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Acta Physica Sinica

Stimulated Raman spectrum and optical pumping in a Λ-type Rb vapor system

Li-Ya Pei, Ruquan Wang, Zuo Zhan-Chun, Ling-An Wu, Panming Fu

Published Jan 1, 2013• 6 citations
Publisher

Abstract

Theoretical and experimental studies of stimulated Raman spectrum with the D1 line of an 85Rb atomic vapor system are performed. In this Λ-type system, we discover that the influence of the pump on the probe field includes two parts: stimulated Raman excitation and optical pumping. Depending on the pump frequency detuning and field intensity conditions, the Raman spectrum can display either gain or loss. When the optically pumped absorption spectrum is taken as background, either a narrow transparent window or an absorption doublet with widely different linewidths appears. Our theoretical analysis agrees well with our experimental observations. Based on these studies, we present an interpretation from a new viewpoint of the physics of electromagnetically induced transparency (with on-resonance pumping) and Autler-Townes splitting (with far off-resonance pumping) in a Λ-type level atomic system, giving the relationship between these two important phenomena and the Raman spectrum.

DOI
10.7498/aps.62.124208

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Journal of Physics B Atomic Molecular and Optical Physics•May 17, 2011•15 citations
Theoretical and experimental studies of stimulated Raman spectrum with the D1 line of an 85Rb atomic vapor system are performed. In this Λ-type system, we discover that the influence of the pump on the probe field includes two parts: stimulated Raman excitation and optical pumping. Depending on the pump frequency detuning and field intensity conditions, the Raman spectrum can display either gain or loss. When the optically pumped absorption spectrum is taken as background, either a narrow transparent window or an absorption doublet with widely different linewidths appears. Our theoretical analysis agrees well with our experimental observations. Based on these studies, we present an interpretation from a new viewpoint of the physics of electromagnetically induced transparency (with on-resonance pumping) and Autler-Townes splitting (with far off-resonance pumping) in a Λ-type level atomic system, giving the relationship between these two important phenomena and the Raman spectrum.
Based on our experimental observations of the resonant stimulated Raman gain and loss spectra in Rb atomic vapor, we propose an alternative interpretation of electromagnetically induced transparency (EIT). We find that, in the presence of a coupling field, a probe beam can exhibit both gain and loss depending on the frequencies of the incident beams. The gain and loss can coexist in a Doppler-broadened system, leading to polarization interference between atoms of different velocities, with a sharp transition from gain to loss as the probe-field frequency is scanned. We use the concept of Raman gain, instead of the commonly accepted quantum Fano interference, to explain the phenomenon of EIT and its transparency window as being due to the suppression of linear absorption by the Raman gain. Using this concept, the phenomena of EIT and Autler-Townes splitting may be explained within the same framework.
We observe experimentally huge enhanced four-wave mixing based on Raman resonance in an 85Rb atomic vapor system. With the decrease of coupling field power or the increase of experimental temperature, the signal tends to be narrowed down in linewidth, and be basically the same spectrum as the stimulated Raman spectroscopy. It is found that the macroscopic polarization interference effect plays a crucial role in determining the nonlinear spectra. Further more, in the Doppler-broadened Λ-type energy level system, there is a strong relationship among electromagnetically induced transparency, four-wave mixing and stimulated Raman spectroscopy. The sharp transparent window in electromagnetically induced transparency can be interpreted as the suppression of Raman gain on the linear absorption of the probe field. However, the four-wave mixing signal is a new field generated by the atomic vapor system, and it comes from the Raman gain which is affected and modified by the absorption and dispersion of the nonlinear optical medium. It shows that in a Λ-type Doppler-broadened system, in essence, both the electromagnetically induced transparency and enhanced four-wave mixing stem from stimulated Raman scattering based on the third-order nonlinear processes, just the spectra of which are from different ways and objects of detection.
Abstract We report the observation of electromagnetically induced transparency (EIT) in a six-level Λ -type system in atomic Rb vapor containing both 87 Rb and 85 Rb. The experimental observation includes five velocity selective optically pumped (VSOP) absorption dips for both 87 Rb and 85 Rb. The EIT signal appears on the background of one such VSOP absorption dips. The measured EIT linewidth ( ) shows sub-natural ( ) values for both lower and higher values of pump Rabi-frequencies. The density matrix based theoretical model for the six-level system is developed and solved numerically by taking into account the Doppler broadening. A complete analytical solution (non perturbative) for a three level Λ -type system has been obtained and compared with the experimentally observed sub-natural EIT linewidth. The simulated spectra are in good agreement with the experimental findings.
Abstract In this paper, we present a theoretical analysis of the burning of a narrow transparency window on the enhanced absorption spectrum of a probe laser in a triple resonance configuration visualized as a V-type plus a two-level system. The transparency window is due to coherence effects causing electromagnetically induced transparency (EIT) in a V-type system (i.e. Autler–Townes splitting plus quantum interference), for atomic vapor at a finite temperature. Additionally, the EIT window is enhanced by velocity induced population oscillation effect, velocity selective saturation effect, optical pumping effect (a source of ground state level population dynamics) and wavelength mismatch of the resonant laser fields forming a V-type configuration.
An electromagnetically induced transparency is observed at one arm of a three-level \ensuremath{\Lambda}-type system in a rubidium D1 line (794.8 nm) with an 85% reduction in absorption, when a pumping field is present at the other arm. This reduction in absorption for the weak probe field is due to the atomic coherence produced by the strong pumping field. This experiment is done in a Rb vapor cell at room temperature with cw diode lasers for both pumping and probe beams in a Doppler-free configuration. A simple theoretical treatment including Doppler broadening is in good qualitative agreement with the experimental measurement.
Atomic coherence resonances such as electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) signals are observed in a Doppler broadened multi-level system for the D 2 line of 85 Rb atoms in a vapour cell considering different types of atom–laser coupling schemes. Besides the coherence resonances, the nonlinear resonances like velocity selective optical pumping (VSOP) dips and velocity selective resonances (VSR) are also observed for the non-zero velocity selective groups of atoms. In the presence of pump, control and probe laser fields, we observe two EIT signals in a double Λ-type system and two EIA signals in a double V -type system. We are able to see the EIA signal for the non-zero velocity group of atoms when the pump laser is locked with an open transition by a small red detuning. We also report the experimental observation of simultaneous EIT and EIA signals in a (Λ+ V )-type system. The effect of frequency tuning of the control laser is also studied in the presence of the frequency locked pump laser. A simple theoretical analysis explains the experimental results.