Fluorescence and absorption by a two-level atom in a bichromatic field with one strong and one weak component

We analyze the fluorescence and absorption spectra of a two-level atom driven by a bichromatic field with frequencies omega(1) and omega(2), separated by omega(2)-omega(1) = 2 delta and Rabi frequencies (at resonance) 2 Omega(1) and 2 Omega(2) such that their ratio alpha = Ohm(2)/Ohm(1)<1. We focus on the case of omega(1) close to the atomic frequency omega(0) and omega(2) near the Rabi sideband frequency omega(1) + 2 Ohm(1); the detunings are denoted by Delta(1) = omega(0)-omega(1) and Delta(2) = omega(1) + 2 Ohm(1)-omega(2). We find that the spectra depend critically on the detuning Delta(2): For large Delta(2), the fluorescence spectrum consists of the well known Mellow triplet, centered at omega(1); for smaller (but nonzero) Delta(2), the spectrum is composed of a triplet at omega(1) together with doublets near the sideband frequencies omega(1) +/- 2 Ohm(1). However, when Delta(2) = 0 (and alpha much less than 1), the spectrum consists of a doublet centered at omega(1) and triplets at omega(1)+/-2 Ohm(1):there is then no fluorescence at omega(1). As alpha increases, additional triplet structures appear in the spectrum at frequencies omega(1) +/- 2n Ohm(1) with intensities proportional to alpha(2(n-1)), n>1, and a line reappears at omega(1), with intensity proportional to alpha(4). The absorption by the system of a weak probe beam is also strongly dependent on the detuning, and the spectrum is composed of emission-dispersion-absorption features located near omega(1) and omega(1) +/- 2n Ohm(1). An analysis in the dressed-atom pic ture is presented which explains the physical origin of all these features, in both fluorescence and absorption.