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Dark surface waves in self-focusing media with diffusion and photovoltaic nonlinearities.

Dark surface waves with photorefractive diffusion and photovoltaic nonlinearities are predicted for the first time. We find it is extraordinary that this type of dark surface waves should be in self-focusing media, which is very different from the surface dark solitons or other nonlinear dark surface waves. An oscillator model is proposed by which the above extraordinary phenomenon is demonstrated. In this model an equivalent force function is established, whose form determines the varieties of surface waves (bright surface waves, dark surface waves or others).