We demonstrate that scattering-type scanning near-field optical microscopy (s-SNOM) allows nanoscale-resolved imaging of objects below transparent surface layers at both visible and mid-infrared wavelengths. We show topography-free subsurface imaging at lambda=633 nm. At lambda=10.7 microm, gold islands buried 50 nm below a polymer surface are imaged with a lateral resolution < 120 nm, corresponding to lambda/90. Studying oxide layers with systematically varied thicknesses we provide experimental evidence of mid-infrared near-field probing in depths > 80 nm.
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Taubner et al. (2005) studied this question.