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Europhysics Letters (EPL)

Dark and bright Doppler-free resonances observed in atomic 87 Rb vapor

Xiaogang Wei, Jin‐Hui Wu, Hai-Hua Wang, Ang Li, Zhi‐Hui Kang, Yun Jiang, Jin-Yue Gao

Published May 1, 2007• 8 citations
Publisher

Abstract

We investigate an inhomogeneously broadened four-level atomic system driven by two strong coherent fields and probed by a weak laser. By carefully adjusting strengths of the driving fields, we can observe a greatly enhanced peak between two dark resonances in the probe absorption spectrum. The enhanced absorption peak will split into two lower ones and then result in a third dark resonance if we gradually weaken one driving field. Ultimately, the whole probe absorption spectrum evolves towards the usual EIT spectrum with only one dark resonance. Based on appropriate numerical calculations and qualitative analysis, we show that the experimental results can be well understood by separately considering the probe absorptions arising from partial atoms with different velocities.

DOI
10.1209/0295-5075/78/44002

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We investigate an inhomogeneously broadened four-level atomic system driven by two strong coherent fields and probed by a weak laser. By carefully adjusting strengths of the driving fields, we can observe a greatly enhanced peak between two dark resonances in the probe absorption spectrum. The enhanced absorption peak will split into two lower ones and then result in a third dark resonance if we gradually weaken one driving field. Ultimately, the whole probe absorption spectrum evolves towards the usual EIT spectrum with only one dark resonance. Based on appropriate numerical calculations and qualitative analysis, we show that the experimental results can be well understood by separately considering the probe absorptions arising from partial atoms with different velocities.
We study both theoretically and experimentally three-photon electromagnetically induced transparency and electromagnetically induced absorption resonances in inhomogeneously broadened 85 Rb atomic vapour driven by probe and drive laser radiations. We observe narrow Doppler-free absorption as well as transmission resonances for the probe field when the driving laser field is redshifted from the D1 or D2 lines of 85Rb; the frequency difference between the drive and probe fields is equal to the hyperfine splitting of the ground state of the atoms, and the probe field is tuned to the centre of the Doppler broadened atomic transition. We theoretically study the spectroscopic effect in both homogeneously and inhomogeneously broadened media. Our numerical simulations are in good agreement with the experimental results.
Abstract We report the observation of two additional sub-natural line width quantum interferences in the D 2 manifold of 39 K vapor, in addition to the usual single Electromagnetically Induced Transparency (EIT) peak. In a typical three level Λ-type system, only one EIT peak is observed. However, here we report observation of two additional line shapes riding on top of the absorption profile. The fact that the hyperfine splitting is smaller than the Doppler width in 39 K allows the probe and control beams to swap their transition pathways in different velocity groups of atoms even when their frequencies are kept constant. Our observations are in striking contrast to standard EIT measurements. These findings are in quantitative agreement with density matrix formalism taking into account velocity-selective two-photon resonances. Owing to the favorably low ground hyperfine splitting (Δ hf ) in 39 K, which allows a significantly large number of atoms with a Doppler shift greater than or equal to the Δ hf , the strength of these additional resonances is strong compared to that of other alkali atoms such as 87 Rb, 133 Cs where these resonances can not be observed. The control photon detuning to atomic transition captures the nature of the coherence; therefore an unusual phenomenon of conversion from perfect transparency to enhanced absorption of the probe photon is observed and explained by utilizing the adiabatic elimination of the excited state in the Master equation. Controlling such dark and bright resonances leads to new applications in quantum technologies such as frequency-offset laser stabilization and long-lived quantum memory.
We report an experimental investigation of the effect of electromagnetically induced transparency (EIT) using bichromatic laser radiation and an extremely thin cell filled with pure Rb with smoothly controllable thickness LLL of the atomic vapor layer in the range $\ensuremath{\sim}780--1600\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, for which LLL is comparable to the laser wavelength $\ensuremath{\lambda}$ resonant with the D2{D}_{2}D2​ line (780nm)(780\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(780nm). It is revealed that the transmission spectrum of the probe laser contains two sub-Doppler peaks that have different linewidths. The narrow peak corresponds to EIT, whereas the broader one results from velocity-selective optical pumping (VSOP) and repumping processes. It is demonstrated that in the case of nonzero detuning of the coupling laser, the EIT resonance and VSOP are shifted with respect to each other on the frequency scale, which makes it possible to observe a competition between the two effects (this is not possible to realize in an ordinary cell). Also, the Dicke-type coherent narrowing effect depending on the ratio $L∕\ensuremath{\lambda}$ influences the absorption spectrum of the probe laser. Formation of an EIT resonance is substantially favored for the atoms with slow normal velocity, caused by their longer interaction time with the bichromatic laser field. As a result of the predominant contribution of these atoms, the observed linewidth of the EIT resonance is only $\ensuremath{\sim}9\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, which is more than ten times narrower than the inverse window-to-window flight time. In an external magnetic field, three and five EIT resonances have been observed for the vapor thickness of $L=2\ensuremath{\lambda}$, on 87Rb^{87}\mathrm{Rb}87Rb and 85Rb^{85}\mathrm{Rb}85Rb, respectively.
We study the phenomenon of electromagnetically induced transparency (EIT) in room-temperature Rb vapour under conditions of a strong probe laser. The atoms form a ladder system with the lower transition coupled by the probe laser and the upper transition coupled by the control laser. Density-matrix analysis of the system shows that, for zero-velocity atoms, the strong probe beam causes a reduction in absorption and broadening of the profile. But the lineshape after thermal averaging shows a splitting of the EIT resonance and enhanced absorption near line centre. The experimental observations in 85Rb^{85}Rb85Rb agree qualitatively with the predicted lineshapes.
We investigate dispersion and absorption properties of a weak probe field in a four level atomic system through interacting dark resonances. We show that two narrow peaks appears in the optical spectra due to the presence of interacting dark resonances. This two narrow peaks leads to the superluminal light propagation with strong absorption. However, if a weak incoherent pump field is exerted to the probe transition, then the peaks structures can be changed such that two absorption peaks transformed to two gain deeps. We Show that both subluminal and superluminal light propagation occur in this system, where controlled by the incoherent pumping strength.
We have experimentally studied electromagnetically induced transparency (EIT) and absorption (EIA) in hot 85Rb atomic vapor using probe and coupling light with comparable power levels. We have shown that strong-probe EIT has different linewidth and appears in fewer configurations than does usual, weak probe EIT. In V-scheme, where optical pumping and saturation are dominant mechanisms, narrow EIT is possible only when a probe is tuned to a closed transition. The width of the EIT resonance increases with laser intensity with non-linear dependence, similar to the weak-probe EIT in Lambda- scheme. The EIT in Lambda- scheme was observed when two transitions had balanced population losses. The EIA was modified for the case of a strong probe as well: in four-level N-scheme with Zeeman sublevels the EIA was observed only for a cycling transition when F'=F+1, where F and F' are the angular momenta of the 5 2S1/2 (ground) and 5 2P3/2 (excited) state hyper-fine levels, respectively. The combination of strong probe and strong coupling laser beam intensities allows observation of an absorption dip due to three-photon resonance in a four-level scheme that involves the Raman transitions via virtual level.