document: Absorption of long waves by nonresonant parametric microstructures Using simple acoustical and mechanical models , we consider the conceptual possibility of designing an active absorbing -LRB- nonreflecting -RRB- coating in the form of a thin layer with small-scale stratification and fast time modulation of parameters . Algorithms for space-time modulation of the controlled-layer structure are studied in detail for a one-dimensional boundary-value problem . These algorithms do not require wave-field measurements , which eliminates the self-excitation problem that is characteristic of active systems . The majority of the considered algorithms of parametric control transform the low-frequency incident wave to high-frequency waves of the technological band for which the waveguiding medium inside the layer is assumed to be opaque -LRB- absorbing -RRB- . The efficient use conditions are found for all the algorithms . It is shown that the absorbing layer can be as thin as desired with respect to the minimum spatial scale of the incident wave and ensures efficient absorption in a wide frequency interval -LRB- starting from zero frequency -RRB- that is bounded from above only by a finite space-time resolution of the parameter-control operations . The structure of a three-dimensional parametric '' ` black '' coating whose efficiency is independent of the angle of incidence of an incoming wave is developed on the basis of the studied one-dimensional problems . The general solution of the problem of diffraction of incident waves from such a coating is obtained . This solution is analyzed in detail for the case of a disk-shaped element
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