The present invention discloses a method and its associated apparatus to retrieve the amplitude and, especially, the phase of nonlinear electromagnetic waves. The application field of the present invention is optical imaging. A sample is probed by coherent electromagnetic radiation, and by a nonlinear interaction such as harmonic generation a nonlinear object wave is emitted. A nonlinear reference wave is generated by interaction of the same nature with the coherent electromagnetic radiation, and an interference between the nonlinear object wave and the nonlinear reference wave is sensed by a detector array. As an example, the technique makes possible real-time nanometric localization and tracking of nonlinear field emitters, such as, but not limited to, nanoparticles.