![]() One intrinsic property of FZPs is that they are designed to operate at a single frequency, as the different radii of the lens depend on the central design wavelength. This π phase change results in a propagation path difference of λ / 2 between consecutive radii, which directly yields to the design equation of the lens: Moreover, in the past years novel designs based on the FZP structure have been presented, which allow the formation of acoustic vortex 27, and bifocal 28, 29 and fractal 30, 31 intensity distributions, increasing the versatility and interest of this type of lenses.įor a FZP lens, the diffracted pressure waves at two consecutive Fresnel regions reach the focus with a complex phase difference of π (in phase opposition), meaning that those pressure waves interfere destructively. Due to their easy design, manufacturing process, and focusing capabilities, this kind of devices are employed in a wide range of fields ranging from optics 15– 17 to X-ray 18, 19, microwaves 20, 21, and acoustics 22– 26. With this method, an arbitrary number of foci and relative acoustic intensities can be multiplexed in the time domain, achieving, therefore, an ultrasonic focal beam that can be controlled in both space and time.įZPs are widely used monofocal planar lenses made of a series of concentric rings, known as Fresnel regions, with decreasing width. In this work, we present a spatio-temporal beam modulation technique to achieve multiple foci using a conventional Fresnel Zone Plate (FZP). However, in contrast to phased arrays, acoustic holograms do not allow to dynamically control the ultrasound beam once the lens is manufactured. A more recent approach to achieve arbitrary pressure fields consists of using acoustic holograms directly coupled to a single ultrasound transducer, which provides a simpler and cheaper, yet very powerful and versatile method to provide complex 3D pressure distributions 9– 14. ![]() Among all the possible options, one of the main and most used methods is employing phased arrays, which allows to generate different acoustic foci by adjusting the time delay of each one of its different transducer elements 6– 8. Different techniques have been devised in the literature to achieve multiple acoustic foci. Focusing ultrasonic waves has multiple applications in various fields, such as non-destructive testing in industrial scenarios 1, biomedical imaging of different kind of tissues 2, or thermal ablation of tumours through High Intensity Focused Ultrasound (HIFU) 3– 5. ![]()
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