Experimental
The Au-hole with a diameter of 200 nm is made by the simple nanosphere-lithography: polystyrene beads of a corresponding diameter are dispersed onto the Si-wafer, and is coated with an Au film of thickness of 50 nm by thermal evaporation coating, and the bead is removed by sonicating in dichloromethane (CH2Cl2). This procedure yields randomly distributed nano-holes with a well-defined diameter. To minimize the tip-induced perturbation (shift in resonance frequency and distortion of the recorded field) in sSNOM,5 we use weakly scattering Si-tip (tapping mode), and use the cross-polarization geometry in which excitation light is polarized orthogonal to the tip-axis (s-polarized) and only the scattered light that is polarized near-parallel to the tipaxis (p-polarized) is detected.
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