Optical Bloch Saturation and Power Broadening

The steady-state behavior of a driven two-level system under the rotating wave approximation can be described by the optical Bloch equations. With a real Rabi frequency Ω and detuning Δ, and constant decay rates, the system reaches an equilibrium state that balances coherent drive and dissipative relaxation.

Experiment

Open the Optical Bloch Saturation interactive module. Observe how the parameters alter the steady state:

1. Set Drive Rabi frequency to 1, Detuning to 0, and Pure dephasing to 0. Note the saturation parameter is 2, and the excited population is roughly 1/3, as calculated above.
2. Increase Drive Rabi frequency to 4. Notice that the excited population approaches 0.5 (specifically ~0.485) and the saturation parameter becomes large (32). The line HWHM also increases dramatically to ~2.87, demonstrating power broadening.
3. Set Drive Rabi frequency to 0. The system falls purely into the ground state (inversion = -1, excited population = 0). The line HWHM reports "No driven line" because there is no coherent drive to produce a measurable spectrum.
4. Introduce Pure dephasing of 0.5 with Drive Rabi frequency 2 and Detuning 1. This increases the total coherence decay rate \Gamma_2 from 0.5 to 1.0, thereby affecting the saturation parameter and the line width.