In this paper we present the first direct observation of incoherence of the diffuse (partial nonspecular) second-harmonic generation (SHG). The surface sensitivity of the SHG technique is used for studying the incoherent and nonspecular SHG radiation originated from spatial fluctuations of linear and nonlinear optical susceptibilities of thin oriented films of bacteriorhodopsin (bR). Bacteriorhodopsin is chosen as an excellent object for investigation of the diffuse SHG due to the resonant behavior of the bR hyperpolarizability at the SHG wavelength (532 nm) and significant spatial fluctuations of the nonlinear susceptibility across the film.
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