Abstract

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.

Highlights

  • The speed of light is one of the basic concepts of physics

  • The maximum advance in time for the reflected pulse is −0.87 μs, corresponding to a group velocity of −0.00029c (−87 km/s). These effects can be explained by atomic dispersion induced by quantum coherence. In this atom-light quantum coherence scheme, when the coupling light is changed from a single forward travelling light, partial standing wave, to a perfect standing wave, the absorption of the probe light is gradually transformed from an electromagnetically induced transparency (EIT) dip, EIT within absorption, to a narrow absorption peak, correspondingly, the dispersion is gradually changed from a normal to a steep anomalous dispersion [37]

  • A gain peak with an absorption dip in the center of resonant frequency is obtained due to resonant four-wave mixing (FWM), leading to an anomalous dispersion, which results in the superluminal propagation of the reflected light

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Summary

Introduction

The speed of light is one of the basic concepts of physics. An actual light pulse with a finite extent travels at a different speed, the velocity at which a peak of optical pulse travels is known as the group velocity υg. The group velocity of a light pulse can be slowed down (i.e. subluminal propagation speed) when the dispersion has a steep positive slope in electromagnetically induced transparency (EIT) media of atoms [2,3,4,5,6,9,10], solids and fibers [11,12]. The manipulation of the group velocity from a subluminal to a superluminal propagation speed has been demonstrated in atomic systems by changing one-photon detuning [17], amplitude [18] and single-into standing-wave [19] of the coupling field, and in optical fibers by changing the cross-gain modulation phase [20]

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