Introduction

Stimulated emission is fundamental to light amplification and thus to the operation of the laser. To understand it, it must be placed in the context of interactions between light and matter. Here, the matter is composed of optically active elements in “solution” in a gas, plasma, solid or liquid medium. These elements can be atoms, ions, molecules, free radicals or electrons (for simplicity, we consider “atoms” in the following). Their energy levels are quantified and are such that light of a certain frequency can interact with the population found in these levels. More precisely, let us consider two energy levels E1 and E2 (E1 is less than E2) whose atoms can interact with light of frequency . The group E1-E2 is called radiative transition if atoms can only pass from E1 to E2 (or from E2 to E1 ) by interacting with light. E1 is called the lower energy level and E2 the upper energy level.