Abstract
The effective operation of certain electronic, medical, industrial, and testing equipment relies on high-quality focusing and imaging capability, which also plays a vital role in the field of wave physics. Therefore, continuously improving the resolution capacity is essential. However, in a homogeneous medium dominated by the diffraction limit, the best resolution for wave focusing and imaging could only reach half the wavelength corresponding to the lowest operating frequency, significantly hindering the relevant application value. The development of phononic crystals (PCs) and acoustic metamaterials (AMMs) has realized sub-wavelength focusing and super-resolution imaging and attracted increasing research attention in physics, mechanics, engineering, and biomedical science. This Tutorial explained the basic principles and traditional methods of acoustic focusing and imaging. Then, the implementation principles and related research progress of sub-wavelength focusing and super-resolution imaging based on artificial acoustic devices, including but not limited to PCs and AMMs, were systematically discussed. Moreover, a method was introduced to realize sub-wavelength or sub-diffraction focusing by integrating these artificial devices into the time-reversal procedure. Finally, the potential development trends and practical application prospects were presented.
Highlights
Focusing and imaging are commonly used in everyday products and medical equipment, as well as in industrial production and testing
The effective operation of certain electronic, medical, industrial, and testing equipment relies on high-quality focusing and imaging capability, which plays a vital role in the field of wave physics
Acoustic focusing can be used for medical diagnosis, ultrasonic lithotripsy, tumor hyperthermia, material defect detection, biosensing, droplet migration, cell migration, and other fields.[1,2,3,4,5,6]
Summary
Focusing and imaging are commonly used in everyday products and medical equipment, as well as in industrial production and testing. The former included gradient cross-sectional lenses with gradient index and Fresnel lenses with wavefront diffraction, while the latter included phased arrays with gradient wavefront phases, and TR focusing with reciprocity principle Artificial structures, such as PCs, AMMs, and AMTs, are used to construct various new-concept focusing lenses, including an anisotropic wave vector hyperlens, a negative index superlens, resonant tunneling, or nearzero index metalens, and a small-sized lens with a gradient index or wavefront phases. Some working principles are shared by newconcept lenses and traditional lenses, while some are unique to new-concept lenses Introducing artificial structures, such as PCs, AMMs, and AMTs, into the design of traditional lens or lens-free focusing and imaging methods is expected to improve the resolution and reduce the size of the devices, providing extensive application potential. It should be noted that the methods discussed in this Tutorial mainly involve acoustic waves in air and water rather than elastic waves in a solid medium
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