Narrowing of resonances in electromagnetically induced transparency and absorption using a Laguerre-Gaussian control beam

We study the phenomenon of electromagnetically induced transparency and absorption (EITA) using a control laser with a Laguerre-Gaussian (LG) profile instead of the usual Gaussian profile, and observe resonances with width up to 20 times below the natural linewidth. Aligning the probe beam to the central hole in the doughnut-shaped LG control beam allows simultaneously a strong control intensity required for high signal-to-noise ratio and a low intensity in the probe region required to get narrow resonances. Experiments with a second-order LG beam show that transit time and orbital angular momentum do not play a significant role. This explanation is borne out by a density-matrix analysis with a radially varying control Rabi frequency. We observe these resonances using degenerate two-level transitions in the $D_2$ line of Rb in a room temperature vapor cell. We show that the linewidth is reduced by about a factor of 4 compared to the use of a Gaussian control beam, which should prove advantageous in all applications of EITA and other kinds of pump-probe spectroscopy.