Real-Space Distribution of Cavity Modes in Single ZnO Nanowires

F. GÜELL; A. R. GOÑI; J.O. OSSÓ; L. A. PÉREZ; E. A. CORONADO; J. R. MORANTE

Boston

Congreso; 2011 MRS Fall Meeting & Exhibit; 2011

Materail Research Society

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. A spatially resolved study of the electromagnetic field distributions of different cavity modes in ZnO NWs is still lacking. In this work, we have used an SNOM to map out the evanescent fields of optically excited ZnO NWs grown 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 are quite different for a ZnO NW of approx. 260 nm in diameter using unpolarized UV (378 nm) and red (633 nm) excitations. 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. In order to interpret the experimental findings, we have performed electrodynamics simulations 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. We used about 890000 dipoles to describe the ZnO NW, out of a total of 1.5 million for taking also the substrate into account. We notice the striking qualitative agreement between calculated and measured field distributions.