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arXiv:1008.3231

Subnatural linewidth in a strongly-driven closed $F \rightarrow F'$ transition

Sapam Ranjita Chanu, Alok K. Singh, Boris Brun, Kanhaiya Pandey, Vasant Natarajan A

Published Aug 19, 2010• 0 citations
arXivHTMLPDF

Abstract

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 $D_2$ line of Rb in room-temperature vapor. The observations can be understood from a density-matrix analysis of the sublevel structure.

DOI
10.48550/arxiv.1008.3231

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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 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 D2{D}_{2}D2​ line of Rb.
We observe resonances with width 20 times below the natural linewidth for a probe laser on an optical transition, when the same transition is driven by a control laser with a Laguerre-Gaussian (LG) profile. The basic mechanism is the phenomenon of electromagnetically induced transparency and absorption (EITA). Aligning the Gaussian probe beam to the central hole in the LG control beam simultaneously allows a strong control intensity required for high signal-to-noise ratio and a low intensity in the probe region required to get narrow resonances. We observe these resonances on the D2D_2D2​ line of Rb in a room-temperature vapor cell and show that the linewidth is reduced by a factor of 4 compared to the use of a Gaussian control beam, which should prove advantageous in all applications of EITA.
We demonstrate extremely narrow resonances for polarization rotation in an atomic vapor. The resonances are created using a strong control laser on the same transition, which polarizes the atoms due to optical pumping among the magnetic sublevels. As the power in the control laser is increased, successively higher-order nested polarization-rotation resonances are created, with progressively narrower linewidths. We study these resonances in the D 2 line of Rb in a room temperature vapor cell, and demonstrate a width of for the third-order rotation. The physical basis for the observed resonances is that optical pumping results in a simplified ΛV-type level structure with differential dressing of the levels by the control laser , which is why the control power has to be sufficiently high for each resonance to appear. This explanation is borne out by a density-matrix analysis of the system. The dispersive lineshape and subnatural width of the resonance lends itself naturally to applications such as laser locking to atomic transitions and precision measurements.
We demonstrate extremely narrow resonances for polarization rotation in an atomic vapor. The resonances are created using a strong control laser on the same transition, which polarizes the atoms due to optical pumping among the magnetic sublevels. As the power in the control laser is increased, successively higher-order nested polarization-rotation resonances are created, with progressively narrower linewidths. We study these resonances in the D-2 line of Rb in a room temperature vapor cell, and demonstrate a width of 0.14 G for the third-order rotation. The physical basis for the observed resonances is that optical pumping results in a simplified. AV-type level structure with differential dressing of the levels by the control laser, which is why the control power has to be sufficiently high for each resonance to appear. This explanation is borne out by a density-matrix analysis of the system. The dispersive lineshape and subnatural width of the resonance lends itself naturally to applications such as laser locking to atomic transitions and precision measurements. Copyright (c) EPLA, 2014
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.
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.