Noise induced bistability of parametric surface waves.

We report an experimental study on the effect of an external multiplicative noise on a subcritical bifurcation leading to the parametric amplification of surface waves. We show that the probability density function of the wave amplitude in the presence of noise has two maxima that do not correspond to any of the deterministic states. When the deterministic forcing is varied in the presence of noise, these most probable values give two new branches in the bifurcation diagram that involve a much larger difference in oscillation amplitude. The bistable region is also strongly enlarged. This noise induced bistability can be understood in the general framework of noise induced transitions.