The spectral “hole” forms due to coherent population oscillations that are set up when an argon-ion laser operating at 514.5 nm is shone on the ruby. The oscillations lead to a rapid variation in the refractive index of the ruby over a very narrow range of wavelengths, and this greatly reduces the group velocity. Moreover, the ruby is transparent over this narrow range of wavelengths.

The authors write that the pulse slowing depends on the intensity and modulation of the light. “By moving the ruby a small distance away from the focus, we could greatly increase the measured delay,” they report. “The group velocity can be controlled by changing the modulation frequency or the input intensity.”

Moreover, because the technique is easy to implement and involves only one laser, the Rochester team reckons it could find practical applications in telecoms. Light pulses passed along transmission lines could be stopped or delayed in crystals to synchronize or store them.