Driving the driven atom: Spectral signatures C. C. Yu, J. R. Bochinski, T. M. V. Kordich, and T. W. Mossberg Oregon Center for Optics and Department of Physics, University of Oregon, Eugene, Oregon 97403 Z. Ficek Department of Physics, The University of Queensland, Brisbane, Queensland 4072, Australia Received 25 August 1997 We have measured the emission spectrum of two-level-like Ba atoms driven by a continuous-wave bichro- matic field containing a strong resonant component and a weaker component detuned from atomic resonance by the strong-field Rabi frequency. With the specified detuning, the weak field resonantly drives a transition of the atom–strong-field dressed states. Observed spectra show that each peak of the normal single-driving-field resonance fluorescence triplet is split into three subpeaks separated by one-half the weak-field Rabi frequency. Also seen is another triplet of peaks displaced from the atomic resonance by twice the strong-field Rabi frequency. Splitting of the normal triplet peaks can be explained through weak-field dressing of the strong-field dressed states. The origin of the additional triplet is less transparent. Comparison with theory is made. S1050-29479751411-9 PACS numbers: 42.50.Hz, 42.62.Fi, 32.70.Jz, 32.80.-t I. INTRODUCTION Theoretical and experimental studies of the spectral and dynamical features of two-level atoms TLA’s under a va- riety of continuous-wave cw strong-field driving conditions have provided fundamental insight into light-matter interac- tions. A significant triumph of quantum optics is the predic- tion 1 and observation 2 of the three-peaked fluorescence spectrum of TLA’s driven by a strong near-resonant mono- chromatic field. Of the three peaks, one occurs at the driving field frequency the center peak, while the other two the sideband peaks are symmetrically displaced about the center peak by the generalized Rabi frequency. The dressed-atom model 3 emerged as a powerful description of the compos- ite system of atom-plus-field. The system eigenstates dressed states form a ladder of doublets, with adjacent dou- blets separated by the driving field frequency and split by the generalized Rabi frequency. In this dressed-atom picture, the fluorescence peaks correspond to the transition frequencies between dressed levels, while peak areas reflect dressed level populations and relative transition strengths. TLA’s display additional absorptive and emissive spectral features and dynamics when exposed to complex or poly- chromatic driving fields 4. These features have been stud- ied in a surprisingly limited number of experiments. Appli- cation of a weak, tunable probe field to monochromatically driven TLA’s demonstrated gain without population inver- sion 5 and observations of cw two-photon optical gain and lasing 6. Spectrally integrated fluorescence intensity mea- surements 7 in the presence of driving field modulation bichromatic excitation showed the presence of parametric resonances. Other bichromatic field studies 8 revealed novel features such as Rabi subharmonic resonances in ab- sorption spectra. Emission spectra dramatically different from the familiar triplet were observed in experiments in- volving TLA’s driven by two equal intensity fields, sym- metrically detuned from the atomic resonance symmetric bichromatic excitation9,10. Observed spectra display ad- ditional peaks, intensity-independent peak separation, intensity-dependent peak quantity, and alternating peak line- widths. There have also been observations of Autler-Townes spectra 11 of TLA’s driven by a 100% amplitude modu- lated field. Recently, Wu et al. 12 have experimentally investigated the transient dynamics of TLA’s driven by bichromatic fields comprised of a strong and weak component. It was found that the weaker field, appropriately tuned, excites transient responses in the atom + strong-field ‘‘molecule’’ that are entirely analogous to those observed in the transient mono- chromatic excitation of purely material systems. We report here the results of a complementary experimental study of spectrally resolved fluorescence from TLA’s under similar but cw driving conditions. Specifically, we make one field component resonant with the TLA resonance frequency  a ) and one component detuned see Fig. 1. The detuning of the nonresonant component is equal in magnitude to the Rabi frequency of the resonant driving component. Observations reveal that many but not all aspects of the emitted spectrum FIG. 1. The system under investigation is a two-level atom driven by a bichromatic field with one resonant,  1 , and one off- resonant,  2 , component. RAPID COMMUNICATIONS PHYSICAL REVIEW A DECEMBER 1997 VOLUME 56, NUMBER 6 56 1050-2947/97/566/43814/$10.00 R4381 © 1997 The American Physical Society