Subcategory: Physics (not Nanoscience)
Matthew Li - University of North Texas
Co-Author(s): Dmytro Shymkiv, University of North Texas, Denton TX; Arkadii Krokhin, University of North Texas, Denton TX
Reciprocity is a fundamental principle following from the time-reversal symmetry (T-symmetry) of the dynamics which govern inviscid fluids. It establishes symmetry between the forwards and backwards propagation of sound. Several different active schemes of breaking T-symmetry have been realized . Here we propose a passive scheme of nonreciprocal transmission through 2D phononic crystals. An advantage of the passive scheme is that it does not require a source of energy. In any real fluid, viscous losses break T-symmetry, creating irreversible fluid dynamics. However, irreversibility alone does not necessarily lead to nonreciprocity. It has been demonstrated  that nonreciprocal acoustic transmission within viscous media additionally requires scatterers possessing broken mirror symmetry (P-symmetry). Here we propose scatterers composed of two semicylinders with contrasting acoustic properties as a novel method of breaking P-symmetry. We prove that such heterogenous scatterers serve as a sufficient prerequisite for non-reciprocity, despite remaining spatially isotropic.Aluminum and acrylic plastic were chosen as materials for the semi-cylinders due to their high acoustic contrast. In general, the level of nonreciprocity may be tuned by the acoustic contrast between the materials. These rods, arranged in a square lattice with period a = 1.5 cm, were nested in water. The orientation of the line separating the semicylinders is a degree of freedom permitting the continuous tuning of transmission. We additionally numerically obtain the band structure for different orientations of such scatterers, allowing for comparison. Note that if this line is parallel to the incident wave, reciprocity is necessarily restored to the system. Calculations along this direction thus served as a control for the accuracy of following data.Nonreciprocity was calculated from transmission coefficients within a 5×5 phononic crystal. To separate true nonreciprocal transmission from time reversible asymmetric transmission, which exists even within inviscid media, calculations were performed for both viscous and ideal fluid. Nonreciprocal transmission displayed strong correlation with the location of corresponding band gaps for a wide range of frequencies and orientations. In future work, we plan to study transmission through spatiotemporal phononic crystals. Temporal modulation can efficiently be achieved by rotating such heterogenous rods, which induce minimal turbulence due to their circular geometry. The proposed structure may serve as a time-dependent nonreciprocal medium, allowing for the realization of topological insulators.References:1.H. Nassar et al., 2020, Nonreciprocity in acoustic and elastic materials, Nat. Rev. Mater. 5. 1-19.2.E. Walker et al., 2018, Nonreciprocal linear transmission of sound in a viscous environment with broken P symmetry, Phys. Rev. Lett. 120.
Funder Acknowledgement(s): This work was supported by EFRI Grant No. 1741677 from the National Science Foundation.
Faculty Advisor: Dr. Arkadii Krokhin, email@example.com
Role: I designed the model and used computational methods to calculate the band structure and transmission coefficients pertinent to the system at hand. Additionally, I employed various sweeps over critical parameters to tune and verify results. Finally, I visualized and analyzed all numerical results with graphing software, which allowed for me to demonstrate the presence of nonreciprocal transmission within the systems described in the abstract.