摘要
This work develops a time-domain approach to explain the origin of superluminality, i.e., an apparent faster-than-light phenomenon in one-dimensional Fabry-Perot cavities. The interferences between multiple-reflection echoes are tracked and analyzed. Under the quasi-static condition, the destructive interference is found to result in the advancement of the superposed pulse as compared to its parent signals. It is the key to the superluminal effect. On the contrary, constructive interference can lead to the retardation of the superposed pulse. For a cavity operated at the off-resonance state, the competition between roundtrip destructive interference (group advancement) and multiple-roundtrip constructive interference (group retardation) determines how short the group delay can be observed. Manipulating the competition between roundtrip interferences opens numerous opportunities in controlling the signal delays or shaping the pulse waveforms.