Optical characterization of holographic acoustic tweezers.

Acoustic tweezers make use of sound waves to trap solid or liquid samples. Ultrasonic transducers can be used to achieve acoustic tweezers in air, giving a trapping point at a target position. This method permits to trap and manipulate single or multiple particles for studying them even $in situ$, without altering their properties. In particular, our experimental setup consists of an $8 \times 8$ flat array of ultrasonic emitters. Trapped particles are styrofoam beads with mm radius. To study their dynamics in the acoustic trap, we use two different optical detection methods. In the first method, we illuminate the trapped particle with a laser beam, tracking down the fluctuations of its shadow on a quadrant photodiode (QPD). With the second procedure, we track the image of our trapped particle with a CCD camera and a telescope, then we extract data on the particle position using a video tracking software. With both methods we detect the particle displacement in $x, y$, and $z$ directions that is the particle dynamics in the trap. We analyze the experimental data to obtain the force constants of the acoustic trap referring to a harmonic-oscillator model.