Hypersonic phononic crystals

Abstract

Manipulation of the distribution of phonons ini a solid is important for both basic science and applications ranging from heat management to reduction of noise in electronic circuits and creating materials with superior acoustic and acousto-optical properties. This thesis explores hypersonic phononic crystals as means to achieve control over high frequency acoustic phonons. An integrated approach to fabrication, measurement and analysis of hypersonic phononic crystals with band gaps in the GHz range is presented.First, the phonon dispersion relation for one dimensional polymeric phononic crystals fabricated by coextrusion of a large number of poly(methylmethacrylate)/poly(carbonate) and poly(methylmethacrylate)/poly(ethylene terephthalate) bilayer pairs is investigated as a function of a lattice constant and composition using Brillouin light scattering and numerical simulations. This set of relatively simple multilayer structures represents an excellent platform to gain a basic understanding of phononic band gap phenomena. In addition, their in-plane phonon dispersion is used to extract information about the elastic constants and glass transition temperatures of individual nanolayers in a periodic multilayer arrangement. Next, two dimensional epoxy/air phononic crystals fabricated in a photoresist using interference lithography are studied. These structures are 2D single crystalline, enabling direction-resolved measurements of their phonon dispersion relation. with performance characteristics that can be adjusted dynamically during operation. The investigations described in this thesis demonstrate both theoretically and experimentally that 1D, 2D and 3D periodic submicron structures have complex phonon dispersion relations at GHz frequencies. As a result, these crystals can be used to manipulate the flow of random thermal phonons as well as externally generated acoustic waves resulting in novel acoustic and thermal properties.

Freeform Phononic Waveguides

“…Large PBGs, comparable to their periodic counterparts, have been identified, accompanied by huge transmission drops in the relevant frequency regions for all possible polarizations. We note that our results should be qualitatively similar for a wide range of material parameters, provided that a strong enough contrast between the cylinders and the matrix is maintained [31][32][33].…”

“…Motivated by the recent progrss in the fabrication of finite-height hypersonic band gap structures [31][32][33], we now consider a realistic 3D slab architecture. For simplicity, we assume that our structure is suspended in air.…”

Granular acoustic switches and logic elements

“…This dynamically varying stiffness mechanism can be used to control acoustic waves when it is combined with the presence of acoustic frequency bandgaps. Similarly to what is seen in traditional phonon theory [25][26][27] , a chain of compressed spheres presents an acoustic band structure for weak travelling waves, with high frequencies decaying exponentially as they travel through the crystal 21 . The cutoff frequency f cutoff of the chain's pass band is determined to be…”

Development of an Acoustic Metamaterials for Aero Acoustic Noise Control

“…In addition, phononic crystals can tailor the allowed modes and their wave speeds inside the material, in such a way that the frequencies of various material loss subjected to different regimes to be matched with the density of some states and frequencies of some modes provide enhanced energy absorption (Thomas et al, 2011). Since mechanical waves propagate in a solid in the form of both longitudinal and transverse waves, a designated structure with complete phononic filters might have band gaps for both waves in the same frequency region (Gorishnyy, 2005).…”