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arXiv:0709.0809 · Journal of Physics B Atomic Molecular and Optical Physics

Group velocity control in the ultraviolet domain via interacting dark-state resonances

Mohammad Mahmoudi, Robert Fleischhaker, Mostafa Sahrai, Jörg Evers

Published Jan 8, 2008• 28 citations
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Abstract

The propagation of a weak probe field in a laser-driven four-level atomic system is investigated. We choose mercury as our model system, where the probe transition is in the ultraviolet region. A high-resolution peak appears in the optical spectra due to the presence of interacting dark resonances. We show that this narrow peak leads to the superluminal light propagation with strong absorption, and thus by itself is only of limited interest. But if in addition a weak incoherent pump field is applied to the probe transition, then the peak structure can be changed such that both sub- and superluminal light propagation or a negative group velocity can be achieved without absorption, controlled by the incoherent pumping strength. A suitable choice of laser propagation directions allows us to preserve these results under Doppler averaging.

DOI
10.1088/0953-4075/41/2/025504

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arXiv:0808.0263•Laser Physics•Jul 1, 2009•8 citations
The propagation of a weak probe field in a laser-driven four-level atomic system is investigated. We choose mercury as our model system, where the probe transition is in the ultraviolet region. A high-resolution peak appears in the optical spectra due to the presence of interacting dark resonances. We show that this narrow peak leads to the superluminal light propagation with strong absorption, and thus by itself is only of limited interest. But if in addition a weak incoherent pump field is applied to the probe transition, then the peak structure can be changed such that both sub- and superluminal light propagation or a negative group velocity can be achieved without absorption, controlled by the incoherent pumping strength. A suitable choice of laser propagation directions allows us to preserve these results under Doppler averaging.
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 analyse the group velocity of a laser pulse in an optically dressed atomic system. A Λ system with an additional close upper level and an incoherent pump is specially investigated. The group velocity of a pulse is strongly dependent on the optical properties of the system which can be modified by changing the amplitude or detuning of a strong coupling field or of an incoherent pump between the two lower levels of the system. Depending on the parameters, the medium may have alternatively absorptive or gain properties and the dispersion can be changed from anomalous into a normal one. The ranges of parameters are especially investigated in which absorption (gain) is not too strong, with dispersion being not too steep. The group velocity of a pulse propagating through a sample with such optical properties can be switched from the subinto superluminal regime. The dynamics of propagation in the case of negative group velocities of a small absolute value is especially interesting. In such a regime the peak of the transmitted pulse exits the sample before the peak of the incoming pulse reaches the medium. The transmitted pulse splits into two pulses - one of them propagates forward behind the medium and the other propagates backward and is cancelled at the entrance of the sample by the incoming pulse. Ranges of parameters are seeked in which the shape of the pulse is changed and the group velocity as a single parameter is not sufficient to describe the pulse propagation.
We theoretically study the coherent interaction of a tripod-type atomic system with three laser fields, i.e., one probe and two coupling, or two probe and one coupling field. Our analytical and numerical results show that, when all three fields are on two-photon resonance, in addition to the Rabi frequencies of the coupling fields, the probe group velocities also depend on the spontaneous decay rates and the initial population distribution. So we can manipulate the probe pulses to achieve controllable ultraslow group velocities by preparing different initial atomic states. With an incoherent pump field applied, a narrow transparent window accompanied by abnormal dispersion may occur in the probe absorption spectrum, which then allows us to switch a probe pulse from ultraslow to superluminal propagation.
A four-level atomic system interacting with two strong drive fields and a weak probe field has been studied. We demonstrate that the group velocity of probe field can be controlled to switch from subluminal to superluminal or even negative values around the probe resonance without any significant absorption. The basic idea is to create a gain with slow light on the probe beam resonance with one drive field while the other drive creates a dip in the gain profile of probe leading to superluminal and negative group velocities. A simple switching from slow light to negative group velocity light can be obtained by changing the relative intensities of drive fields. We also describe a control to tune the position of slow light frequency continuously simply by adjusting the relative intensities of both the driving fields.
We study the manipulation of Goos-H\"anchen (GH) shifts for the reflected and transmitted probe light pulses injected into a cavity containing four-level configuration mercury atoms where the probe transition is in the ultraviolet (UV) region with a wavelength of 253.7nm253.7\phantom{\rule{0.28em}{0ex}}\mathrm{nm}253.7nm. Different behaviors of the GH shifts can be observed in the absence, or presence, of two driving fields as well as an incoherent pump field. When neither coherent driving fields nor incoherent pumping is turned on, we realize negative reflected GH shifts for anomalous dispersion. Including only one driving field leads to subluminal-based light propagation with positive lateral shifts at certain incident angles. Taking into account the impact of both driving fields, negative GH shifts reappear in the reflected part of the incident light. The origin of this defect is attributed to interacting double dark resonances due to a high-resolution absorption peaks with a very steep negative slope of dispersion in the susceptibility profile. We then show that one can surpass this defect by applying a weak incoherent pumping field to obtain positive GH shifts for both reflected and transmitted light beams. Finally, using the $6{\phantom{\rule{0.16em}{0ex}}}^{1}{P}_{1}\ensuremath{\leftrightarrow}6{\phantom{\rule{0.16em}{0ex}}}^{1}{S}_{0}$ transition of Hg, we generalize our study to the case where the wavelength of the probe transition is 185nm185\phantom{\rule{0.28em}{0ex}}\mathrm{nm}185nm which is in the vacuum-ultraviolet domain. Although the number of oscillations is now increased, however, similar results are reported with respect to the case of UV transition.