Leveraging nonlinear wave mixing in rough contacts-based phononic diodes for tunable nonreciprocal waves

In recent years, phononic material-based diodes or “phononic diodes” have been explored to enable nonreciprocal wave propagation for novel applications such as unidirectional filters and waveguides. Despite extensive studies, the functionality and tunability of these diodes have been restricted to specific frequencies. Here, we exploit mixing of double-frequency (pump and probe) waves in nonlinear phononic materials to design a diode that can control the frequency, mode, and direction of energy propagation of the nonreciprocal waves. Our one-dimensional phononic diode is based on local and periodic nonlinearity, and global asymmetry. The weak nonlinearity is incorporated in the form of precompressed rough contacts, where contacts exhibit a quadratic force–displacement relationship. Using finite element models, we demonstrate tuning of the dispersion branch associated with the nonreciprocal wave by controlling the frequency of the pump waves. This allows propagation of nonreciprocal waves at frequencies lower or higher than the probe wave frequencies. Further, we show the ability to switch between reciprocal and nonreciprocal wave propagation, where the nonreciprocal response can cause either unidirectional or bidirectional energy propagation. This switching is possible through suitable selections of a pair of frequencies, and external precompression that controls the band gaps. The proposed phononic diode exhibits nonreciprocal wave propagation over a broad frequency range yet is simple in design and purely mechanical in construction. Moreover, due to its ability to filter and tune the frequencies of nonreciprocal waves, this diode will have potential applications as mechanical sensors and devices.