The invention relates to a device for determining a deflection of the test beam (12) caused by interaction with a sample (10), comprising an oscillator unit (28) that periodically deflects the test beam and a detector unit (24) relative to one another. The detector unit is designed in such a way that the sensitive surface of the detector covered by the test beam and thus the corresponding measurement signal changes in a non-linear manner as a function of the deflection in the event of relative deflection of the test beam and of the detector unit. An evaluation of the detector signal on the basis of a Fourier spectral analysis and the theory of second harmonics generation then enables the definition of the smallest beam shifts caused by interaction with the sample having high dynamics. In an alternative design, instead of the beam deflection the effect of the sample to be measured is periodically modulated.