Achromatic reflected metalens for highly directional and long-distance acoustic probing

Abstract

Simultaneous temporal and spatial focusing of a pulse is of significance for detection and imaging.Here, an achromatic reflected metalens is designed using hybrid resonance and anti-resonance. The theoretical result demonstrates that the anti-resonance provides an extra degree of freedom to control local phases of reflected waves, yielding an achromatic lens of thickness equal to one half of central wavelength. To overcome the shortcoming of traditional approach to design lenses (neglecting the intercell coupling), a boundary integral method is proposed to alleviate the focus deviation over a broadband. The achromatic feature of designed lens is then verified in the frequency range from 2800 to 5600 Hz by an experiment. Owing to a very weak frequency dependence of focal point and a high reflected focusing efficiency over a broadband, a highly directional and long-distance acoustic probing scheme (the mainlobe width about 8 0 ) is proposed with the aid of achromatic reflected metalens and being confirmed by another experiment, where a signal processing method using triple sensors separated by a subwavelength interval is adopted to eliminate the interferences between incident waves and reflected waves. Our result may find its application in a long-distance underwater acoustic probing.

Ultrabroadband and Reconfigurable Transmissive Acoustic Metascreen

“…There are also few studies focusing on broadband sound manipulations. [37][38][39] However, they either only work for reflected waves [4,9,39,40] or on a narrow frequency range. Hence, flexible and broadband transmitted wave manipulations using reconfigurable acoustic metastructure are still highly desired.…”

Achromatic acoustic generalized phase-reversal zone plates

“…Up to now, a number of potential applications related to wave focusing have been demonstrated [7][8][9][10][11][12][13][14][15][16][17], including the nondiffracting beams [9,15], the self-bending beam [11], achromatic metalenses for full-color detection and imaging [18][19][20][21], etc. However, to obtain achromatic focusing in acoustics [22][23][24][25][26][27][28] is still a challenge, especially for the design of transmitted metalens. Although an achromatic acoustic metasurface is recently proposed for broadband focusing [28] using a bottom-up inverse-design paradigm, considering its size (thickness being on the order of a wavelength) and its focusing efficiency (transmission coefficient |t 0 | > 0.6 for each unit cell), it is interesting to achieve an achromatic high efficient focusing by a thinner metasurface.…”

Self-bending airborne ultrasound beam using a binary lens based on the Airy function

“…In addition, some structures developed for AMs can convey the energy of an evanescent wave. 13,14) However, AMs are difficult to manufacture for ultrasonic wave propagation because the required structures, such as coiled structures, 13,19) helical structures, 13,14,20) and resonator-like cells, 13,[15][16][17][18][21][22][23] are too complex to manufacture with general 3D printers.…”

“…Among the various acoustic lens that can focus acoustic power, [23][24][25][26][27][28][29][30] one promising type is the acoustic Fresnel zone plate (FZP). An acoustic FZP generally has a binary structure with two types of regions that are transparent and opaque to sound propagation, allowing wavefronts propagating in phase to be collected.…”