Pump-probe spectroscopy in degenerate two-level atoms with arbitrarily strong fields T. Zigdon, a A. D. Wilson-Gordon, a C. Goren, b M. Rosenbluh, b and H. Friedmann a a Department of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel; b Department of Physics, Bar-Ilan University, Ramat Gan 52900, Israel ABSTRACT We review our previous work on pump-probe spectroscopy in realistic degenerate two-level systems and model systems. In particular, we discuss the role of transfer of coherence (TOC) between the ground and excited hyperfine states in producing electromagnetically-induced transparency (EIA) peaks in the probe spectrum, when an F g → F e = F g +1 transition in an alkali-metal atom interacts with a strong pump and weak probe that have perpendicular polarizations. When the pump is σ + polarized and the probe π polarized, this system can be modelled by an N system. We also discuss the role of transfer of population (TOP) between the Zeeman levels of the ground hyperfine state in producing EIA peaks when the pump and probe have the same polarization. This system can be modelled using a double two-level system. The role of Doppler broadening and phase-changing collisions in modifying the EIA-TOC and EIA-TOP absorption and refraction spectra is also discussed. All these spectra were calculated using MATLAB programs that both construct and solve the relevant Bloch equations. In our recent work, we consider the effect of a strong probe on the pump absorption and refraction spectra when the pump and probe polarizations are linear and perpendicular. It is difficult to solve this problem numerically due to the large number frequencies involved. In order to simplify the problem, we considered two cases: (i) σ + polarized pump and π polarized probe, and (ii) σ + polarized pump and σ − polarized probe, and investigated a series of transitions in both Rb and Cs, using modified versions of the MATLAB programs devised for the weak- probe case. A number of interesting differences from the weak-probe case were found. For example, when the probe is sufficiently strong, we found the pump and probe spectra to show complementary behavior. In addition, as the number of Zeeman levels increase, the EIA peaks become progressively sharper, and are accompanied by steeper dispersion. Keywords: Degenerate two-level systems, electromagnetically induced transparency, electromagnetically in- duced absorption, transfer of coherence, transfer of population 1. INTRODUCTION The probe absorption spectrum of an atomic degenerate two-level system (TLS), interacting with a strong pump and weak probe, can exhibit narrow features at line center where the pump and probe have equal frequencies. 1–3 When the probe absorption spectrum is characterized by a sharp dip, the phenomenon is called electromagneti- cally induced transparency. 4 However, when the absorption spectrum is characterized by a sharp peak, the effect is called electromagnetically induced absorption (EIA). 5, 6 Electromagnetically induced absorption (EIA) has been studied extensively, both experimentally and theoretically. In the original experiments, EIA was obtained for perpendicularly polarized pump and probe lasers, interacting with a cycling degenerate two-level transition in which F e = F g + 1 and F g > 0. 5, 6 It was also observed in the Hanle configuration. 7–10 For the case of perpendicularly polarized pump and probe lasers, EIA has been shown to be due to transfer of coherence (TOC), via spontaneous emission, from the excited state to the ground state. 11, 12 In our first paper on EIA, 13 we gave a detailed explanation of why the TOC that leads to EIA can only take place in systems where ground-state population trapping does not occur, that is, when F e = F g +1. We also explained why EIA-TOC is observed in open systems. 7, 14, 15 EIA has also been predicted for a three-level Λ system interacting with a standing wave, 16 and for a nearly degenerate tripod system interacting with a σ polarized pump and a π polarized probe. 17 The simplest system that can be used to model EIA due to TOC is the N system. 11, 12, 18 In this scheme, which Send correspondence to ADWG; e-mail: gordon@mail.biu.ac.il Invited Paper 14th International School on Quantum Electronics: Laser Physics and Applications, edited by Peter A. Atanasov, Tanja N. Dreischuh, Sanka V. Gateva, Lubomir M. Kovachev, Proc. of SPIE Vol. 6604, 660402, (2007) 0277-786X/07/$15 doi: 10.1117/12.726791 Proc. of SPIE Vol. 6604 660402-1