A method and device for detecting a resonant non-linear optical signal induced in a sample, the sample including a resonant medium and a non-resonant medium forming an interface. The device includes an emission source of at least one first excitation light beam, called a pump beam, at a given angular frequency ωp for the excitation of the resonant medium of said sample, an optical detection module for detecting a non-linear optical signal resulting from the interaction of said pump beam with an axial interface between the resonant and non-resonant media of the sample, in at least two symmetrical directions ({right arrow over (k)}, {right arrow over (k)}′) relative to the optical axis of said excitation beam incident in the sample, and a processing unit for processing signals (IFwd({right arrow over (k)}), IFwd({right arrow over (k)}′)) thus detected, allowing the difference between said signals to be obtained.