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Extremely narrow resonances in electromagnetically induced transparency and absorption using a Laguerre-Gaussian control beam

Sapam Ranjita Chanu, Vasant Natarajan A

Published Oct 29, 2011• 0 citations
OpenAlex

Abstract

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 $D_2$ 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.

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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.
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.
Electromagnetically induced transparency (EIT) in a molecular three-level Λ system with permanent dipole moments (PDMs) and the control beam in Laguerre–Gaussian (LG) mode is investigated. The results are compared with the atomic system where the use of LG mode yields EIT linewidth narrower than that with Gaussian mode. It is shown that in contrast to the atomic system, the linewidth of EIT in the dipolar molecular system with LG mode can be narrower as well as broader than that with Gaussian mode depending on the strength of the effective laser-molecule coupling. Peculiar space dependence of Bessel function in the laser-molecule coupling is at the origin of this distinctive feature. The results are further augmented by a discussion on PDM-mediated multi-photon absorption and multi-photon EIT profiles with LG and Gaussian modes.
We study the influence of laser profile on the linewidth of the optical spectrum of multi-photon resonance phenomena. First, we investigate the dependence of the absorption spectrum on the laser profile in a two-level system. Thanks to the Laguerre–Gaussian field, the linewidth of the one-photon optical pumping and two-photon absorption peaks are explicitly narrower than that obtained with a Gaussian field. In the next section, it is shown that, compared to the Gaussian fields, the Laguerre–Gaussian ones reduce the linewidth of the optical spectrum in the coherent population trapping. Interestingly, it turns out that the use of a Laguerre–Gaussian beam makes the linewidth of the spectrum narrower as compared with a Gaussian one in Doppler-broadened electromagnetically induced transparency. Moreover, we study the effect of the laser profile on the Autler–Townes doublet structure in the absorption spectrum for a laser-driven four-level atomic system. We also consider the different values of the Laguerre–Gaussian mode beam waist, and, perhaps more remarkably, we find that for the small waist values, the Autler–Townes doublet can be removed and a prominent narrow central peak appears in the absorption spectrum. Finally, we investigate the effect of the laser profile on the linewidth of the sub-natural three-photon absorption peak of double dark resonance. The differences in the linewidth are quite large, offering potential applications in metrology and isotope separation methods. Our results can be used for super ultra-high resolution laser spectroscopy and to improve the resolution of the technology of isotope/isomer separation and photo-biology even at essential overlap of the spectra of the different particles.
We report an experimental observation of the variation in linewidth of the electromagnetically induced transparency (EIT) resonance in a three-level ?-type system for several laser bandwidths in a Rb vapor cell, with and without a buffer gas. It is found, using narrow bandwidth (about 20 kHz) diode laser for both coupling and probe beams, that the linewidth of the EIT resonance can be significantly narrowed in the Rb vapor cell with the buffer gas. The results are in good qualitative agreement with a simple theoretical calculation.
Counterpropagating, spatially separated hollow pump and coaxial probe laser beams generate narrow Zeeman electromagnetically induced transparency (EIT) resonances in the vacuum Rb cell. The lasers are locked to D2 line transition Fg=2→Fe=1 of Rb87. For the probe laser beam intensity between 0.1 and 3.0 mW/cm2 this Ramsey-type configuration yields dual-structured resonances having a narrow peak on top of a broader pedestal. Linewidths of the narrow peak are nearly independent of the probe laser beam intensity and of the probe diameter (for diameters 0.8 and 2.7 mm), provided that the dark region between the pump and the probe beams is fixed. At the probe laser beam intensities below 0.1 mW/cm2 Zeeman EIT is a single narrow resonance. With this geometry of laser beams, and at low probe intensity, the presence of the pump enables the probe EIT, i.e., the probe transmission becomes enhanced in a narrow spectral window. Accompanying theoretical model showed good quantitative agreement with the measurements.