Ultrafast observation of the abnormal time delay of femtosecond laser pulses in a quartz crystal

Light with different frequencies has different propagation speeds when propagating in a medium. Nonlinear optics indicates that the refractive index of the medium also varies with the intensity of light. In this article, we have discovered an anomalous slow light phenomenon that is independent of light intensity through multi-frame ultrafast imaging. The experimental results show that when coaxial 800-nm and 400-nm pulses simultaneously enter the quartz crystal, the time delay between the 800-nm and 400-nm laser pulses is up to 5 times the normal value calculated by the dispersion theory. Moreover, this delay is independent of the laser intensity, indicating that the anomalous slow light phenomenon does not originate from refractive index changes in nonlinear optics. This abnormal time delay effect may find more applications in different fields, and its physical mechanism still needs further exploration.