Ultrasmall penetration depth in nanoscale thermal radiation

Near-field thermal radiation can significantly exceed that predicted by the Stefan–Boltzmann law, especially when surface polaritons are excited such that the energy transfer is through photon tunneling. The penetration depth, or skin depth, of evanescent waves is usually a few tenths of a wavelength. This letter demonstrates that an extremely small skin depth (on the order of a nanometer) can exist for nanoscale thermal radiation between two plates separated by a vacuum gap, even though the dominant wavelengths are in the infrared. Furthermore, the skin depth is proportional to the separation distance.