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[en] Transformation-optics devices of arbitrary shapes usually result in material parameters inside the device that feature level sets of different shapes. Consequently, these devices cannot easily be implemented using a layered architecture and thus are, generally, more difficult to realize in practice. We present a method of designing two-dimensional transformation-optics devices of arbitrary shapes characterized by material parameters of same-level sets, thus suitable to be implemented through concentric layers, each layer being made of a single type of material or metamaterial. Remarkably, we show that transformations leading to such designs are obtained from solutions to the well-known eikonal equation. This approach allows fabrication techniques developed for cylindrical designs of circular cross section to be directly applied to devices of other shapes.
[en] We show that complex coordinates used in conjunction with transformation optics offer an extra degree of freedom that allows control over not only the propagation direction of electromagnetic waves but also their amplitude. We illustrate this idea in two applications. First, we show that in an n-dimensional space one can manipulate the field amplitude for up to n different amplitude distributions in regions that are critical to the performance of the device under consideration, and thus reduce the device's sensitivity to design imperfections in these regions. Second, we expand previous work on reflectionless perfectly matched layers and show how complex coordinates and transformation optics are a natural choice for designing perfectly matched layers of arbitrary shape.
[en] Optimization techniques are efficient methods to simplify the design of transformation optics devices. Asymmetric devices obtained through these methods typically lose omnidirectionality and perform well only when illuminated from a finite set of directions. We present here an optimization approach that results in simplified, omnidirectional designs of fairly large bandwidths. The method leverages a class of coordinate transformations that result in transformation media whose material parameters follow identical isocontours. We show that discretizing these media in a finite number of layers that follow the common isocontours and optimizing the material parameters inside each layer is an effective way to significantly simplify the design complexity while preserving most of the original performance including omnidirectionality. (paper)
[en] The ability to create linear systems that manifest broadband nonreciprocal wave propagation would provide for exquisite control over acoustic signals for electronic filtering in communication and noise control. Acoustic nonreciprocity has predominately been achieved by approaches that introduce nonlinear interaction, mean-flow biasing, smart skins, and spatio-temporal parametric modulation into the system. Each approach suffers from at least one of the following drawbacks: the introduction of modulation tones, narrow band filtering, and the interruption of mean flow in fluid acoustics. We now show that an acoustic media that is non-local and active provides a new means to break reciprocity in a linear fashion without these deleterious effects. We realize this media using a distributed network of interlaced subwavelength sensor–actuator pairs with unidirectional signal transport. We exploit this new design space to create a stable metamaterial with non-even dispersion relations and electronically tunable nonreciprocal behavior over a broad range of frequencies. (paper)
[en] Zetea lake grading project comprises an earthen dam, made out of local materials, having as main purposes water supply, flood control and protection against flooding. The paper analyzes the possibility of building a small hydropower plant at the base of the dam, using private investment resources, in order to put to good use the water flow evacuated from the storage lake. (authors)
[en] We present here two diffractive acoustic lenses with subwavelength thickness, planar profile, and broad operation bandwidth. Tapered labyrinthine unit cells with their inherently broadband effective material properties are exploited in our design. Both the measured and the simulated results are showcased to demonstrate the lensing effect over more than 40% of the central frequency. The focusing of a propagating Gaussian modulated sinusoidal pulse is also demonstrated. This work paves the way for designing diffractive acoustic lenses and more generalized phase engineering diffractive elements with labyrinthine acoustic metamaterials.