Quasimonochromatic emission spectra in the near field by polar semiconductor thermal sources

The near-field spectra of the electromagnetic field emitted from a planar surface are theoretically investigated for a number of polar semiconductor and dielectric materials that support surface phonon polaritons. All of the studied materials, InP, GaAs, GaN, SiC, and alpha-Al2O3 (sapphire), exhibit quasimonochromatic thermal emission symbolized by strong peaks of evanescent modes at well-defined frequencies in the near field that correspond to the appropriate peaks in the density of states for surface phonon polaritons. It is also found that the materials with lower polariton frequencies (e.g., InP and GaAs) generally demonstrate a higher peak spectral energy density compared to those with higher frequencies (e.g., SiC). This trend is maintained over the entire range of temperature (300-600 K) and the distance from the surface (<= 10 mu m) considered in the calculation. Thus, the results clearly indicate that among the studied materials InP and GaAs are the best candidates to provide the quasicoherent thermal emission for potential use as a nanoscale thermal source. The energy density stored in the evanescent peaks, when close to the surface, is estimated to be many orders of magnitude larger than that in the blackbody radiation. (c) 2007 American Institute of Physics.112sciescopu