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.

光群速度的操纵在进一步的信息处理和量子网络中非常重要。众所周知，通过控制实验介质的色散特性可以获得亚光速和超光速的光传播。为了实现同一频率下从正常色散到反常色散的切换，我们研究了四能级原子系统的色散和吸收特性。通过调整非相干泵浦的强度，我们得到透明的正常色散和无损的异常色散，分别导致亚光速和超光速光传播。对于强非相干泵浦，也可以利用相干耦合场的强度来实现类似的切换效果。