PowerBroadening

Power broadening is the increase in the observed spectral linewidth of a quantum transition as the intensity of the driving light is raised. It is a consequence of saturation and the coherent dynamics of a driven system, and it sets a limit on the spectral resolution achievable in laser spectroscopy and related measurements.

In the prototypical picture of a two-level system driven near resonance, the driving field induces a Rabi frequency Omega that grows with the square root of the light intensity. As the drive strengthens, the excited-state population saturates and the emission spectrum broadens beyond the natural linewidth gamma. A common phenomenological description uses a saturation parameter s, proportional to intensity, with the effective linewidth often described as gamma_eff ≈ gamma sqrt(1 + s). In the low-power limit (s much less than 1), the natural linewidth dominates; at higher powers, gamma_eff increases roughly with the square root of the drive strength.

At sufficiently strong driving, more complex spectral features arise, such as the Mollow triplet, which reflects dressed-state dynamics rather than simple Lorentzian broadening. Power broadening is a homogeneous broadening mechanism, in contrast to Doppler or inhomogeneous broadening, and it is relevant in atomic and ionic transitions, quantum dots, color centers, and other solid-state emitters where high-intensity driving is used or unavoidable.

The effect influences precision measurements, linewidth-based calibrations, and the operation of driven quantum devices, necessitating a balance between signal strength and spectral resolution.