A thermal light-emitting source, such as a black body or the incandescent filament of a light bulb, is often presented as a typical example of an incoherent source and is in marked contrast to a laser. Whereas a laser is highly monochromatic and very directional, a thermal source has a broad spectrum and is usually quasi-isotropic. However, as is the case with many systems, different behaviour can be expected on a microscopic scale. It has been shown recently that the field emitted by a thermal source made of a polar material is enhanced by more than four orders of magnitude and is partially coherent at a distance of the order of 10 to 100nm. Here we demonstrate that by introducing a periodic microstructure into such a polar material (SiC) a thermal infrared source can be fabricated that is coherent over large distances (many wavelengths) and radiates in well defined directions. Narrow angular emission lobes similar to antenna lobes are observed and the emission spectra of the source depends on the observation angle--the so-called Wolf effect. The origin of the coherent emission lies in the diffraction of surface-phonon polaritons by the grating.

We review in this article the influence of surface waves on the thermally excited electromagnetic field. We study in particular the field emitted at subwalength distances of material surfaces. After reviewing the main properties of surface waves, we introduce the fluctuation-dissipation theorem that allows to model the fluctuating electromagnetic fields. We then analyse the contribution of these waves in a variety of phenomena. They give a leading contribution to the density of electromagnetic states, they produce both temporal coherence and spatial coherence in the near field of planar thermal sources. They can be used to modify radiative properties of surfaces and to design partially spatially coherent sources. Finally, we discuss the role of surface waves in the radiative heat transfer and the theory of dispersion forces at the subwavelength scale.

We propose in this article an unambiguous definition of the local density of electromagnetic states (LDOS) in a vacuum near an interface in an equilibrium situation at temperature T . We show that the LDOS depends only on the electric field Green function of the system but does not reduce in general to the trace of its imaginary part as often used in the literature. We illustrate this result by a study of the LDOS variations with the distance to an interface and point out deviations from the standard definition. We show nevertheless that this definition remains correct at frequencies close to the material resonances such as surface polaritons. We also study the feasability of detecting such a LDOS with apetureless SNOM techniques. We first show that a thermal near-field emission spectrum above a sample should be detectable and that this measurement could give access to the electromagnetic LDOS. It is further shown that the apertureless SNOM is the optical analog of the scanning tunneling microscope which is known to detect the electronic LDOS. We also discuss some recent SNOM experiments aimed at detecting the electromagnetic LDOS.