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Conference on Lasers and Electro-Optics

A narrow Rabi frequency window for competition between coherent population trapping and Raman absorption

Yi-Chi Lee, Ray-Yuan Chang, Zong-Syun He, Ming‐Tsung Lee, Wei-Chia Fang, Hsiang‐Chen Chui, Chin‐Chun Tsai

Published Jan 1, 2009• 0 citations
Publisher

Abstract

We demonstrate the competition mechanism between the coherent population trapping and two-photon Raman absorption in a ladder-type system of cesium cell. 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, a decoherence created by the strong probe field takes place and two-photon Raman absorption dominates the interaction process.

DOI
10.1364/cleo.2009.jwa104

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arXiv:1707.09485•Scientific Reports•Jul 13, 2017•24 citations
We demonstrate the competition mechanism between the coherent population trapping and two-photon Raman absorption in a ladder-type system of cesium cell. 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, a decoherence created by the strong probe field takes place and two-photon Raman absorption dominates the interaction process.
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.
Ladder-type electromagnetically induced transparency (EIT) and two-photon absorption (TPA) under a low-light level of probe (0.2 μW/cm2(0.06Γ2)) and weak coupling power for a cesium atom at room temperature are investigated. Reduction of the fluorescence on the TPA in a three-level system via EIT interference is clearly observed and analyzed under the low probe Rabi frequency of about 0.30 MHz to avoid the affects from the vicinity of intermediate hyperfine states. The transparency ratio of EIT derived from the reduction of fluorescence is about 25%. Additionally, the EIT linewidth observed can be as narrow as 2.64 MHz, while the coupling Rabi frequency is around 2.66 MHz. By solving the steady-state optical Bloch equations, the numerical simulation spectra are in good agreement with EIT and TPA. According to our investigations, the Doppler velocity averaging effect over the thermal atoms reducing the linewidth of the EIT signal proves the advantage of observing the EIT in the room-temperature cell.
The effect of the collisions on the control of coherent population transfer in a closed inverse YYY-type four-level system driven by three laser fields is investigated with the stimulated Raman adiabatic passage technique. An arbitrary coherent superposition between the initial state and target state can be created either by varying the Rabi frequency of the control laser field or by tuning the one-photon detunings of the pump and Stokes laser fields. When the three laser fields with nearly equal peak Rabi frequencies are each tuned to resonance with the respective transitions, the low transfer efficiency due to the nonadiabatic coupling between two degenerate adiabatic states could be enhanced dramatically with the increase of the collision-induced coherence decay rates to the value nearly comparable to the Rabi frequency of the control field. The enhanced transfer efficiency results from the collision-controlled electromagnetically induced transparency in the $\ensuremath{\Lambda}$-type three-level subsystem and electromagnetically induced absorption in the ladder-type three-level subsystem of the four-level system.
Employing an intense laser field coupling to the degenerate two-level system of cesium 6 2S_ 1/2(F=4)- 6 2P_ 3/2(F′=5) transition in a cesium vapor cell, electromagnetically induced absorption (EIA) has been demonstrated via probe absorption spectra, at the same time, electromagnetically induced transparency (EIT) of V-type three-level scheme has been demonstrated on the F=4-F′=3 and F=4-F′=4 transition frequency. The EIA is affected by the intensity and detuning of the coupling laser. By analyzing the effect of optical pumping between Zeeman sub-levels on the EIA and the coherent interactions between multiple degenerate two-level atoms, it is shown that the optical pumping affects the population of different Zeeman sub-levels and the number of the involved two-level atoms, the coherent time between the sub-levels is then changed and the linewidth and intensity of the EIA are affected accordingly.
The absorption spectrum of a weak probe, interacting with a driven degenerate two-level atomic system, whose ground and excited hyperfine states are Fg,e,{\mathrm{F}}_{g,e},Fg,e​, can exhibit narrow peaks at line center. When the pump and probe polarizations are different, Fe=Fg+1{F}_{e}{=F}_{g}+1Fe​=Fg​+1 and Fg>0,{F}_{g}>0,Fg​>0, the electromagnetically induced absorption (EIA) peak has been shown to be due to the transfer of coherence (TOC) between the excited and ground states via spontaneous decay. We give a detailed explanation of why the TOC that leads to EIA (EIA-TOC) can only take place when ground-state population trapping does not occur, that is, when Fe=Fg+1.{F}_{e}{=F}_{g}+1.Fe​=Fg​+1. We also explain why EIA-TOC is observed in open systems. We show that EIA can also occur when the pump and probe polarizations are identical and Fe=Fg+1.{F}_{e}{=F}_{g}+1.Fe​=Fg​+1. This EIA is analogous to an effect that occurs in simple two-level systems when the collisional transfer of population (TOP) from the ground state to a reservoir is greater than that from the excited state. For a degenerate two-level system, the reservoir consists of the Zeeman sublevels of the ground hyperfine state, and of other nearby hyperfine states that do not interact with the pump. We will also discuss the four-wave mixing spectrum under the conditions where EIA-TOC and EIA-TOP occur.
We experimentally demonstrate coherent amplification of probe field in a tripod-type atoms driven by strong coupling, signal and weak probe fields. We suppress linear and nonlinear atomic absorptions for resonant and near resonant probe via double electromagnetically induced transparency (DEIT). Combining these advantages of suppressed absorption along with temperature- or atomic-density-controlled transfer of population(ToP) between hyperfine ground states, we can induce near-resonant amplification of probe through stimulated Raman scattering(SRS) pumped by low-intensity signal field. The increased population difference of initial and final states of SRS due to increased ToP rate, together with reduced absorption at the second EIT window in an optically thick Cesium vapor, gives rise to highly effective coherent amplification.