Auto-focusing acoustic-vortex tweezers for obstacle-circumventing manipulation

Abstract

Contactless manipulation of hard-to-reach objects, especially those behind obstacles, is highly demanded in many applications. In this work, auto-focusing acoustic-vortex (A-FAV) tweezers are developed based on-axis-symmetrically arranged and azimuthally phase-coded Half-Bessel (HB) beams, which can self-bend themselves along an arc-shaped trajectory and, hence, circumvent obstacles that are located on the beam path. To build such tweezers, active phase control and passive binary phase modulation for a simplified ring-array of sectorial planar transducers are integrated to build A-FAV beams of arbitrary order. Both theoretical and experimental results prove that the zero-force of A-FAV tweezers enables the stable object manipulation at the focus with improved axial and radial resolutions. Benefiting from the non-diffracting self-bending of HB beams, the enclosed bullet-shaped cavity whose inside pressure is close-to-zero can be created around the beam axis, defining the upper size limit of obstacles that can be bypassed. Moreover, A-FAV beams are capable of self-healing even parts of the mainlobes that are blocked. More importantly, the A-FAV tweezer with an obstacle-circumvention cavity is easily adjustable, exhibiting prosperous perspectives in obstacle-avoidant ultrasound imaging, ultrasound therapy, acoustic levitation, and object manipulation.