Third-order nonlinear spectra of a strong bichromatic field interacting with a three-level atom in a “V” configuration. I

Abstract

Abstract This investigation is concerned with the interaction of a three-level atom in resonance with two strong laser fields of frequencies ωa and ωb. The system is in a “V” configuration and the feature of interest is the excitation spectrum near the frequency regions 3ωa, ωa + 2ωb, 3ωb and ωb + 2ωa. In order to treat this problem, which involves nonlinear three-photon processes, it is necessary to work beyond the rotating wave approximation. An appropriate Hamiltonian and the Green function formalism are used to derive expressions for the excitation spectrum near the frequency regions 3ωa and ωa + 2ωb . The Green functions, which define the third-order nonlinear susceptability of the system, describe physical processes where one of the excited states of the atom absorbs two photons from each laser field and produces the frequencies near 3ωa and ωa + 2ωb, respectively, while the other excited state absorbs one photon from each laser field and generates the frequency near ωa + 2ωb. Analytical expressions for the spectral functions are derived for three special cases of the Rabi frequencies ηa and ηb describing the laser-atom interactions: ηa⪢ ηb, ηa ⪡ ηb and ηa = ηb. The derived results are presented graphically for different values of the Rabi frequencies. The results for the frequency regions 3ωb and ωb + 2ωa immediately follow from the application of symmetry arguments.