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Conference on Lasers and Electro-Optics

Quasi-degenerate phase conjugation spectroscopy of cold cesium

George C. Cardoso, A. Lezama, J. W. R. Tabosa

Published Aug 29, 2005• 0 citations

Abstract

Summary form only given. Resonant cold atomic samples are known to provide an efficient medium for the study of nonlinear optical processes such as four-wave-mixing (FWM) and phase conjugation (PC). Although this system is frequently modeled as an homogeneous sample of two-level atoms, the actual nonlinear response strongly depends on the internal structure (Zeeman degeneracy) of the atomic energy levels and consequently on the polarizations of the optical fields. We have studied the spectra of quasi-degenerate PC on the 6S/sub 1/2/(F=4)-6P/sub 3/2/(F=5) transition of cold Cs atoms for different optical polarizations. The atomic sample was obtained in a magneto-optical trap. The spectra where recorded within a 1 ms time interval during which the trapping light beams were blocked and the quadrupole magnetic field was turned off. We used a backwards FWM scheme with two counter-propagating pump beams and a probe beam. The PC signal propagates in opposite direction respect to the probe beam. Typical PC spectra as a function of frequency offset are shown for probe beam polarization parallel and perpendicular to pump polarization. Large differences are observed between these two cases demonstrating the crucial role of the Zeeman degeneracy on the atomic response. The experimental spectra are in good agreement with the theoretical predictions for FWM generation in degenerate two-level atoms.

DOI: 10.1109/iqec.2000.907788

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Summary form only given. Resonant cold atomic samples are known to provide an efficient medium for the study of nonlinear optical processes such as four-wave-mixing (FWM) and phase conjugation (PC). Although this system is frequently modeled as an homogeneous sample of two-level atoms, the actual nonlinear response strongly depends on the internal structure (Zeeman degeneracy) of the atomic energy levels and consequently on the polarizations of the optical fields. We have studied the spectra of quasi-degenerate PC on the 6S/sub 1/2/(F=4)-6P/sub 3/2/(F=5) transition of cold Cs atoms for different optical polarizations. The atomic sample was obtained in a magneto-optical trap. The spectra where recorded within a 1 ms time interval during which the trapping light beams were blocked and the quadrupole magnetic field was turned off. We used a backwards FWM scheme with two counter-propagating pump beams and a probe beam. The PC signal propagates in opposite direction respect to the probe beam. Typical PC spectra as a function of frequency offset are shown for probe beam polarization parallel and perpendicular to pump polarization. Large differences are observed between these two cases demonstrating the crucial role of the Zeeman degeneracy on the atomic response. The experimental spectra are in good agreement with the theoretical predictions for FWM generation in degenerate two-level atoms.

Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate two-level atomic transition is theoretically and experimentally examined. Using the model presented by Lezama et al. [Phys. Rev. A 61, 013801 (2000)] the NDFWM spectra corresponding to different pump and probe polarization cases are calculated and discussed. The calculated spectra are compared to the observation of NDFWM within the ${6S}_{1/2}(F=4)\ensuremath{\rightarrow}{6P}_{3/2}(F=5)$ transition of cesium in a phase conjugation experiment using magneto-optically cooled atoms.

We have employed a sample of cold cesium atoms in a magneto-optical trap to study nonlinear spectroscopy and four-wave-mixing optical phase conjugation using a noncycling transition. Reflectivities of the order of 1% have been measured for moderate levels of pump power and large angular aperture. In particular, we have performed nonlinear spectroscopy in a three-level V-type system and observed directly the ac Stark splitting of the cesium

{6\mathrm{S}}_{1\mathrm{/}2}

, F=4 ground state.

We present a study of atomic coherence effects in resonant degenerate four-wave mixing. We find for energy levels having magnetic degeneracies, the third-order nonlinear response resulting from two counterpropagating pump waves orthogonally polarized to an incident probe wave generates a signal whose polarization is phase conjugate to that of the incident probe wave. This effect is due to the coupling of different magnetic states resulting from coherent excitation giving rise to a net electric quadrupole. Experimental studies in Na on the D2 line at 5890 Å confirm the analysis.

We study phase-conjugate degenerate four-wave mixing (DFWM) in an optically pumped, two-level atomic system (the

3s\phantom{\rule{0.2em}{0ex}}^{2}S_{1∕2}

,

F=2

, ${m}_{F}=2\ensuremath{\rightarrow}3p\phantom{\rule{0.2em}{0ex}}^{2}P_{3∕2}$,

F=3

,

{m}_{F}=3

transition in a diffuse, collision-free, thermal beam of sodium) using phase-modulated laser beams. In this investigation, we study the detailed line shape of the four-wave mixing signal as a function of the temporal delay between the probe field and the pump fields at different intensities of the cw pump waves and modulation frequencies. We show that phase modulation of the pump and probe beams leads to strong amplitude modulation of the phase-conjugate beam and report excellent agreement between our experimental observations of the DFWM spectra and the results of numerical calculations. Our numerical results suggest an enhancement of the ``local'' DFWM signal when the sidebands of the phase-modulated laser are resonant with dressed-state transitions, but that this resonance is less obvious in the total signal generated. We also show that the complex line shapes depend sensitively upon the delay of the probe beam, with the peak DFWM signal occurring when the probe beam is advanced compared to the pump beam.