Not sure if this is relevant: In 2004, researchers at UC Berkeley first demonstrated slow light in a semiconductor, with a group velocity 9.6 kilometers per se

Not sure if this is relevant: In 2004, researchers at UC Berkeley first demonstrated slow light in a semiconductor, with a group velocity 9.6 kilometers per second.[5] Hau and her colleagues later succeeded in stopping light completely, and developed methods by which it can be stopped and later restarted. https://en.wikipedia.org/wiki/Slow_light https://en.wikipedia.org/wiki/Slow_light