Acoustic trapping of particles using a Chinese taiji lens

Focused acoustic vortex (FAV) beams can trap particles in a contactless and non-destructive way and the method has thereby attracted much attention. In contrast to the traditional complex and expensive transducer array, we propose a fast and cheap method to generate FAV beams in water using an ultrasonic holographic lens engraved with the Chinese taiji pattern. This method can obtain high transmission efficiency, and hence the strong trapping force makes the particles trapped stably in a straight line. The formation of the FAV beams derives from a superposition of the spiral phase of a Laguerre Gaussian beam and the focusing phase. Because of the phase singularity of this beam, the intensity of the ultrasonic field on the beam axis is zero, thereby forming a strong gradient surround the beam axis. The trapping and manipulation of polystyrene particles with a radius of 150 μm is realized in the gradient field of the FAV beam. The proposed single beam acoustic trapping method does not depend on the reflector, making it more suitable for the manipulation of cells in vivo.