Observing ‘back-to-the-future ’ phenomenon with photonic chip

The possibility to engineer the group velocity (vg) of light has attracted much attention in the last couple of years. One of the most exotic phenomena in this research field is the negative vg phenomenon. Negative vg implies that if we send light pulse into the optical medium, the peak of the output pulse will leave the output before the peak of the input pulse entering the medium, i.e. a pulse ‘advancement ’ or negative delay. This paper will discuss such counter-intuitive ‘back-to-the-future ’ phenomenon and its direct time-domain experimental observations on a real photonic chip using measurement equipments available in a typical optical telecommunication laboratory. Comments on the consistency of the phenomenon with the causality principles as well as possible application will also be briefly discussed