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Acta Sinica Quantum Optica

Subluminal and Superluminal Propagation of Light in a Ladder-type Four-level Atomic System

Nian Liu

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

We study nonlinear optical behaviors of a ladder-type four-level cold atomics,where a radio-frequency field couples upper two-folded levels and double dark resonances can arise.Using appropriately chosen parameters,the system will show the transition between the normal chromatic dispersion and the abnormal chromatic dispersion.Changing the intensities of the coupling field and radio-frequency field can manipulate the shift between the subluminal and the superluminal.

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We study nonlinear optical behaviors of a ladder-type four-level cold atomics,where a radio-frequency field couples upper two-folded levels and double dark resonances can arise.Using appropriately chosen parameters,the system will show the transition between the normal chromatic dispersion and the abnormal chromatic dispersion.Changing the intensities of the coupling field and radio-frequency field can manipulate the shift between the subluminal and the superluminal.
Abstract The manipulation of the group velocity of light is very important in further information processing and quantum network. It is well known that subluminal and superluminal light propagation can be obtained by controlling the dispersion property of the experimental medium. In order to realize the switching from normal dispersion to anomalous dispersion at the same frequency, we investigate the dispersion and absorption properties of a four-level atomic system. By adjusting the intensity of an incoherent pump, we get transparent normal dispersion and lossless anomalous dispersion, which lead to subluminal and superluminal light propagation respectively. For a strong incoherent pump, the intensity of the coherent coupling field also can be used to realize a similar switching effect.
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.
A scheme is proposed for control of pulse propagation from subluminal to superluminal in a four-level Λ-type atomic system. With the relatively intense probe laser and the appropriate detuning of control field, the medium exhibits much lower absorption in the spectral ranges where the dispersion changes from normal to abnormal in the double electromagnetically induced transparency system than in the single electromagnetically induced transparency system. The transmission intensity can be enhanced by several times for the subluminal pulse propagation and by several orders of magnitude for the superluminal pulse propagation without the large distortion in the former compared with those in the latter. We attribute the dramatic absorption reduction to the enhanced nonlinear effects.
We report the experimental manipulation of the group velocities of reflected and transmitted light pulses in a degenerate two-level atomic system driven by a standing wave, which is created by two counter-propagating light beams of equal frequencies but variable amplitudes. It is shown that the light pulse is reflected with superluminal group velocity while the transmitted pulse propagates from subluminal to superluminal velocities via changing the power of the backward coupling field. We find that the simultaneous superluminal light reflection and transmission can be reached when the power of the backward field becomes closer or equal to the forward power, in this case the periodical absorption modulation for photonic structure is established in atoms. The theoretical discussion shows that the anomalous dispersion associated with a resonant absorption dip within the gain peak due to four-wave mixing leads to the superluminal reflection, while the varying dispersion from normal to anomalous at transparency, transparency within absorption, and electromagnetically induced absorption windows leads to the subluminal to superluminal transmission.
The quantum effects of a tripod-type four-level atom system interacting with three light fileds are studied.Under certain conditions,the author's calculations show that with the change of Rabi frequencies of the driving field,the curve of dispersion can transformed from abnormal chromatic dispersion to normal chromatic dispersion,thus the optical switching effect has appeared.When the parameter is certain,there is a phenomenon of electromagnetic induction transparent in resonance-near resonance case.