Nanophotonics devices for thermal emission

The first electric light sources were developed for visible-light applications by Joseph Swan and Thomas Edison at the end of the 19th century. The operation of these incandescent bulbs was based on the fact that any hot object has a propensity to emit electromagnetic radiation determined by the laws of thermodynamics. In the second half of the twentieth century, the invention of laser sources and LEDs made it possible to achieve properties impossible to achieve with incandescent bulbs, such as directionality or spectral finesse, as well as much higher power thresholds and energy yields. The advent of nanophotonics in the 21st century has reopened up major prospects for these so-called "thermal" sources. The idea that it was possible to give a coherent character both spatially and temporally to thermal radiation, which had hitherto been considered the prerogative of laser radiation, received a decisive boost at the beginning of our century.

This article presents the advances and developments made in infrared thermal sources, as well as their potential applications. In the first section, the main infrared light sources are presented, along with the physical mechanisms involved in each case: LEDS, quantum cascade lasers, OPOs and thermal sources. Each of these sources has its own limitations, whether in terms of achievable spectral bandwidth, energy efficiency or cost.

In the second section, the optimization of a heat source is described from both an electromagnetic and a thermal point of view. A hot object tends towards thermodynamic equilibrium via three possible channels: thermal conduction, convection and thermal radiation. The challenge of thermal optimization is to favor the third channel while minimizing the other two. Electromagnetic optimization, based on Kirchhoff's law at both macroscopic and microscopic scales, enables the selection of spectral and angular radiation bands.

In the third section, we present the main families of thermal sources born of the use of nanophotonic devices: directional sources, narrow spectral band sources and rapidly time-modulated sources (currently around MHz).

Finally, the main existing or developing applications for these sources are described (gas detection, light source, thermophotovoltaic generation and frequency conversion, image coding and radiative cooling).

At the end of the article, readers will find a glossary and a list of the symbols and acronyms used.