Long-Sheng Zeng

Ultrasonic contrast agent bubbles are a good medium for particle manipulation due to their high acoustic contrast factor. As a kind of widely used acoustic contrast agent, the surface tension of lipid bubbles is complicated, so the solution of its oscillation is normally obtained numerically rather than analytically. Another issue is that the Gorkov's formula for acoustic radiation force (ARF) used for the manipulation of small particles do not apply to bubbles in some frequency ranges. In this paper, the analytical solution of the oscillation of lipid-coated bubbles excited by ultrasound of different frequencies is given, and the frequency dependent ARF of lipid bubbles is obtained theoretically and verified by experiments. The theory complements the widely used Gorkov force, addressing the limitation that Gorkov force is only applicable when the particle density and compressibility are similar to the medium. The results have theoretical significance for the biomedical application of lipid bubbles and may provide theoretical basis for particle manipulation, particularly for the emerging field of manipulating biological particles containing gas bubbles.

Overcoming the diffraction limit has been a long-lasting pursuit for researchers owing to the great potential it offers in going beyond the fundamental resolution restriction in imaging processes. In acoustics, meta-lenses have been a promising way to achieve sub-wavelength imaging, the practical application of which, however, has been limited by expensive material manufacturing, complex system setup, and material loss. Here, we propose a set of procedures equivalent to a virtual super-lens that selectively amplifies the evanescent wave components in the measured acoustic field spectrum, thereby enabling super-resolution imaging without any auxiliary setups or purposely designed super-lens. The proposed virtual super-lens is experimentally verified by considering the imaging of an irregularly shaped sample with sub-wavelength features. We further demonstrate the robustness of the high-quality imaging performance remains acceptable with some environment background noises. In the light of the simple experimental setup involved, our proposed method is flexible and can be readily applied to various practical imaging scenarios.

Ultrasonic imaging technologies possess extensive applications in modern medical diagnosis and treatment. In this paper, a designed artificial bubble array is utilized to establish a multi-scattering sound field model to describe the nonlinear vibration of the artificial bubble array, combining nonlinear acoustics and multi-scattering theory. With the results of the effective medium model, the correctness and reliability of the multi-scattering model are verified. Furthermore, under different incident frequencies and different bubble array parameters, the distributions of fundamental and second harmonic sound pressure fields produced by nonlinear vibration of the artificial bubble array are investigated. Via a judicious design of structural parameters, both fundamental and second harmonic acoustic focusing at specific locations inside the bubble array and second harmonic acoustic focusing outside the array under a specific incident frequency are realized.

Breaking the diffraction limit to achieve super-resolution focusing is a long-sought goal in the field of acoustic imaging and detection. Here, we demonstrated super-resolution focusing of high-frequency ultrasound (5 MHz) based on an acoustic meta-lens with four centrosymmetric dart-like sharp-edge apertures. For the diffracted ultrasound carrying high spatial frequencies, super-resolution focusing is generated in the far field with mainlobe size breaking the diffraction limit (λ). Different from previous far-field super-resolution focusing, where the sidelobe intensity is close to or even larger than the mainlobe intensity, the sidelobe intensity in our case is weak. We further conducted fine defect detection (∼200 μm) by utilizing our designed far-field super-resolution focusing to verify its nondestructive testing performance.