Directly Probing the Anisotropic Optical Emission of Individual ZnO Nanorods

We report on the anisotropic optical emission of individual ZnO nanorods directly measured by angular-resolved photoluminescence imaging and spectroscopy with independent excitation and detection optics. A reduced diameter at the subwavelength scale results in degraded waveguiding efficiency and more divergent emission at the top ends of individual ZnO nanorods grown by chemical vapor deposition. The near-band-edge (NBE) emission from the nanorod ends exhibits strong angular dependence in both emission intensity and spectral line shape. A continuous intensity increase and spectral red shift of the NBE emission have been observed as the detection direction varies from the radial direction to the axial direction. The pronounced red shift (>60 meV) is primarily attributed to the transition from free exciton (FX) emission to its 2LO phonon replica (FX-2LO) emission. Our results support an exciton-polariton model for the waveguiding behavior of ZnO nanorods and demonstrate the dominant role of the EX-2LO exciton-phonon interaction in the waveguiding process. We also show that the excitation angle has little influence on the NBE emission but can greatly modify the deep level emissions.