupling states of exciton and phonons. It is more interesting that a
new group of peaks, including a ZPL and several SLs, are observed. The
observations can be explained with a newly developed theory in which this group
is attributed to the ground excitonic polaron state and the other group is from
the excited polaron states with LO phonon components partially decaying into
environal phonon modes. Besides these spectral features showing the
quasiparticle properties of exciton-phonon coupling system, the first-order SL
is found to exhibit characteristic Fano lineshape, caused by quantum
interference between the LO components of excitonic polarons and the environal
phonons. These findings lead to a new insight into fundamental effects of
exciton-phonon interaction.
