A method and a device for optically ascertaining the mutual binding of a first and a second reactant are described. Use is made of the fact that a dipolar layer (7) having a non-linear optical behavior is formed upon a reaction between the two reactants. The formation of such a layer is ascertained by illuminating the substance with radiation (8) having one or two frequencies, and by detecting whether radiation (10) from the substance contains a component at a frequency which is twice the frequency, or a mixed term of the frequency of the radiation incident on the substance.
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