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In this work, we numerically and experimentally investigate topological rainbow trapping and energy amplification of acoustic waves in a gradient phononic crystal (PC) structure. Thanks to the acoustic valley Hall effect, topological interface states (TISs) are generated along the interface between two PCs with different topological phases. To achieve rainbow trapping, we introduce the gradient into a 3D-printed PC structure by varying the geometrical parameter of scatterers along the interface. The incident acoustic waves at different frequencies split, stop, and, hence, are significantly amplified at different positions. Notably, the rainbow trapping of TISs is immune to random structural disorders. The topological rainbow trapping is promising for the design of broadband energy harvesters with excellent robustness. [ABSTRACT FROM AUTHOR]

A phoxonic crystal is a periodically patterned material that can simultaneously localize optical and acoustic modes. The acousto-optical coupling in two-dimensional air-slot phoxonic crystal cavities is investigated numerically. The photons can be well confined in the slot owing to the large electric field discontinuity at the air/dielectric interfaces. Besides, the surface acoustic modes lead to the localization of the phonons near the air-slot. The high overlap of the photonic and phononic cavity modes near the slot results in a significant enhancement of the moving interface effect, and thus strengthens the total acousto-optical interaction. The results of two cavities with different slot widths show that the coupling strength is dependent on the slot width. It is expected to achieve a strong acousto-optical/optomechanical coupling in air-slot phoxonic crystal structures by utilizing surface acoustic modes. [ABSTRACT FROM AUTHOR]

The phoxonic crystal sensor is theoretically investigated in this paper. Based on the phoxonic crystal with dual surface mode bandgaps, both optical and acoustic waves can be confined near the surface. Thus dual cavities, one optical cavity and one acoustic cavity, are designed and integrated in the same phoxonic device. The sensor aims at determining the refractive index and sound velocity of different liquids by utilizing surface optical and acoustic waves. The sensitivities can be estimated by the variations of the cavity resonant frequencies in the response spectra. The relation between the frequency and the measuring physical parameters, i.e., the refractive index or the sound velocity, can be approximated by a linear function. By optimizing each cavity the photonic and phononic sensitivities can reach 199 nm/RIU and 2.79 MHz/ms −1 , respectively. This work can serve as a reference for the design of novel phoxonic sensors. [ABSTRACT FROM AUTHOR]

The propagation of elastic waves in a piezoelectric slab covered with periodically structured coatings or the so-called stubbed phononic crystal slab is investigated. Four different models are selected and the effects of distribution forms and geometrical parameters of the structured coatings on complete bandgaps are discussed. The phononic crystal slab with symmetric coatings can generate wider complete bandgaps while that with asymmetric coatings is favorable for the generation of multi-bandgaps. The complete bandgaps, which are induced by locally resonant effects, change significantly as the geometry of the coatings changes. Moreover, the piezoelectric effects benefit the opening of the complete bandgaps. [ABSTRACT FROM AUTHOR]

In this study, we propose the three-dimensional (3D) phononic crystal-based coupled resonator waveguides (PnCCRWs) for the guidance of acoustic waves along complex routes. The proposed 3D PnC exhibits a complete acoustic band-gap. The point defect mode with a frequency in the band-gap is achieved via the introduction of a point defect into the perfect PnC. The 3D PnCCRWs are constructed by appropriately setting the PnC point defects in the designed paths. These PnC point defects interact with the neighboring ones, which leads to the propagation of the acoustic wave energy along the designated PnCCRWs. Notably, the interaction between the neighboring PnC point defects can be well described by the tight-binding model. The waveguiding ability is demonstrated numerically and experimentally for three different PnCCRWs integrated in the same PnC structure. In addition, the viscothermal effect on the transmission properties of the PnCCRWs is also investigated. The proposed 3D PnCCRWs exhibit their flexibility, which may stimulate additional design rules for complex 3D phononic circuits. • The design of phononic crystal-based coupled resonator waveguides is proposed. • The waveguiding along 3D complex routes is verified numerically and experimentally. • The tight-binding model describes the interaction between phononic crystal defects. • The viscothermal effect on the transmission property is investigated. [ABSTRACT FROM AUTHOR]

In this paper, a phononic crystal (PnC) Luneburg lens is designed for harvesting the mechanical energy of Rayleigh surface waves. The square-latticed PnC is formed by drilling holes periodically in an aluminum matrix. The PnC Luneburg lens for Rayleigh surface waves is constructed by spatially modulating the hole radius of each PnC unit-cell according to the refractive index profile of a conventional Luneburg lens. To convert mechanical energy into electricity, a piezoelectric disk is attached at the focal point of the PnC Luneburg lens, where the Rayleigh surface waves are focused. Compared with the reference harvester on a homogeneous aluminum matrix, the electric power outputs are significantly enhanced by the PnC Luneburg lens harvester for two typical directions, namely the Γ X and Γ M directions. For either the Γ X or Γ M direction, the amplification ratio of the PnC Luneburg lens harvester is over 14. [Display omitted] • The phononic crystal Luneburg lens is proposed for focusing Rayleigh surface waves. • The piezoelectric energy harvester based on the phononic crystal Luneburg lens possesses high amplification ratios. • The phononic crystal Luneburg lens exhibits high rotational isotropy in the piezoelectric energy harvesting. • The phononic crystal Luneburg lens can also be used as a collimator for Rayleigh surface waves. [ABSTRACT FROM AUTHOR]

Phoxonic crystals (PXCs) which exhibit simultaneous phononic and photonic bandgaps are promising artificial materials for optomechanical and acousto-optical devices. In this paper, we theoretically investigate the phononic and photonic guided modes in the three-dimensional topology-type PXCs with a line defect. By varying the geometrical parameters, simultaneous guidance of the slow elastic and light (electromagnetic) waves can be realized. Both elastic and optical energies can be highly confined in and near the defect region. Small elastic and optical group velocities with small group velocity dispersions can be achieved. The group velocities are about 10 and 20 times smaller than the transverse velocity of the elastic waves in silicon and the speed of light in vacuum, respectively. [ABSTRACT FROM AUTHOR]