Retrieving the spatial distribution of cavity modes in ZnO nanowires by near-field imaging and electrodynamics simulations

Scanning near-field optical microscopy (SNOM) has become nowadays a very powerful technique for investigating the optical properties of nanostructures with a sub-wavelength spatial resolution below 100 nm, such as waveguiding effects in ZnO nanowires (NWs). A spatially resolved study of the electromagnetic field distributions of different cavity modes in ZnO NWs is still lacking. In this work, a near-field optical microscope was used to map out the evanescent fields of optically excited single-crystal ZnO NWs grown on quartz substrates by the vapour transport method using Au as catalyst. The SNOM measurements were performed at room temperature in transmission-collection mode using four different laser wavelengths (378, 514, 633 and 785 nm). They reveal a different spatial distribution of the electromagnetic fields associated to each cavity mode, which are unique properties of the NWs depending primarily on their size and the wavelength of the mode. The SNOM patterns obtained are quite different. Whereas for UV illumination the pattern exhibits two well defined bright lines running along the edges of the upper hexagonal facet of the wire, for red laser excitation the SNOM pattern displays a strong but wider maximum at the center of the facet. The latter also exhibits a periodic modulation of the near-field intensity all along the axis of the wire. In order to interpret the experimental findings, electrodynamics simulations were perforemd using the discrete dipole approximation (DDA), which is an accurate numerical method in which the object of interest is represented as a cubic lattice of N polarizable points. About 890000 dipoles were used to describe the ZnO NW, out of a total of 1.5 million for taking also the substrate into account.