Abstract
Funneling acoustic waves through largely mismatched channels is of fundamental importance to tailor and transmit sound for a variety of applications. In electromagnetics, zero-permittivity metamaterials have been used to enhance the coupling of energy in and out of ultranarrow channels, based on a phenomenon known as supercoupling. These metamaterial channels can support total transmission and complete phase uniformity, independent of the channel length, despite being geometrically mismatched with their input and output ports. In the field of acoustics, this phenomenon is challenging to achieve, since it requires zero-density metamaterials, typically realized with waveguides periodically loaded with membranes or resonators. Compared to electromagnetics, the additional challenge is due to the fact that conventional acoustic waveguides do not support a cut-off for the dominant mode of propagation, and therefore zero-index can be achieved only based on a collective resonance of the loading elements. Here we propose and experimentally realize acoustic supercoupling in a dual regime, using a compressibility-near-zero acoustic channel. Rather than engineering the channel with subwavelength inclusions, we operate at the cut-off of a higher-order acoustic mode, demonstrating the realization and efficient excitation of a zero-compressibility waveguide with effective soft boundaries. We experimentally verify strong transmission through a largely mismatched channel and uniform phase distribution, independent of the channel length. Our results open interesting pathways towards the realization of extreme acoustic parameters and their implementation in relevant applications, such as ultrasound imaging, acoustic transduction and sensing, nondestructive evaluation, and sound communications.
Highlights
Over the past decade, significant attention has been paid to zero-index metamaterials, due their extreme capabilities for wave manipulation [1]
Despite the use of simple materials with physically hard boundaries, we show that the excitation of a higher-order mode may synthesize effective soft boundary waveguide channels that support a cut-off at finite frequency and enable this unusual tunneling phenomenon
In order to demonstrate the effect of supercoupling for sound in a simple waveguide geometry, we consider the configuration of Figure 1, in which we sandwich an intermediate channel with large cross-sectional area S2, length L, and modal impedance Z2 between two narrow input/output channels, each with acoustic impedance Z1 and much narrower crosssectional area S1
Summary
Significant attention has been paid to zero-index metamaterials, due their extreme capabilities for wave manipulation [1] These materials can be described by governing equations that are temporally and spatially decoupled, due to the unusual physics enabled by nearzero constitutive parameters. Remarkable properties of zero-index metamaterials have been proposed and experimentally validated These metamaterials have been used for cloaking [25,26,27,28], improving the directivity in radiation and scattering [3, 29, 30], and in enhancing the transmission through geometrically mismatched channels, a phenomenon dubbed as supercoupling [4, 27, 31, 32]. The dual phenomenon, tunneling through a geometrically mismatched large channel filled with permeability-near-zero materials, Research was theoretically proposed [17] and experimentally verified for radio-frequency waves [27]. ENZ supercoupling has been proposed at higher frequencies as an effective way to boost optical nonlinearities in plasmonic channels [14] and local density of states and quantum super-radiance [33]
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