国内教材翻译的不对,群速度可以超光速。你可以参考Claus Klingshirn的Semiconductor Optics,3rd Ed第98页最后一段: KiG/XnS
For a discussion of the fact, that both the phase- and group velocities can exceed for some frequenciyes the vacuum speed of light. It should be noted, however, that the signal and energy transport velocities never exceed c. D`r_ Dz
老一辈中英语不好真是害死后代啊!