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Quantum Electronics and Laser Science Conference

Intensity-fluctuation-induced spectral structure

Abraham G. Kofman

Published Jan 20, 2003• 0 citations
Publisher

Abstract

Summary form only given. It is well known that strong resonant laser radiation can significantly modify absorption and emission spectra of matter, producing such features, as the Autler-Towns doublet, the dark resonance, etc. Laser fluctuations, which are often inevitable, usually tend to smear out the field-induced spectral features. Here, however, we show that in an experimental scheme, usually used to obtain electromagnetically induced transparency (EIT), intensity fluctuations of a strong coupling field can induce a narrow peak in the probe absorption spectrum.

DOI
10.1109/qels.1999.807664

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•Jan 1, 2011•5 citations
Summary form only given. It is well known that strong resonant laser radiation can significantly modify absorption and emission spectra of matter, producing such features, as the Autler-Towns doublet, the dark resonance, etc. Laser fluctuations, which are often inevitable, usually tend to smear out the field-induced spectral features. Here, however, we show that in an experimental scheme, usually used to obtain electromagnetically induced transparency (EIT), intensity fluctuations of a strong coupling field can induce a narrow peak in the probe absorption spectrum.
Electromagnetically induced transparency (EIT) with chaotic field is considered for the constructive-interference case ( i.e. ,when the material transition is much narrower than the coupled one). The field intensity fluctuations are shown to induce a narrow peak at the material frequency, in addition to the familiar Autler-Towns doublet. The dependence of the fluctuation-induced peak on the field bandwidth ν is obtained analytically and numerically. EIT is shown to increase with ν for sufficiently small ν.
We report an experimental observation of narrow and high-contrast spectra, which are induced by interacting dark resonances and have been predicted in Phys. Rev. A 60, 3225 (1999). Spectra are measured with cold 87Rb{}^{87}\mathrm{Rb}87Rb atoms produced by a magneto-optical trap. In this experimental system, a coupling laser and a weak probe laser form a three-level \ensuremath{\Lambda}-type configuration of electromagnetically induced transparency (EIT); a microwave drives a magnetic-dipole transition between the fourth level and the ground state that is coupled with the excited state by the coupling laser. The observed spectral profile of probe absorption exhibits a very sharp peak emerging inside a narrow EIT dip. Such spectral feature provides more opportunities in manipulating atomic-optical response.
This paper deals with electromagnetically induced transparency (EIT), i.e., a significant suppression of absorption at a material transition (in the form of a dip in an absorption spectrum), due to a strong laser field, coupling the excited level with an auxiliary excited (unpopulated) level. A comprehensive theory of EIT is developed for the cases of coherent and Markovian phase- and amplitude-phase-fluctuating coupling fields. For a coherent coupling field, a shift of the absorption minimum from the two-photon resonance is revealed in the off-resonance case. Two models of amplitude-phase fluctuating fields are considered: the chaotic field and the uncorrelated-jump field. Closed analytical expressions for EIT line shape are derived and exact limits of different regimes of EIT are obtained, the emphasis being on the near-resonance case. The main conclusion is that an amplitude-phase-fluctuating field can induce significant transparency, though reduced in comparison to a phase-fluctuating field of the same average intensity and bandwidth. EIT decreases with the increase of the bandwidth for all stochastic models considered. EIT with a chaotic field is generally less pronounced than EIT with an uncorrelated-jump field of the same intensity and bandwidth, the difference increasing with the field intensity. The possibility of experimental verification of the results obtained is discussed.
Summary form only given. The phenomenon of electromagnetically induced transparency (EIT) has been the subject of extensive experimental and theoretical studies in recent years, since it has a fundamental significance and a host of important applications. To produce EIT one requires a strong laser field, which often undergoes random fluctuations. The case of amplitude-phase fluctuations appears to be much more complicated and richer than the case of Markovian phase fluctuations. Here we consider transparency induced by a Gaussian-Markovian amplitude-phase fluctuating (chaotic) field. The problem involves five parameters: the coupling field rms Rabi frequency, bandwidth, detuning, and the widths of the material and coupled transitions.
We report an experimental observation of Autler-Townes doublet splitting in electromagnetically induced transparency (EIT) resonance. The splitting is introduced by a coherent microwave field, which perturbs a three-level A-type EIT system through an auxiliary level. The doublet splitting of EIT resonance is demonstrated in two cases, where the microwave field shares a common lower level with coupled or probe transition, respectively. The dependence of doublet splitting on microwave field intensity and detuning is measured. This may provide a feasible way of manipulating the atomic states with both laser and microwave field.
We initially discuss the theory of three-level systems using the dressed state formalism. One of the dressed states, containing a ground state and a Rydberg state, does not couple with the probe laser; thus, the medium becomes transparent to the latter. This phenomenon is known as electro- magnetically induced transparency (EIT) and this dressed state is known as a Rydberg dark state. We show that EIT can be used to extract the reduced dipole matrix element for transitions to a Rydberg state. However, a prob- lem with three-level Rydberg EIT in a vapour cell is the occurrence of space charges caused by photoelectric ionisation of Rb metal deposited inside the cell. To avoid this problem, we consider adding a third laser resonant with a fourth level. This is to avoid using the laser whose wavelength is less than the threshold wavelength. In cold atoms, the effect of the third laser is to split the usual EIT resonance into a doublet. In thermal atoms, we observe narrow features due to electromagnetically induced absorption and electro- magnetically induced transparency in the Doppler-free configuration. Next we consider the action of a far off-resonance radio frequency (rf) field in the three-level system. We demonstrate the formation of rf-dressed EIT reso- nances in a thermal Rb vapour and show that such states exhibit enhanced sensitivity to dc electric fields compared to their bare counterparts. Fitting the corresponding EIT profile enables precise measurements of the dc field in- dependent of laser frequency fluctuations. We further investigate the theory of rf-dressed Rydberg EIT using the Floquet approach in order to understand the formation of the sideband structure of the Rydberg state. We find that if the time scale of the rf interaction is much shorter than that of the system evolution and decoherence, the sideband structure is well resolved. We also show that the intermediate state exhibits a sideband structure, induced by the Rydberg state, when the Rabi frequency of coupling laser is larger than twice the modulation frequency. Finally we consider resonant microwave cou- pling between the Rydberg states which leads to an Autler-Townes splitting of the EIT resonance in cold atoms. This splitting can be employed to vary the group index by ±10^5 allowing independent control of the absorptive and dispersive properties of the medium, i.e., one can switch the transparency of the medium or control the group velocity of a pulse propagation by tuning on and off the microwave field.