RESONANCE-FLUORESCENCE AND ABSORPTION-SPECTRA OF A 2-LEVEL ATOM DRIVEN BY A STRONG BICHROMATIC FIELD

We examine the radiative properties of a two-level atom driven by a strong bichromatic field with frequencies omega1=omega0-delta1 and omega2=omega0+delta2, which can be asymmetrically placed about the atomic transition frequency omega0 and can have different Rabi frequencies OMEGA1 and OMEGA2. Applying the optical Bloch equations for the bichromatic excitation, we derive an infinite set of equations of motion for the time evolution of the atomic variables. The equations are solved numerically by matrix inversion and Laplace transforms. Using the quantum regression theorem, we then solve for the steady-state total fluorescence intensity, and the resonance-fluorescence and absorption spectra. The spectra are found to depend on the frequency difference 2delta=delta1+delta2, the average detuning DELTA=omega0- 1/2(omega1+omega2), and the Rabi frequencies of the driving fields. For OMEGA1=OMEGA2=OMEGA, the total fluorescent intensity displays a series of maxima for DELTA=+/-ndelta-/+OMEGA2/4delta, where the n are the odd integers. The intensity-dependent shift from the resonances ndelta is explained as an analog pf the generalized Bloch-Siegert shift. The resonance-fluorescence spectrum for DELTA not-equal O appears to contain more peaks than that for DELTA=0. This is due to the splitting of the central peak and the even sidebands into doublets. The absorption spectrum of a weak-probe beam for DELTA=0 and equal Rabi frequencies consists of a symmetric series of dispersionlike sidebands separated from omega0 by integer multiples of delta2 together with a central absorption peak at omega0, whose amplitude oscillates with OMEGA. For DELTA not-equal O and/or OMEGA1 not-equal OMEGA2, the odd sidebands remain dispersionlike, while the central peak and the even sidebands split into absorption-emission doublets. A simple physical interpretation of the spectral features is given in terms of the dressed-atom model.