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  • 2013 Conference on Lasers and Electro-Optics - International Quantum Electronics Conference
  • (Optica Publishing Group, 2013),
  • paper CE_3_3

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

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Abstract

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) [1–2]. 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 a near-field optical microscope 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. Figures 1(a) and 1(b) show an example of the SNOM measurements performed on a ZnO NW of approx. 260 nm in diameter at normal incidence from the substrate side, using unpolarized UV (378 nm) and red (633 nm) excitations. The SNOM patterns 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, 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 [3]. 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. Figures 1(c) and 1(d) show the results for the leading modes at the corresponding laser wavelengths. The plots represent the field distribution in cross section of a hexagonal wire supported by a quartz substrate on one of its facets and with true dimensions, as obtained from the topography SNOM profiles. The false color scale corresponds to the field intensity normalized to that of the incident light. For clarity we only show the field intensity outside the wire, for this is the magnitude sensed by the SNOM probe. We notice the striking qualitative agreement between calculated and measured field distributions.

© 2013 IEEE

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