Interactions in weak and strong coupling regimes (I)

Quantum electrodynamics shows us that light-matter interactions form a much richer field of possibilities than the semi-classical approach would suggest. To understand the physics involved, we need to recall the properties of the vacuum according to quantum electrodynamics. Light-matter interactions can be classified into two distinct regimes, the so-called weak and strong coupling regimes. In the weak coupling regime, the radiative properties of molecules can be modified, as Karl Drexhage demonstrated as early as 1968. He studied the radiative properties of a Europium complex at different distances from a mirror and showed that fluorescence lifetime and intensity vary as a function of the density of vacuum states. In the strong coupling regime, the interactions are so strong that hybrid light-matter states, known as polaritonic states, are formed even in the darkness of the vacuum field. Rabi's separation of polaritonic states is treated theoretically by the Jaynes-Cummings model. When many molecules are strongly coupled to the same mode of an optical cavity, the polaritonic states are collective, and Rabi splitting can reach energies in the electronvolt range.