Optics of material media - Atoms, molecules, solids and photons

Engineers and scientists routinely use light in a myriad of systems, either as a "witness", for diagnostics or detection, or increasingly as a means of action, for machining, surface treatments, purification, surgery, not to mention renewable energies such as solar power. To understand these techniques, we need to know how light and matter interact. The different constituents of matter - atoms and molecules - define the characteristics of these interactions, and this is what we call the optics of material media, which we discuss in this article. We now know in great detail how quantum mechanics governs all this. It was largely born of the observation of the "line" spectra of atoms, and has given them their accepted explanation for the past century. Our compass here is to propose a general understanding of light-matter interaction by analyzing matter's degrees of freedom, the particles that carry them and the energy distributions they adopt: electrons, atoms, molecular vibrations, solid "bands". We show the underlying physical models in simplified form. We also mention the quantum nature of the photon itself, which is clearly destined to enrich the palette of future instruments and machines used in cryptography or in the complex systems simulation machines currently being developed around the world. More concretely, the use of light sources such as lamps, LEDs, lasers, filaments, etc. provides useful examples of conversion between electrical and optical forms of energy or information. We also talk about detectors, in particular the countless semiconductor-based elements that irrigate our everyday means of information, whether fiber or camera. The theme of spectroscopy is also omnipresent, so much so that we have to acknowledge that it is inescapable, because basically, the admirer of the rainbow or the medical staff who analyze an image by nuclear magnetic resonance (MRI) are both practicing forms of spectroscopy.

Finally, from an environmental point of view, in addition to the question of solar energy already mentioned, when questions of pollution arise, unfortunately all too frequently, it is very often remarkable optical interactions that provide the right detection schemes (fluorescence, lidar, etc.). In a similar vein, at a time of anthropogenic climate change, an understanding of thermal radiation is bound to be useful to any engineer or scientist, whether it's a question of "energy-efficient" comfort in the home or understanding the interactions of gases with infrared radiation. This last point will have to guide the way we control the main aerosols and, of course, the greenhouse gases emitted by the activities of our various agro-industrial sectors.

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