Plenary Lecture

Design ab-initio of Coherent Thermal Sources

Professor Philippe Ben-Abdallah
Laboratoire de Thermocinetique
CNRS UMR 6607, Ecole Polytechnique de l’Universite de Nantes
44 306 Nantes cedex 03, France
Email: pba@univ-nantes.fr

Abstract: Controlling the spatial or temporal coherence of thermal light a hot body emits when it relaxes to lower states is undoubtedly one of major objectives for improving the efficiency of numerous actual technologies such as thermophotovoltaic conversion devices, radiative cooling systems, infrared gas sensors and highly directional/narrow band thermal radiators. Until recently thermal sources were considered as objects that were able to emit light only over a broad band of the infrared spectrum. Today we know this paradigm is wrong and several partially coherent thermal sources have been already fabricated. The physical origin of these unusual behaviors comes from the structures at the wavelength scale of materials used to fabricate these sources and from the presence of surface waves. Roughly speaking, in the first generation of partially coherent thermal sources, polar materials surmounted by an appropriate surface grating were used to diffract the surface phonon-polaritons into the far field. This principle has opened new prospects for engineering the radiative properties of these media. One of the best achievements in the design of coherent thermal sources has been obtained later with photonic crystals. These periodic dielectric structures-also known as photonic band gap (PBG) materials have, for almost two decades, attracted much attention because of their high potentiality in numerous applied and theoretical fields. At sufficient refractive index contrast, PBG forbid photons to propagate through them at certain frequencies, irrespective of propagation direction in space and polarization. Coupled with frequency selective surfaces photonic crystals have recently allowed the construction of narrow bands IR emitters. These last years promising results have opened prospects for the fabrication of temporally coherent IR sources when a defect is introduced into a photonic crystal. Such defects act like waveguides with a confinement achieved by means of the photonic band gap and not by total internal reflection as in traditional wave guides. The latest generation of partially coherent thermal sources, has been engineered by coupling polar layers with photonic crystals. These structures exhibit highly directional and narrow bands emission patterns for both p- and s-polarization states of the thermal light. Similar antenna-like emission patterns also have been achieved with completely different physical mechanisms using simple thin fims and more recently resonant cavities coupled with metallic layers. Another direction of research has been recently explored for designing thermal antennae with left-handed material (LHM) which are engineered from one-dimensional periodic metallic structures. Near the plasmon resonance of these structures, the effective optical index is close to zero. Therefore, in accordance with the Snell-Descartes laws, the radiation emitted by a source embedded in this medium is expected to be refracted around the normal to the surface. However, no LHM material have been built so far to operate in the infrared range. Moreover, although these structures make it possible to consider many applications at localized frequencies they seem, because of the dispersion, much more difficult to exploit for designing spatially coherent thermal sources over a broad spectral band.
All distinct approaches mentioned above have led to highly directional, narrow band partially coherent thermal sources. However it is not known whether the corresponding structures truly achieve the maximum permissible coherence degree. This is due on the fact that only heuristic strategies based on trial-and-error have been followed for engineering such sources. In this lecture we present a general method for the ab initio design of coherent thermal sources by using only the first principles of optics. The ability to artificially grow, from modern deposition techniques, complex structural configurations of planar heterogeneous metallic/dielectric materials raises the issue of the best achievable thermal emitter that is with the highest directivity and/or with the narrowest band of emission in a given spectral range. This engineering design problem is formally a type of mathematical inverse problem. After an overview on the actual coherent thermal sources, I will present a new strategy to solve this problem. From our current research I will present two examples of multilayered thermal sources. These planar structures involves dielectric and metallic films only without gratings and can be used to realize coherent emission for either polarization both in the far field and in the near-field. The first example I will present is a quasi-isotropic source that has been imagined to radiate in the far field in a narrow spectral band for both polarization states at ambient temperature. It was found that the designed structure can be interpreted as a phonon-polariton resonant guide which converts any photon into atomic vibration at a localized frequency for both polarization states. The second example is a multilayered metallic source which strongly enhance the near-field thermal emission. I will conclude by raising some significant questions.