The theory of Hawking radiation can be tested in laboratory analogues of black holes. Recently, we have used light pulses in nonlinear fiber optics to establish artificial event horizons. Each pulse generates a moving perturbation of the refractive index via the Kerr effect. Probe light perceives this as an event horizon when its group velocity, slowed down by the perturbation, matches the speed of the pulse. In our experiment we have observed that the probe stimulates Hawking radiation with positive and with negative frequencies in the co-moving frame.

In the talk I will present our recent observation, being the first detection of stimulated negative frequency Hawking radiation in optics.