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Ultrasonic imaging is crucial in the fields of biomedical engineering for its deep penetration capabilities and non-ionizing nature. However, traditional techniques heavily rely on impedance differences within objects, resulting in poor contrast when imaging acoustically transparent targets. Here, we propose a compact spatial differentiator for underwater isotropic edge-enhanced imaging, which enhances the imaging contrast without the need for contrast agents or external physical fields. This design incorporates an amplitude meta-grating for linear transmission along the radial direction, combined with a phase meta-grating that utilizes focus and spiral phases with a first-order topological charge. Through theoretical analysis, numerical simulations, and experimental validation, we substantiate the effectiveness of our technique in distinguishing amplitude objects with isotropic edge enhancements. Importantly, this method also enables the accurate detection of both phase objects and artificial biological models. This breakthrough creates new opportunities for applications in medical diagnosis and nondestructive testing. High-contrast ultrasonic imaging holds significant importance in biomedical and engineering applications. Here, the authors present a compact spatial differentiator tailored for underwater isotropic edge-enhanced imaging, facilitating the realization of high-contrast ultrasonic imaging. [ABSTRACT FROM AUTHOR]

The plasmon-exciton couplings in Ag-CuCl-coated nanowires (ACNWs) have been investigated by using the scattering theory and the finite element method. It is found with increasing the shell thickness that the dipole plasmon resonance of the Ag nanowire can be tuned through the exciton resonance in the CuCl shell. The strong coupling between the plasmon resonance in the Ag nanowire and the exciton resonance in the CuCl shell leads to two new hybridized plexcitonic modes. The dispersion curves of the plexcitonic modes in the ACNWs are studied, and the obtained splitting energy is about 88 meV. We further find that the destructive interference between the plasmon and exciton resonances results in the plasmon-exciton induced transparency. As the TE wave propagates through the ACNW array, an extraordinary transmittance can be found in the ACNW array at the scattering dip of the ACNW. [ABSTRACT FROM AUTHOR]

Fano resonances in the symmetry-broken gold-SiO-gold (BGSG) nanotubes and the associated dimers have been investigated based on the finite element method. In the BGSG nanotube, the symmetry breaking induced the interactions of the inner gold core and outer gold nanoshell plasmons of all multipolar orders and hence the red-shifts of the plasmon resonance modes and the enhanced quadrupole mode peaks were observed. The interference of the quadrupole mode peak with the subradiant dipole mode caused a Fano-dip in the scattering spectrum. By increasing the core offset-value in the BGSG nanotube, the Fano dip with low energy showed a red-shift and became deeper. Unexpectedly the plasmon coupling between a GSG nanotube and a BGSG nanotube can lead to two strong Fano dips in the scattering spectra of the dimer. It was further noted that the thin side of the BGSG nanotube located at two sides of the dimer gap can lead to the strong near-field coupling between two BGSG nanotubes and hence a deeper and broader Fano dip. [ABSTRACT FROM AUTHOR]

The influence of the spherical anisotropy (SA) of a middle layer on the plasmon resonance couplings in the sandwiched gold nanoshell (Au/SA/Au) has been investigated by means of a modified Mie theory. It is found that the plasmon couplings in the Au/SA/Au nanoshells are more sensitive to the permittivity along the radial direction of SA layer than the permittivity along the tangential direction. With increasing the anisotropic value of the middle layer, the dipole peaks of antisymmetric ω mode and symmetric ω mode both show blue-shifts, while the shift of the antisymmetric ω mode is larger than that for the symmetric ω mode. The larger anisotropic value of the SA layer induces the stronger near-field outside the nanoparticles for the antisymmetric ω mode, while the smaller anisotropic value makes the larger near-field for the symmetric ω mode. We further have found that the middle SA layer with smaller anisotropic value is helpful to obtain larger electric fields inside the nanoshells, which may be useful for their potential applications in nonlinear optics. [ABSTRACT FROM AUTHOR]

The extinction spectroscopy and near-field enhancement of dielectric-silver nanoshell coated by tetrahedral amorphous carbon [ta-C] layer (DSC) have been calculated by using Mie theory. With decreasing the Ag layer thickness, the localized surface plasmon resonance (LSPR) of DSC nanoshell moves from the visible region into the near-infrared region and the corresponding local field factor (LFF) increases first and then decreases. In addition, the increase of ta-C shell thickness leads to red-shift of LSPR and the decrease of LFF in DSC nanoshell. We further find that the increase of the dielectric constant for the outer shell can induce a significant enhancement of near-field. Based on the simulation analysis, we show that the DSC nanoshell can provide strong near-field in near-infrared region and may be suitable for the biomedical applications in vivo. [ABSTRACT FROM AUTHOR]