T. Taubner, F. Keilmann, and R. Hillenbrand
Optics Express 13, 8893 (2005)
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 λ=633 nm. At λ=10.7 μm, gold islands buried 50 nm below a polymer surface are imaged with a lateral resolution < 120 nm, corresponding to λ/90. Studying oxide layers with systematically varied thicknesses we provide experimental evidence of mid-infrared near-field probing in depths > 80 nm.