  Radiative thermal engineering with subwavelength metallic bodies is a key
element for heat and energy management applications, communication and sensing.
Here, we numerically and experimentally demonstrate metallic thermal emitters
with narrowband but extremely stable emission spectra, whose resonant frequency
does not shift with changes on the nanofilm thickness, the angle of observation
and/or polarization. Our devices are based on epsilon-near-zero (ENZ)
substrates acting as material-based high-impedance substrates. They do not
require from complex nanofabrication processes, thus being compatible with
large-area and low-cost applications.
