Threshold lowering for subharmonic generation in Cantor-like composite structures

We show evidence of extremely low thresholds for subharmonic generation in one-dimensional artificial composites with hierarchical structure, as compared to the corresponding homogeneous and periodic structures. The displacement profiles of the anharmonically coupled normal modes are probed experimentally, indicating that the enhanced non-linear interaction is due to the fact that, for a given fundamental mode, a second mode with much more favourable frequency and spatial matching is found in the hierarchical composite than in the non-fractal structures.