They first shot a relatively slowmoving laser pulse through the fibre, and then sent a faster “probe wave” chasing after it. The first pulse distorts the optical properties of the fibre simply by travelling through it. This distortion forces the speedy probe wave to slow down dramatically when it catches up with the slower pulse and tries to move through it. In fact, the probe wave becomes trapped and can never overtake the pulse’s leading edge, which effectively becomes a black hole event horizon, beyond which light cannot escape.
Saw that on slashdot a few minutes ago. I am constantly amazed at what the human mind can figure out. It is just incredible that they can not only recognize such phenomenon but that they can actually devise a way to reconstruct it in a lab.