Abstract
The hypersonic phonon propagation in large-area two-dimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering. The dispersion relation of thermally populated Lamb waves reveals multiband filtering due to three distinct types of acoustic band gaps. We find Bragg gaps accompanied by two types of hybridization gaps in both sub- and superwavelength regimes resulting from contact-based resonances and nanoparticle eigenmodes, respectively. The operating GHz frequencies can be tuned by particle size and depend on the adhesion at the contact interfaces. The experimental dispersion relations are well represented by a finite element method model enabling identification of observed modes. The presented approach also allows for contactless study of the contact stiffness of submicrometer particles, which reveals size effect deviating from macroscopic predictions.
Highlights
The hypersonic phonon propagation in large-area twodimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering
Structured materials that take advantage of the wave-like nature of phonons, such as phononic crystals (PnCs) and acoustic metamaterials, have been shown to enable new mechanical and acoustic features, including negative effective moduli and densities, frequency filtering, and acoustic cloaking.[2−7] Recently, this field has experienced a tremendous growth of research in topological PnCs showing unidirectional propagation of sound immune to structural imperfections.[8,9]
Propagation of GHz phonons in threedimensional (3D) self-assembled PnCs, typically known as colloidal crystals, composed of glass or polymer nanoparticles was initially investigated by means of Brillouin light scattering (BLS).[13−17] More recently, contact-based modes of colloidal crystals have been extensively studied using the pump−probe technique.[18−25] This approach, unlike BLS, is limited to subGHz frequencies and often requires additional fabrication of a transducer for the acoustic wave generation
Summary
The hypersonic phonon propagation in large-area twodimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering. We find significant modification of the phonon dispersion revealing three types of band gaps that are related to nanoparticle: membrane contact resonance (hybridization), lattice period (Bragg), and local resonances of nanoparticles (hybridization).
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