Through an alternative paradigm, a predictive design of a Dirac-like point is introduced in a linear periodic metamaterial for the spatial guidance of acoustic waves. Dirac conelike dispersion at the Г point (for k→=0) in a Brillouin zone is called a “Dirac-like cone,” which seldom occurs due to accidental degeneracy. However, a deaf band-based predictive model shows incredible potential to achieve an engineered Dirac cone at a predictive pivoted frequency. A targeted Dirac cone at a higher frequency is carried out in this article validating the orthogonal energy transport in a spiral pattern. The dominance of asymmetric deaf band modes triggers total internal reflection and guiding of acoustic waves inside phononic crystals. To elucidate the versatility of this methodology, experimental validation of orthogonal wave transport is presented.
Exotic acoustical features, like acoustic transparency, ultrasonic beam focusing, acoustic band gap and super lensing capability using a single metamaterial architecture is unconventional and unprecedented in the literature, demonstrated herein. Conventional metamaterials can focus an ultrasonic beam at specific frequency which results into unwanted distortion of the output wave fields at neighboring sonic frequencies in the host medium. However, ultrasonic wave focusing by virtue of negative refraction and simultaneous transparency of the metamaterial at sonic frequencies are uncommon due to their frequency disparity. To circumvent this problem and to avoid the unwanted distortion of wave at sonic frequencies, metamaterial with an array of butterfly-shaped thin ring resonators are proposed to achieve the beam focusing at ultrasonic frequency (37.3 kHz) and keep the structure transparent to the sonic frequencies (<20 kHz). The butterfly metamaterial with local ring resonators or butterfly crystals (BC) were previously proposed to create wide band gaps (∼7 kHz) at ultrasonic frequencies above 20 kHz. However, in this study a unique sub-wavelength scale wave focusing capability of the butterfly metamaterial utilizing the negative refraction phenomenon is demonstrated, while keeping the metamaterial block transparent to the propagating wave at lower sonic frequencies below the previously reported bandgaps.
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