In this presentation, I will introduce our recent advances on controlling acoustic wave propagation via spinning media. We will start from a review of the theoretical framework for the scattering problem by a rotation object. Then we will introduce a generalized scattering cancellation theory (SCT) to cloak spinning objects from static observers. In another example, we will study the torque and force a spinning cylindrical column of fluid experiences in evanescent acoustic fields, and show that the resulting discontinuity can scatter sound in unusual ways, e.g., a negative radiation force.
In this paper, we propose a general inverse-design strategy based on genetic algorithm optimization to achieve ‘on demand’ manipulation of light in one-dimensional (1D) and two-dimensional (2D) non-Hermitian systems. The optimization process faithfully creates non-Hermitian potentials from any given arbitrary real (or imaginary) permittivity distribution for the desired frequency selective and broadband asymmetric response in 1D multilayer structures. As a demonstration in 2D, we design periodic and aperiodic complex permittivity spatial distributions to create "sink-type" concentrators of light around a desired area. The proposed inverse-design approach to generate non-Hermitian potentials represents an alternative to the Hilbert Transform (HT) generalizing the Kramers Kronig relations in space, additionally being selective in spectrum.
A feasible restricted Hilbert Transform (HT) is presented to solve the challenging practical realization of non-Hermitian systems, restricting the complex susceptibility within practical limits. Beyond closed-conservative systems, the physics of non-Hermitian systems has become the playground to uncover unusual phenomena. Whilst Kramers Kronig relations break the temporal symmetry leading to causality, we proposed an analogous generalized Hilbert Transform (HT) to engineer complex media holding a non-isotropic response, thus breaking the spatial symmetry. Applications of such HT range from tailoring the field flows in arbitrary dimensions with particular application on VCSELS and edge-emitting lasers to cloaking arbitrary objects.
In this talk, I will present our practical design of a three-dimensional (3D) acoustic DZIM, which is made of a cubic lattice of metal rods. We demonstrate a four-fold degenerate point with conical dispersion can be induced at the Brillouin zone center, at which the effective mass density and compressibility simultaneously turning zero. To demonstrate the extraordinary functionalities of the DZIM, we have fabricated an acoustic "periscope" with two 90° turns to achieve almost perfect tunneling of a normally incident plane wave. The experimental results agree well with our theoretical predictions and numerical simulations.
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