Bidirectional Asymmetric Frequency Conversion in Nonlinear Phononic Crystals

Beyond conservative systems, altering the wave propagation frequency emerges as a crucial factor across diverse physical domains. This Letter presents a demonstration of bidirectional asymmetric frequency conversion-either upward or downward-depending on the excitation direction, surpassing conventional unidirectional mechanisms. We numerically and experimentally demonstrate its practical realization in a model system of cylindrical granular crystals with intrinsic local resonance coupling. This novel wave transport mechanism arises from the interplay of nonlinear contact, spatial asymmetry, and coupled local resonance. In particular, we show that local resonance coupling induces wavenumber-dependent wave dynamics, including frequency conversion. Given that this local resonance exemplifies avoided crossings (i.e., strong coupling), this Letter may inspire studies on nonlinear systems supporting material or structural resonance.