1. A few basics

The internal electronic dynamics of optically active atoms have long been played out on time scales ranging from nanoseconds to fractions of a second. Knowledge in this field has therefore progressed in parallel with advances in pulse sources enabling the pumping of atomic energy levels in gases, liquids and solids, i.e. mainly with the laser tools made available to researchers, particularly effective both in terms of their intensity and their dual frequency and time selectivity. In reality, we couldn't stop there, and efforts to find even faster sources, in the sub-nanosecond range, gradually led us to break through the picosecond time scale (1 ps = 10 ^–12 s) and then the femtosecond time scale (1 fs = 10 ^–15 s), i.e. the time scale for exchanges of atoms or groups of atoms in many-body systems...