Figure 1 - Schematic of the 'dressing' of electron energy levels by a strong coupling beam⁹.

a, When driven at _cE₁₂, a two-level system evolves into two doublets, each split by the Rabi energy, _Rabi, denoted by the red arrows, which itself increases monotonically with the coupling beam intensity. b, The three-level system whose 'bare' transition energies are denoted by the solid black arrows. Most (80%) of the electrons remaining in state |1 and the gain arising from coherences appearing in the upper two states, which have been split into Rabi doublets by the off-resonant coupling beam. Upward green arrows represent the coupling photons, the downward dashed arrows are the transitions generating the optical gain and the blue arrows denote the frequency, ₀, where extra loss appears.