Disclosed is an array for modifying a wavefront of an optical beam (3) having a beam axis (15). Said array comprises an influencing unit (1) which can be introduced into the beam path of the optical beam (3) and a heat source (12) that generates a thermal pattern and acts upon the influencing unit (1). The influencing unit (1) is provided with at least one planar cooling plate (7a, 7b) that extends transversal to the optical axis (15) of the incident beam (3) and a fluid layer or gel layer (9) which has a two-dimensional expansion, is disposed on the cooling plate with a basal surface, and absorbs the heat of the heat source. The two-dimensional expansion is large enough to receive approximately the entire cross section of the beam. The thickness of the fluid layer or gel layer (9) is embodied in such a small manner that only a minute amount of heat can flow perpendicular to the beam axis (15) while being thick enough for the wavefront of the incident beam (3) to be modified by the heat pattern resulting from thermal impingement of the fluid or gel (9). The wavefront of a beam can thus be influenced in an inexpensive and tailor-made manner. Beams that are influenced in such a way can preferably be used in high-power lasers for creating custom modes.