Observation of band gaps in the gigahertz range and deaf bands in a hypersonic aluminum nitride phononic crystal slab

Abstract

International audienceWe report on the observation of elastic waves propagating in a two-dimensional phononic crystal composed of air holes drilled in an aluminum nitride membrane. The theoretical band structure indicates the existence of an acoustic band gap centered around 800 MHz with a relative bandwidth of 6.5% that is confirmed by gigahertz optical images of the surface displacement. Further electrical measurements and computation of the transmission reveal a much wider attenuation band that is explained by the deaf character of certain bands resulting from the orthogonality of their polarization with that of the source

Phonon dispersion in hypersonic two-dimensional phononic crystal membranes

“…4 and 5. As it was shown for similar PnCs, a full acoustic band gap, desirable for potential applications, can be reached simply by increasing the hole diameter [20,35]. Furthermore, the central frequency of the acoustic band gap can be adjusted by rescaling the PnC characteristic sizes, i.e., keeping the thickness to diameter ratio fixed.…”

“…Various one-, two-, three-dimensional PnCs designed to tune the propagation of bulk [2][3][4][13][14][15]24], surface (Rayleigh, Sezawa) [6,[25][26][27], and plate (Lamb) waves [5,16,18,28] have been investigated both theoretically and experimentally in a wide range of sizes and frequencies. However, the direct measurements of the hypersonic phonon dispersion were performed mostly for bulk [2][3][4]24,29,30] and surface PnCs [6,[31][32][33], while the influence of the phononic patterning in thin membranes was studied theoretically and experimentally by means of transmission measurements, where the acoustic waves are generated and detected electrically [5,18,34,35] or optically [28] in the kHz-MHz range.…”

Bloch wave deafness and modal conversion at a phononic crystal boundary

“…16 Recently, they have also been observed in phononic crystal slabs. [17][18][19] Deaf Bloch waves manifest themselves as dips in the transmission, much as band gaps do, and thus lead to discrepancies between the theoretical band structure and the experimental result. This discrepancy is soon explained by observing that a plane wave incident normally on the PC is symmetric with respect to the propagation direction; if a particular Bloch wave is antisymmetric with respect to the same axis, then it cannot be excited and it cannot contribute to transmission through the PC.…”

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

“…21 Soliman et al improved this result by demonstrating a directional PnBG in the 1 GHz-1.7 GHz range using W inclusions in a silicon (Si) membrane. 22 Furthermore, holebased PnC membranes composed of an aluminum nitride (AlN) memrbane 23 and a stacked AlN and SiO 2 membrane 24 have been shown to have PnBGs around 800 MHz, 900 MHz, and 1.1 GHz along the C-X direction of the square lattice. While these membrane-based PnCs support PnBGs at the GHz frequency range, the reported experimental results for PnBGs in pillar-based structures have been limited to lower frequencies.…”