Acoustic Holograms – a new technology

Ultrasound is used in many disciplines. Sound fields can be created to exert forces, transfer power, and for medical imaging. Ultrasound is typically shaped with lenses to create a single focus, but also by combining many transducers, so called phased array transducers. The Micro, Nano and Molecular Systems group has developed a new concept to shape sound fields with complexities several orders of magnitude higher than what has been achieved by traditional methods. Our approach only requires a single transducer and a structured phase plate — the acoustic hologram. The hologram promises new tools for directed assembly, and in the use of shaped ultrasound fields in medical treatment and diagnostics, which we are exploring.

The speed of sound depends on the mechanical properties of the medium. A wave travelling through a material of a certain thickness will emerge with a phase difference compared to the wave travelling the same distance through the surrounding medium (say water or air). By modulating the thickness profile of the material, we can encode complex, designed wave fronts in a thin plate. This “phase plate” is called a hologram as it encodes the entire field that emerges via diffraction.

Current 3D printing technologies are suitable for rapid and cheap fabrication of the phase plates – the acoustic holograms. Only one plane-wave transducer is needed when exciting the hologram, to create high-fidelity sound fields requiring minimal electronic equipment. We are advancing this new technology.

We also develop new computational tools for the efficient design of acoustic holograms and for their use in liquid and solid environments. Our research focuses at extending these tools to include non-linear effects such as radiation force on particles, fluid streaming and localized heating.

Acoustic holograms are a general concept applicable to any field that ultrasound is used in. We work on:

• Computational tools for 3D sound field synthesis
• Materials for holograms and suitable fabrication methods
• Generation of amplitude and phase holograms
• Reconfigurable (dynamic) holograms
• Medical applications of acoustic holograms

A related field is the imaging of 3D scenes using a structured phase plate and a single sensor. This is known as compressed imaging in optics, and we explore compressed ultrasound imaging with the goal of miniaturized high-frequency imaging for medical applications.

Melde, K., Mark, A. G., Qiu, T., & Fischer, P. (2016). Holograms for acoustics. Nature, 537(7621), 518-522. doi:10.1038/nature19755

Melde, K., Qiu, T., Mark, A., & Fischer, P. (2017). Akustische Hologramme steuern Partikel. Physik in unserer Zeit, 48(1), 9-10. doi:10.1002/piuz.201790014