Acoustoelectric conversion and deep-subwavelength acoustic focusing based on Fresnel zone plates

Abstract

Fresnel zone plates (FZP) with quasi-planar structure and ultrathin thickness are capable of focusing light or acoustic waves efficiently. In this paper, we employ the FZP to manipulate the wave fields and suggest two acoustic functionalities. On one hand, by placing the moving coil-type microphone near the focal point of the FZP, enhanced acoustoelectric conversion is realized. The measured output voltage and power attained are 18.5 mV and 130 nW, respectively, which are 5.1 and 27.3 times that obtained without the FZP. On the other hand, by setting a tapered tube near the focal point of the FZP, deep-subwavelength and efficient localization of acoustic waves at the tube end (the mode size is around λ/20) are suggested theoretically. The cascaded focusing scheme leads to an enhancement of the acoustic intensity of the hotspot by more than 8700 times. The results are useful for realizing acoustic energy harvesting, strong mode localization, and acoustic-field enhancement.