Phase-independent spectrum of atomic resonance fluorescence from a driven three-level atom in ladder configuration

We investigate the phase-dependent squeezing of resonance fluorescence emitted from a coherently excited three-level atom with a ladder configuration of energy levels. Quantum fluctuation spectra of the optimum-phase quadratures and atomic populations and coherences are calculated. We find that two-photon absorption greatly modifies the steady-state atomic populations and coherence which can lead to squeezing of the total field of fluorescent light, but is not favourable for frequency-dependent squeezing spectra. Optimized squeezing can be attained and controlled with the phase of quadrature of fluorescent light, intensity and detuning of the driving fields and the decay rate of the controlling transition. Analytical picture is provided in dressed states representation for supporting our numerical results in the strong driving field limit.