Analog of the coherent population trapping state in a polychromatic field

TL;DR

This paper theoretically investigates how a three-level atom interacts with a polychromatic field, revealing that the coherent population trapping state is only an approximation due to additional couplings and light shifts, especially in femtosecond laser fields.

Contribution

It provides a theoretical model showing the interaction of a three-level atom with a polychromatic field reduces to a bichromatic interaction with added light shifts and couplings, clarifying the nature of the trapping state.

Findings

Interaction reduces to bichromatic field with light shifts

Dark state is not an exact eigenstate in general

Model explains atomic state formation in femtosecond laser fields

Abstract

The interaction between a three-level atom and a polychromatic field with an equidistant spectrum (\Lambda-scheme of the atom--field interaction) has been studied theoretically. It is shown that the interaction of an atom with such a field can be reduced to its interaction with a bichromatic field with additional light shifts of transition frequencies and an additional coupling of the lower atomic levels, which is proportional to the field intensity. Owing to this coupling, the idea of the coherent population trapping can be considered only as an approximation, because the dark state is not an eigenstate of the effective Hamiltonian in the general case of arbitrary dipole moments. The analyzed model gives a simple theoretical interpretation for the formation of the atomic state, which is close to the coherent population trapping, in the radiation field of a femtosecond laser.