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Physical Review A

Resonant stimulated Raman gain and loss spectroscopy in Rb atomic vapor

Li-Ya Pei, Xiaogang Lu, Jin-Hai Bai, Xingxu Miao, Ruquan Wang, Ling-An Wu, Shiwei Ren, Zhi-Yong Jiao, Huafeng Zhu, Panming Fu, Zhanchun Zuo

Published Jun 14, 2013• 22 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.87.063822

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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.
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.
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.
This investigation clarifies the transition phenomenon between the electromagnetically induced transparency (EIT) and Raman absorption in a ladder-type system of Doppler-broadened cesium vapor. A competition window of this transition was found to be as narrow as 2 MHz defined by the probe Rabi frequency. For a weak probe, the spectrum of EIT associated with quantum interference suggests that the effect of the Doppler velocity on the spectrum is negligible. When the Rabi frequency of the probe becomes comparable with the effective decay rate, an electromagnetically induced absorption (EIA) dip emerges at the center of the power broadened EIT peak. While the Rabi frequency of the probe exceeds the effective decay rate, decoherence that is generated by the intensified probe field occurs and Raman absorption dominates the interaction process, yielding a pure absorption spectrum; the Doppler velocity plays an important role in the interaction. A theory that is based on density matrix simulation, with or without the Doppler effect, can qualitatively fit the experimental data. In this work, the coherence of atom-photon interactions is created or destroyed using the probe Rabi frequency as a decoherence source.
The evolution of enhanced four-wave mixing based on Raman coherence as the probe intensity increases is observed experimentally in an 85 R b atomic vapor system. We propose that for a Doppler-broadened Λ -type level system, the generated anti-Stokes field from one-photon resonance will be suppressed by the dressed-states resonance and can be controlled by the probe intensity, which influences the effective transverse relaxation rate of Raman coherence. When the probe field is not so weak, the double resonance for dressed states will greatly suppress the anti-Stokes emission at the central frequency, and the signal peak will be split. We show that this phenomenon is a result of the destructive polarization interference between atoms with different velocities from the two types of resonance. Based on our model, we present a new, to the best of our knowledge, interpretation of four-wave mixing enhancement in the conventional Λ -type electromagnetically induced transparency system.
We have theoretically and experimentally investigated coherent pump-probe spectra for the 3S1/23{S}_{1/2}3S1/2​-3P1/23{P}_{1/2}3P1/2​ D1D1D1 transition of sodium atomic vapor. Probe transmission spectra in the presence of a coupling beam exhibit dramatic changes depending on experimental conditions. In the weak-excitation, low-atomic-density limit, the spectra are mainly characterized by electromagnetically induced transparency (EIT) and saturated absorption, but for the strong-excitation, high-density case, parametric amplification (PA) is dominant, featuring high probe gain and Stokes-wave generation. We have developed a theory that can explain these two seemingly totally different phenomena (EIT and PA) within the same theoretical framework by manipulating a few experimentally controllable parameters, and have successfully reproduced the observed spectra. Other than the main spectral features, many other interesting physical processes have been predicted and observed.