Engineering the Hypersonic Phononic Band Gap of Hybrid Bragg Stacks

Abstract

We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.

Phonon dispersion in hypersonic two-dimensional phononic crystal membranes

“…Both periodic modulation of the elastic properties in phononic crystals (PnCs) [1][2][3][4][5][6] and reduction of the characteristic dimensions as in, e.g., thin membranes, thin films, and nanowires [7][8][9][10] lead to acoustic phonon propagation which is quite different than for bulk systems. The artificial, second-order periodicity introduced in PnCs results in the modification of the phonon dispersion and, optionally, in complete frequency band gaps due to Bragg reflections and/or local resonances, which can be controlled by geometry and material properties [1,3,11,12].…”

“…The first experimental studies on PnCs, limited by fabrication capabilities (spacing of mm), were focused on sound (kHz) and ultrasound (MHz) waves propagation and intended for applications in acoustic filtering, sensing, and wave-guiding [13][14][15][16][17][18]. Recent advances in fabrication methods have allowed reduction of the characteristic sizes of PnCs to nm scale, enabling the control of hypersonic (GHz) phonons [2,4,6,19] and heat transport [20,21]. Therefore, such structures have become attractive for optomechanics, radio frequency (RF), and thermoelectric applications [17,22,23].…”

“…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.…”

Band structure of cavity-type hypersonic phononic crystals fabricated by femtosecond laser-induced two-photon polymerization

“…The uniqueness of the velocity values can be verified by a more stringent criterion, which is the prediction of the BLS intensity. 12,37 Since the samples are transparent, we use a simple expression of the Brillouin intensity based only on the elasto-optic (or photoelastic) mechanism of light-matter interaction, 38…”

Complete Band Gaps in Nano-Piezoelectric Phononic Crystals

“…Phononic crystals (PCs) are artificial periodic composites designed to exhibit phononic band gaps and they have been quite studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . There are no mechanical (elastic or acoustic) propagating waves in phononic band gaps, only evanescent waves.…”