Compensation of ac Stark and Zeeman shifts in Doppler-free nonlinear Faraday rotation in rubidium vapor

摘要

Nonlinear Faraday rotation (NFR) was studied for 87{sup left}Rb atomic D{sub}2 F{sub}g = 2-F{sub}e =1,2,3 transitions with crossed polarizers, with varying longitudinal magnetic field strength B = 0 - 10 G. The experiment was realized in counter-propagating beam Doppler-free geometry. Our results were obtained for forward probe beam power (P{sub}(FW) ～150 μW) higher than the backward pump power (P{sub}(BW) ～30 μW) to induce a large value of Faraday rotation and eliminate the influence of polarization rotation due to pump induced anisotropy in the absence of magnetic field. This enabled the first observations of NFR for each hyperfine and crossover resonances. Fig.1 shows our experimental configuration and experimentally obtained spectra. The zero frequency spectrum of (a) corresponds to the F{sub}g = 2-F{sub}e =1 transition. For closed F{sub}g = 2-F{sub}e =3 transition the signal has a nearly "W-shape", while the signal shape for crossover resonances F{sub}g = 2-F{sub}e =2,3 (CO23) and F{sub}g = 2-F{sub}e =1,3 (CO13) is nearly dispersive. Our results were explained taking into account the optical pumping of atoms by the backward pump beam and different transitions probabilities for σ{sup}+ and σ{sup}- polarization components of probe beam, leading to "W-shape" frequency profile of Faraday rotation signal for genuine resonance, as well as effects of coherent population trapping (CPT) [1] for dispersive signals at crossover resonances. We consider the compensation effect of ac Stark and Zeeman shifts of magnetic sublevels, which for single beam NFR was studied in [2]. In our experiment the light shift is determined predominantly by the more intense probe beam. The light shift acts against the Zeeman shift and decreases or compensates the two photon Raman detuning caused by external magnetic filed and CPT can still exist at B=6 G.