Engineering the band structure of one-dimensional hypersonic phononic crystals

Abstract

Phononic crystals—the mechanical analogues of photonic crystals—have attracted increasing interest and have been widely studied within the past decade. The phononic dispersion relation at hypersonic frequencies can be directly measured by the powerful non-destructive technique of high resolution spontaneous Brillouin-light-scattering (BLS) [W. Cheng et al., Nature Mater. 2006, 5, 830]. Due to the vector nature of the elastic wave propagation, theoretical phononic band structures can be uniquely verified at low dimensionality, and hence 1D phononic crystals constitute model systems for fundamental studies. Such hybrid Bragg stacks, composed of alternating layers of silica and poly(methyl methacrylate) (PMMA), respectively, exhibit clear hypersonic phononic band gaps [Gomopoulos et al., Nano Lett. 2010, 10, 980]. In this paper, we report on the fabrication, characterization, and both experimental and theoretical dispersion diagrams along and normal to the periodicity direction of silica/PMMA multilayers. The width of the gap, the phonon frequencies, and their intensities near the first Brillouin zone are sensitive probes of the longitudinal moduli and elasto-optic constants of the individual layers and structural parameters. Mixing with layer modes under oblique incidence conditions allows access to the shear moduli of the two layers.