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arXiv:0711.0064 · Physical Review A

Subnatural linewidth for probe absorption in an electromagnetically-induced-transparency medium due to Doppler averaging

S. M. Iftiquar, Gautam Karve, Vasant Natarajan A

Published Jun 9, 2008• 77 citations
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Abstract

We obtain subnatural linewidth (i.e., $<\ensuremath{\Gamma}$) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a $\ensuremath{\Lambda}$ system. For stationary atoms, the EIT dip for a resonant control laser is roughly one-half as wide as the control Rabi frequency ${\ensuremath{\Omega}}_{c}$. But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when ${\ensuremath{\Omega}}_{c}⪢\ensuremath{\Gamma}$. We observe a linewidth of $\ensuremath{\Gamma}/4$ for ${\ensuremath{\Omega}}_{c}=8\ensuremath{\Gamma}$ in the ${D}_{2}$ line of Rb.

DOI
10.1103/physreva.77.063807

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We obtain subnatural linewidth (i.e., <\ensuremathΓ<\ensuremath{\Gamma}<\ensuremathΓ) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ system. For stationary atoms, the EIT dip for a resonant control laser is roughly one-half as wide as the control Rabi frequency \ensuremathΩc{\ensuremath{\Omega}}_{c}\ensuremathΩc​. But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when \ensuremathΩc⪢\ensuremathΓ{\ensuremath{\Omega}}_{c}⪢\ensuremath{\Gamma}\ensuremathΩc​⪢\ensuremathΓ. We observe a linewidth of \ensuremathΓ/4\ensuremath{\Gamma}/4\ensuremathΓ/4 for \ensuremathΩc=8\ensuremathΓ{\ensuremath{\Omega}}_{c}=8\ensuremath{\Gamma}\ensuremathΩc​=8\ensuremathΓ in the D2{D}_{2}D2​ line of Rb.
Subnatural linewidth in a strongly-driven closed F→F′F \rightarrow F'F→F′ transition
We observe linewidths below the natural linewidth for a probe laser on a {\it two-level system}, when the same transition is driven by a strong control laser. We take advantage of the fact that each level is made of multiple magnetic sublevels, and use the phenomenon of electromagnetically induced transparency or absorption in multilevel systems. Optical pumping by the control laser redistributes the population so that only a few sublevels contribute to the probe absorption. We observe more than a factor of 3 reduction in linewidth in the D2D_2D2​ line of Rb in room-temperature vapor. The observations can be understood from a density-matrix analysis of the sublevel structure.
We study the phenomenon of electromagnetically induced transparency (EIT) in room-temperature Rb vapour under conditions of a strong probe laser. The atoms form a ladder system with the lower transition coupled by the probe laser and the upper transition coupled by the control laser. Density-matrix analysis of the system shows that, for zero-velocity atoms, the strong probe beam causes a reduction in absorption and broadening of the profile. But the lineshape after thermal averaging shows a splitting of the EIT resonance and enhanced absorption near line centre. The experimental observations in 85Rb^{85}Rb85Rb agree qualitatively with the predicted lineshapes.
Electromagnetically induced transparency (EIT) experiments in $\ensuremath{\Lambda}$-type systems benefit from the use of hot vapor where the thermal averaging results in reducing the width of the EIT resonance well below the natural linewidth. Here, we demonstrate a technique for further reducing the EIT width in room-temperature vapor by the application of a small longitudinal magnetic field. The Zeeman shift of the energy levels results in the formation of several shifted subsystems; the net effect is to create multiple EIT dips each of which is significantly narrower than the original resonance. We observe a reduction by a factor of 3 in the D2{D}_{2}D2​ line of 87Rb^{87}\mathrm{Rb}87Rb with a field of 3.2G3.2\phantom{\rule{0.3em}{0ex}}\mathrm{G}3.2G.
We show that it is possible to change from a subnatural electromagnetically induced transparency (EIT) feature to a subnatural electromagnetically induced absorption (EIA) feature in a (degenerate) three-level Λ system. The change is effected by turning on a second control beam counter-propagating with respect to the first beam. We observe this change in the D 2 line of Rb in a room temperature vapor cell. The observations are supported by density-matrix analysis of the complete sublevel structure including the effect of Doppler averaging, but can be understood qualitatively as arising due to the formation of N -type systems with the two control beams. Since many of the applications of EIT and EIA rely on the anomalous dispersion near the resonances, this introduces a new ability to control the sign of the dispersion.
We show that it is possible to change from a subnatural electromagnetically induced transparency (EIT) feature to a subnatural electromagnetically induced absorption (EIA) feature in a (degenerate) three-level. system. The change is effected by turning on a second control beam counter-propagating with respect to the first beam. We observe this change in the D-2 line of Rb in a room temperature vapor cell. The observations are supported by density-matrix analysis of the complete sublevel structure including the effect of Doppler averaging, but can be understood qualitatively as arising due to the formation of N-type systems with the two control beams. Since many of the applications of EIT and EIA rely on the anomalous dispersion near the resonances, this introduces a new ability to control the sign of the dispersion. Copyright (C) EPLA, 2012
We demonstrate two ways of obtaining subnatural linewidth for probe absorption through room-temperature Rb vapor. Both techniques use a control laser that drives the transition from a different ground state. The coherent drive splits the excited state into two dressed states (Autler-Townes doublet), which have asymmetric linewidths when the control laser is detuned from resonance. In the first technique, the laser has a large detuning of 1.18 GHz to reduce the linewidth to 5.1 MHz from the Doppler width of 560 MHz. In the second technique, we use a counterpropagating pump beam to eliminate the first-order Doppler effect. The unperturbed probe linewidth is about 13 MHz, which is reduced below 3 MHz $(0.5\ensuremath{\Gamma})$ at a detuning of 11.5 MHz.