Holographic techniques are fundamental to applications such as volumetric displays1, high-density data storage and optical tweezers that require spatial control of intricate optical2 or acoustic fields3,4 within a three-dimensional volume. The basis of holography is spatial storage of the phase and/or amplitude profile of the desired wavefront5,6 in a manner that allows that wavefront to be reconstructed by interference when the hologram is illuminated with a suitable coherent source. Modern computer-generated holography7 skips the process of recording a hologram from a physical scene, and instead calculates the required phase profile before rendering it for reconstruction. In ultrasound applications, the phase profile is typically generated by discrete and independently driven ultrasound sources3,4,8,9,10,11,12; however, these can only be used in small numbers, which limits the complexity or degrees of freedom that can be attained in the wavefront. Here we introduce monolithic acoustic holograms, which can reconstruct diffraction-limited acoustic pressure fields and thus arbitrary ultrasound beams. We use rapid fabrication to craft the holograms and achieve reconstruction degrees of freedom two orders of magnitude higher than commercial phased array sources. The technique is inexpensive, appropriate for both transmission and reflection elements, and scales well to higher information content, larger aperture size and higher power. The complex three-dimensional pressure and phase distributions produced by these acoustic holograms allow us to demonstrate new approaches to controlled ultrasonic manipulation of solids in water, and of liquids and solids in air. We expect that acoustic holograms will enable new capabilities in beam-steering and the contactless transfer of power, improve medical imaging, and drive new applications of ultrasound.
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We thank P. Weber and M. Fratz for suggestions. This work was in part supported by the Max Planck Society and by the European Research Council (ERC grant agreement 278213).
Extended data figures
The container, filled with a suspension of silicone particles in water, is positioned above the acoustic hologram with the transducer located in the back. The scene is observed from the top. Initially the sound field is off and the particles are at rest. When the transducer is turned on, the particles collect at the top window of the container (towards observer) and assemble in the form of the “Dove of Peace”. The trapping sites are defined by the projected sound pressure image. When the system is turned off the ensemble collapses and particles settle to the bottom of the container.
Two objects, in the form of spherical caps, move along circular paths of opposite direction. The scene is first observed from the side then from above. The outer object has a diameter of 4 mm and the inner object a diameter of 2 mm, with both heights being equal to 0.5 mm. The radii for the inner and outer trajectory are 8 mm and 16 mm, respectively, and the projected acoustic phase gradient for both paths is about 1 rad/mm. This corresponds to a topological charge of +8 for the inner path and -16 for the outer path. The objects will follow the closed contour indefinitely until the transducer is turned off. The same projected phase gradient is then used to propel different objects. The last part demonstrates the effect of an open contour. Observed from above, a blue spherical cap of 4 mm diameter and 0.5 mm height is manually placed at the start of the open trajectory with a phase gradient of about 1 rad/mm. The yellow line marks the tracked particle position in each frame. At the end of the contour the particle is ejected and free to float over the water surface.
This video shows the amplitude (left) and phase (right) plot of the calculated, propagating acoustic field for the open contour phase gradient image. The video shows the field while scanning along the z-direction. The z coordinate is displayed above the amplitude plot. The image plane is located at z = 30 mm.
Close up view of the air cavity with the hologram positioned at the top and the transducer at the bottom. Two water droplets are manually loaded into the traps using a syringe needle. Their diameters are approximately 1.1 mm.