  A novel photonic thermal diode concept operating in the near field and
capitalizing on the temperature-dependence of coupled surface polariton modes
in nanostructures is proposed. The diode concept utilizes terminals made of the
same material supporting surface polariton modes in the infrared, but with
dissimilar structures. The specific diode design analyzed in this Letter
involves a thin film and a bulk, both made of 3C silicon carbide, separated by
a subwavelength vacuum gap. High rectification efficiency is obtained by tuning
the antisymmetric resonant modes of the thin film, resulting from surface
phonon-polariton coupling, on- and off-resonance with the resonant mode of the
bulk as a function of the temperature bias direction. Rectification efficiency
is investigated by varying structural parameters, namely the vacuum gap size,
the dielectric function of the substrate onto which the film is coated, and the
film thickness to gap size ratio. Calculations based on fluctuational
electrodynamics reveal that high rectification efficiencies in the 80% to 87%
range can be maintained in a wide temperature band (~ 700 K to 1000 K). The
rectification efficiency of the proposed diode concept can be potentially
further enhanced by investigating more complex nanostructures such as gratings
and multilayered media.
