Figure 2. Near-field images of three different samples.

From left to right, the subfigures display: sketches of the sample, topography images obtained by atomic-force microscopy (scale bars, 10 μm), near-field signals as functions of the probe-sample distance z for selected wavelengths (an offset is added for better comparison), as well as near-field images as described in the following. (a) For SrRuO₃ objects (orange rectangle) on a SrTiO₃ substrate we image second- and third-harmonic near-field signals (NF_2Ω and NF_3Ω) using a CO₂ laser (λ=10.6 μm). (b,c) For both types of superlenses we depict NF_3Ω at two different wavelengths being λ=17.5 and 14.6 μm for the symmetric superlens (b) and λ=17.3 and 14.1 μm for the asymmetric lens (c). The red and green curves in the distance curves correspond to areas with and without SrRuO₃ objects on the opposite side of the lens, respectively. Such distance curves and the near-field images show strong signals at both wavelengths, but only at the shorter wavelengths we observe a contrast beyond the diffraction limit due to the superlensing effect.