Abstract
Gradient-index phononic crystals (GRIN-PC), characterized by layers with spatially changing refractive indices, have recently been investigated as part of the effort to realize flat lenses in acoustic and elastic regimes. Such gradient-index lens must be inversely designed from the corresponding refractive indices in order to manipulate the target wave. Unfortunately, estimating the index of this type of lens is not straightforward and requires substantial iterative computation in general, which greatly limits the applicability of GRIN-PC to flat lenses. In this work, we propose a novel design of a GRIN-PC in which neighboring layers are separated by partitions, thus preventing waves in each layer from interacting with other layers. This partitioned GRIN-PC design enables us readily to control the phase gradient accurately at the lens’ end, resulting in direct calculation of indices for target wave manipulation. A detailed methodology for partitioned GRIN-PC based collimator and Bessel-beam generator is proposed and experimentally validated to confirm the versatile use of our design in wave engineering applications.
Highlights
Gradient-index phononic crystals (GRIN-PC), characterized by layers with spatially changing refractive indices, have recently been investigated as part of the effort to realize flat lenses in acoustic and elastic regimes
In order to link and control the phase at the transmitted end of the lens directly using the refractive indices in the gradient-index phononic crystal (GRIN-PC), we introduce partitions between the neighboring layers in the GRIN-PC system
The partitioned GRIN-PC design allows the generalized Snell’s law, commonly used when designing metasurfaces[2,4,22,24,25,26,27,28], for PC-designers to readily calculate the phase shifts based on the target wave modulation
Summary
Gradient-index phononic crystals (GRIN-PC), characterized by layers with spatially changing refractive indices, have recently been investigated as part of the effort to realize flat lenses in acoustic and elastic regimes. We propose a novel design of a GRIN-PC in which neighboring layers are separated by partitions, preventing waves in each layer from interacting with other layers This partitioned GRIN-PC design enables us readily to control the phase gradient accurately at the lens’ end, resulting in direct calculation of indices for target wave manipulation. If the index in the GRIN-PC system is readily and accurately designed to control the target phase at the lens’ end without the need for iterative calculations, this advance will open a new avenue for wave-focusing and for the general wavemodulation applications using the GRIN-PC lens. We introduce a novel GRIN-PC design that utilizes the concept of a partition, which enables facile estimations of the refractive indices for wave manipulation without the need for complex iterative
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