![]() 16 Furthermore, in some systems or media, such as fibers, 17,18 semiconductor waveguides, 19,20 nonlinear wave-mixing processes, 21 solids at room temperature, 22,23 and gain slabs, 24 positive and negative group delays can been demonstrated simultaneously in one system. 2 The effect of fast light has been observed in different systems including left-handed media, 10 the samples of GaP:N, 11 quantum wells, 12 double-Lorentzian fiber grating, 13 weakly absorbing slabs, 14 atomic media, 15 and ruby. It should be emphasized that fast light does not violate causality or special relativity because group velocity should not be regarded as a signal velocity. 9 Meanwhile, group velocity is also possible to be much larger than c or even becomes negative in this situation, it is then called as fast light with a very small or even negative group delay. 5 The approaches for realizing this delay have been widely investigated in various circumstances, such as ensembles of warm atoms (i.e., Rb and Cs) in vapor cells, 5 photonic crystal waveguides, 6–8 and multiple quantum wells. The large positive group delay has potential applications in realizing optical delay lines 4 and a high-efficiency memory for optical pulses. 1–4 When group velocity is much smaller than the speed of light c in vacuum, it often refers to slow light with a large value of group delay. Controlling the group delay (or group velocity) of a light pulse has been extensively studied in both theories and experiments for many years.
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