Experimental observation of TM propagated modes in nanocoax structures*

The nanoscale manipulation of light has become one of the most important research areas in the last years.\footnote{R. R. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L.Dai, G. Bartal, X. Zhang, Nature 461, 629-632 (2009).} Several studies in nanoscale waveguides have been done and the coaxial waveguide is among the most promising due to its broadband properties.\footnote{D. Pozar, D. ``Microwave Engineering,'' 3rd. Edition. John Wiley and Sons, Inc. USA, 2005.} Here, we report the experimental observation of photonic and plasmonic transverse magnetic mode propagation in a nanocoax structure by use of leakage radiation microscopy and near-field scanning optical microscopy in the visible and near-infrared ranges of the electromagnetic spectrum. Numerical calculations are consistent with our experimental results and suggest that the propagated modes are mainly TM$_{10}$-like (plasmonic) and TM$_{11}$ (photonic) modes, confirming theoretical results previously reported.\footnote{ Peng Y., Wang W., Kempa K., Opt. Express. 3, 1758-1763 (2008).}