Figure 4. Spectral response of the asymmetric superlens.

Near-field spectra as well as near-field images and NF_3Ω-distance curves for selected wavelengths (scale bars,10 μm). (a) For λ=13.7–15.2 μm, an imaging contrast exists because of the localized polariton mode. (b) Shows the near-field spectrum for a fixed distance of z=20 nm. (c) For λ=15.4–18.4 μm, no imaging contrast is observed although the near-field signal is enhanced by the non-localized polariton mode. The red and green curves in the distance curves of a, c and the spectrum b correspond to the near-field signals on areas with and without SrRuO₃ objects on the opposite side of the lens, respectively.