Two of the most fundamental concepts in wave propagation and signal transmission are the closely related principles of reciprocity and time-reversal symmetry. Reciprocity requires that a wave traveling in one direction can just as well travel in the opposite direction, while time-reversal symmetry provides the same relationship when time is reversed. Recent advances in engineering have shown that either or both principles can be violated under special conditions, for instance in the presence of moving fluid and solid elements. This award supports fundamental research to demonstrate novel methods for realizing non-reciprocal behavior through the design of heterogeneous acoustic, elastic, and electro-mechanical systems. Technology that violates these fundamental rules opens the possibility of changing the standard operating procedures for measuring and utilizing acoustic and elastic waves. The work pairs these new concepts with robust materials design methodologies and additive manufacturing expertise to help redirect the nation?s technological advances in acoustics, structural vibration, ultrasonic inspection, seismic protection, and biomedical imaging. Research efforts supported in this award specifically provide opportunities for underrepresented undergraduate students to participate in knowledge acquisition and exploration via multidisciplinary projects conducted in parallel at the collaborating institutions.
One approach to breaking time-reversal symmetry in linear systems to be considered is based on the dynamic coupling of momentum and strain using excitation on fast and slow time scales for acoustic and elastic media with spatially asymmetric microstructure. Oscillations from a pump excitation provide a quasi-static momentum bias that enables non-reciprocal signal propagation. Other routes to achieving non-reciprocal response include elastically nonlinear up-conversion from the drive frequency to higher harmonics. Optimal damping and absorption in elastic waveguides, using an unexplored powerful relation between exceptional points and damping, will be examined. Other areas to be investigated include active control via shunted piezoelectric elements and hybrid time-varying circuits. Materials design is at the center of the DEPARTED project with a view towards efficient design space exploration and employing state-of-the art additive manufacturing techniques to develop optimal microstructural topology of these structures.