In this work, we reveal the physical origin of three resonances that are at the root of observed characteristics of four-level N-Scheme atomic systems. Explicit excitation pathways are associated with the resonances of interest.
In this work, we theoretically study the absorption and dispersion coefficients of a probe field in four-level N-scheme atomic systems driven by two additional fields. We separate the steady-state solution of the appropriate density matrix element into the sum of three resonances. The effect of each resonance on the overall absorption and dispersion experienced by the probe is studied as a function of the strengths of the two other fields. In the limit of low saturation, we reveal the physics of these resonances and associate them with atomic excitation pathways, which we explicitly explain in the bare-states picture.
We present three examples of pressure-induced extra resonances that can be observed in nonlinear spectroscopy: fluorescence of a ``three-level atom'' driven by two laser fields, two-photon ionization of a ``three-level atom'' (plus continuum) driven by two laser fields, and excitation of a ``four-level atom'' driven by four laser fields. We show that all these extra resonances can be interpreted in terms of quantum pathways, each pathway involving a collisionally aided excitation. We also demonstrate that the two first extra resonances can be obtained with incoherent fields, while relatively coherent fields are required in the last example.
We investigate the steady optical properties of a three-level open V system (i.e. a four-level closed atomic system) through both analytical and numerical calculations. Our results show that the resonant probe gain can be greatly suppressed to generate a narrow and deep transparency window associated with very steep abnormal dispersion. The underlying physics is identified as the perfect quantum destructive interference between a pair of dressed state transition pathways, which is usually believed to be absent in three-level closed V systems. In addition, we show that the transparency window and its associated abnormal dispersion can be used to realize a striking propagation dynamics where a light pulse may have a negative group velocity of small absolute values and experience little gain or absorption.