Experimental evidence of high-frequency complete elastic bandgap in pillar-based phononic slabs

Abstract

We present strong experimental evidence for the existence of a complete phononic bandgap, for Lamb waves, in the high frequency regime (i.e., 800 MHz) for a pillar-based phononic crystal (PnC) membrane with a triangular lattice of gold pillars on top. The membrane is composed of an aluminum nitride film stacked on thin molybdenum and silicon layers. Experimental characterization shows a large attenuation of at least 20 dB in the three major crystallographic directions of the PnC lattice in the frequency range of 760 MHz–820 MHz, which is in agreement with our finite element simulations of the PnC bandgap. The results of experiments are analyzed and the physics behind the attenuation in different spectral windows is explained methodically by assessing the type of Bloch modes and the in-plane symmetry of the displacement profile.

Focusing of the lowest-order antisymmetric Lamb mode behind a gradient-index acoustic metalens with local resonators

“…Besides, depending on the shape and dimensions of the resonant inclusions, the resonances may arise at a very low frequency as compared to the Bragg gap, in a region of the reduced Brillouin zone where effective theories apply. This has been demonstrated both theoretically and experimentally, with 2D PCs made of an array of cylindrical pillars regularly erected on a homogeneous thin slab [44][45][46][47][48][49][50][51][52]. This last structure deserves special attention.…”

“…Metamaterials with local resonances generally fulfill this latter requirement since they may display very slow sound velocity, at least near resonances [9,[18][19][20][21][42][43][44][45][46][47][48][49][50][51][52]. Besides, depending on the shape and dimensions of the resonant inclusions, the resonances may arise at a very low frequency as compared to the Bragg gap, in a region of the reduced Brillouin zone where effective theories apply.…”

Integrated phononic crystal resonators based on adiabatically-terminated phononic crystal waveguides

“…1,2 The periodic structure of PnCs can result in phononic bandgaps (PnBGs), i.e., a range of frequencies with no acoustic mode allowed to propagate inside the structure. While most initial demonstrations of PnC have used hole-based membrane PnCs 3,4 (i.e., formed by etching a periodic array of holes, e.g., in silicon, and potentially infiltrated with another material), more recent demonstrations have considered pillar-based structures [5][6][7][8] (i.e., an array of metallic or dielectric pillars on top of a thin membrane) due to more design flexibility and possibility of operation at higher frequencies. 8 By adding point and line defects to a perfect PnC, PnC resonators 9,10 and waveguides 11,12 with unique properties can be created.…”

“…While most initial demonstrations of PnC have used hole-based membrane PnCs 3,4 (i.e., formed by etching a periodic array of holes, e.g., in silicon, and potentially infiltrated with another material), more recent demonstrations have considered pillar-based structures [5][6][7][8] (i.e., an array of metallic or dielectric pillars on top of a thin membrane) due to more design flexibility and possibility of operation at higher frequencies. 8 By adding point and line defects to a perfect PnC, PnC resonators 9,10 and waveguides 11,12 with unique properties can be created. The possibility of guiding and confining acoustic waves by PnBGs in PnC waveguides and resonators can be used to form more complex function integrated phononic devices 13 (e.g., filters, multiplexers, etc.…”

Pillar-type acoustic metasurface

“…This has been demonstrated both numerically and experimentally, with 1D stripes periodically engraved on the surface of a lithium niobate substrate 8 and more recently with 2D phononic crystals made of a periodical array of cylindrical pillars deposited on a thin and homogeneous slab. [9][10][11][12][13][14][15][16][17][18] This last structure deserves special attention. Indeed, a pillar exhibits both compressional (monopolar) and bending (dipolar) resonances that may lead to negative dynamic effective modulus and mass density respectively.…”