Characterizing core-shell nanostructures through photoacoustic response based on theoretical model in the frequency domain.

Core-shell nanostructures are widely used, and their photoacoustic (PA) properties are important for applications. However, the relations between their structural parameters and the properties of the PA spectrum are indirect because most theoretical models have been reported for them in the time domain. In this study, we develop a complete model in the frequency domain to analyze the PA response of core-shell particles. As in the case of solid spheres, the core-shell particles have pronounced resonant modes. The PA mode varies with the thickness of the shell and the radius of the core. Under single-pulse irradiation, PA signals of gold-silica nanospheres obtained by our theory agreed with those of the theory in the time domain and experiments. Under multi-pulse irradiation, the magnitude of the PA signals peaked whether the repeated excitation itself or its harmonic was equal to the PA mode. The structure could thus be monitored by the PA signals. These findings enrich PA theory and may inspire new techniques for the noninvasive characterization of nanoparticles.

Acoustic radiation forces on three-layered drug particles in focused Gaussian beams.

Drug delivery by acoustic waves is a crucial technology for targeted therapy. Recently, a three-layered drug micro-particle was proposed and fabricated, the second shell of which greatly improves both the encapsulation of the drug and the flexibility in its release rate. In this work, the acoustic radiation force (ARF) of an acoustic focused Gaussian beam on a three-layered particle comprising an inner drug core (D), a middle layer of poly(lactide-co-glycolide) (PLGA), and an outer chitosan shell (CS) is investigated. A three-layered elastic shell (TES) mimics the D-PLGA-CS structure, and the acoustic scattering from and ARF of the D-PLGA-CS are studied using Mie theory. This paper focuses on how the geometry and acoustic parameters of the outer shell influence the ARF, finding that the Poisson's ratio of the outer shell affects the ARF more than does the density or Young's modulus. In addition, this paper finds that the choice of the inner drug has little effect on the ARF acting on the D-PLGA-CS particle. The present work may benefit the acoustic manipulation of both TESs and three-layered drugs.

Dynamic generation and modulation of acoustic bottle-beams by metasurfaces.

Acoustic bottle-beams have been realized by acoustic metasurfaces (AMs) composed of space-coiling subunits. By manipulating the transmitted acoustical phase, the special AM can generate two intersecting accelerating beams along the designed convex trajectories, forming the acoustic bottle-beam. The transmitted acoustic bottle-beams are investigated theoretically and demonstrated numerically. We find that the shape and area of the acoustic bottle-beam could be statically controlled by designing the AM as well as dynamically modulated by the incident angles. In addition, the highly efficient acoustic focusing could be obtained at the convergence point of the bottle-beams, which also could be adjusted dynamically by the incident angles. It is further found that this focusing is robust against the obstacle scattering. The realization and manipulation of acoustic bottle-beams may have potential applications in biomedical imaging/therapy and non-destructive evaluation.

Broadband acoustic subwavelength imaging by rapidly modulated stratified media.

An acoustic anisotropic lens (AAL) based on large mass-density modulation depth (LMMD) medium is proposed for subwavelength imaging. The underlying mechanism for converting evanescent components into propagating waves is attributed to the strong suppression of the transverse velocity field component in LMMD medium. In addition, the proposed lens can operate in a broadband manner, which is more flexible in practical applications. Both transfer matrix method and finite element method are used to corroborate the subwavelength imaging capabilities of the proposed lens. The numerical simulations demonstrate that the proposed lens can clearly distinguish two Gaussian sources with equal width of λ0/25 and separation of λ0/5 in a broad frequency bandwidth. Medium losses decrease the transmission but cannot compromise the resolution of the lens.

Non-diffraction propagation of acoustic waves in a rapidly modulated stratified medium.

A rapidly modulated stratified medium with a large mass density modulation depth (LMMD) is proposed to achieve non-diffraction propagation (NDP) of acoustic waves. It is found that the NDP in LMMD medium is independent of the incident angle and can be operated in a broad-band manner. Such an NDP is robust and is unhampered by medium losses. An effective medium theory (EMT) is developed for acoustic waves propagating in the LMMD medium based on the first-principles method. The LMMD EMT is verified by using the transfer-matrix method (TMM) for both propagating and evanescent waves. Furthermore, we discuss the influence of the geometry on NDP, and finite element simulations are conducted to verify the NDP in the LMMD medium.