Excitonic Polaron States and Fano Interference in Phonon-Assisted Photoluminescence of Bound Excitons in ZnO

Abstract

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.