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arXiv:physics/0204023 · Physical Review A

Subnatural linewidth in room-temperature Rb vapor using a control laser

Umakant D. Rapol, Ajay Wasan, Vasant Natarajan A

Published May 15, 2003• 30 citations
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Abstract

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.

DOI
10.1103/physreva.67.053802

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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\ensuremathΓ)(0.5\ensuremath{\Gamma})(0.5\ensuremathΓ) at a detuning of 11.5 MHz.
We investigate, theoretically and experimentally, absorption on an excited-state atomic transition in a thermal vapor where the lower state is coherently pumped. We find that the transition linewidth can be subnatural, that is, less than the combined linewidth of the lower and upper state. For the specific case of the $6{P}_{3/2}\ensuremath{\rightarrow}7{S}_{1/2}$ transition in room temperature cesium vapor, we measure a minimum linewidth of 6.6 MHz compared with the natural width of 8.5 MHz. Using perturbation techniques, an expression for the complex susceptibility is obtained which provides excellent agreement with the measured spectra.
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 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
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 demonstrate a technique for precisely measuring hyperfine intervals in alkali atoms. The atoms form a three-level Λ system in the presence of a strong control laser and a weak probe laser. The dressed states created by the control laser show significant linewidth reduction. We have developed a technique for Doppler-free spectroscopy that enables the separation between the dressed states to be measured with high accuracy even in room temperature atoms. The states go through an avoided crossing as the detuning of the control laser is changed from positive to negative. By studying the separation as a function of detuning, the center of the level-crossing diagram is determined with high precision, which yields the hyperfine interval. Using room temperature Rb vapor, we obtain a precision of 44 kHz. This is a significant improvement over the current precision of ∼ 1 MHz.
Doppler broadening presents a major limitation for high-resolution spectroscopy and nonlinear optics in room-temperature atomic vapors. Here, we demonstrate the suppression of Doppler broadening accompanied by pronounced absorption on the upper transition of a three-level ladder system, achieved by dressing the intermediate state with a strong control field. As a concrete realization, we study a hot vapor of 87^{87}87Rb where the lower transition is driven by a strong control field resonant with the D2 line at a wavelength of 780 nm, while a weak counter-propagating probe field at the telecom C-band wavelength of 1529 nm (5P(3/2)↔4D(5/2)5P_{(3/2)}\leftrightarrow 4D_{(5/2)}5P(3/2)​↔4D(5/2)​) interrogates the dressed states. We observe absorption features with a resonant optical depth of approximately 4 and a full width at half maximum of about 17 MHz. Remarkably, this corresponds to an order-of-magnitude reduction relative to the Doppler width, while the optical depth on the upper transition of the ladder scheme exceeds that of the Doppler-broadened lower transition. The measured spectra are in good agreement with theoretical modeling. Combining high optical density with sub-Doppler-width absorption lines typically requires laser-cooled atoms, while our approach profits from the experimental simplicity of a hot-vapor platform.