Pulses of 'fast light' and the signal velocity

Daniel J. Gauthier, Michael D. Stenner

Zhenshan S. Yang

We study the physical mechanism and the information velocity of fast light based on the time-domain propagator governing light propagation in a dispersive system. We show that the output pulse can be regarded as the superposition of multiple ``copies'' of the input pulse, each traveling at a well-defined signal velocity no greater than the speed of light in vacuum, and the interference between the front edges of these copies generates the peak of the output pulse. The information velocity is defined as the ratio of the propagation distance to the mini-mum time required to complete any given information transfer process, directly reflecting how fast the signal of interest, i.e., the part of the pulse that carries the information of interest, propagates. With the peak of the output pulse representing the information, as is typical in slow- and fast-light applications, we find that the information velocity is significantly smaller than the speed of a discontinuity in the pulse, which was traditionally proposed to be the signal velocity of fast light. The analysis can also be applied to slow-light propagation.

Makoto Tomita

Datang Xu, Zhengyang Bai, Guoxiang Huang

We propose a scheme to realize the storage and retrieval of light pulses in a fast-light medium via a mechanism of active Raman gain (ARG). The system under consideration is a four-level atomic gas interacting with three (pump, signal, and control) laser fields. We show that a stable propagation of signal light pulses with superluminal velocity (i.e., fast-light pulses) is possible in such a system through the ARG contributed by the pump field and the quantum interference effect induced by the control field. We further show that a robust storage and retrieval of light pulses in such a fast-light medium can be implemented by switching on and off the pump and the control fields simultaneously. The results reported here may have potential applications for light information processing and transmission using fast-light media.

Aaron Schweinsberg, Matthew S. Bigelow, Nikolay N. Lepeshkin +2 more

We demonstrate pump-power-tunable slow and superluminal pulse propagation at room temperature in an erbium-doped fiber amplifier. Signal pulses at 1550 nm were sent through an EDF with various pump intensities, obtaining signal group velocities as slow as 19,000 m/s and as fast as -16,000 m/s.