Reflective Acoustic Holograms for Fine Manipulation of Sound

Intricate spatial manipulation of acoustic pressure fields is critical for a wide range of applications, including particle manipulation, ultrasonic energy transfer, ultrasonic imaging, and neural stimulation. As such, the developments in sound manipulation techniques can significantly improve the potential of the existing applications. Currently, acoustic waves manipulation is achieved mainly using (a) active phased arrays transducers (PATs) and (b) passive metamaterials. In PATs, a large number of acoustic transducers are individually synchronized and driven to obtain a particular target image pattern. Although PATs capabilities were successfully demonstrated in applications such as acoustic tweezers, acoustic volumetric displays, and particle manipulation, this technique scales poorly for higher frequencies and high-resolution patterning. PATs quickly become costly and will require very sophisticated and power-consuming circuitry. In addition, when the transducer size is bigger than the wavelength, especially in underwater high-frequency applications, it severely limits the accuracy and resolution of the control over the sound field.

More recently, acoustic metamaterials gained attention for their wavefront manipulation capabilities. It is demonstrated that they easily scale for larger apertures and high-resolution patterning compared to PATs. Metamaterials showed exotic properties and effects such as wave steering, acoustic cloaking, and extraordinary reflection. However, most of these capabilities have been demonstrated in the air for a range of frequencies between 3-5 kHz. Extending these materials for higher frequencies requires advanced manufacturing techniques since metamaterials are made of labyrinths or arrays of unit cells of sub-wavelength size. Moreover, more complexities arise for different propagation mediums, such as water. These two drawbacks severely compromise the potential of metamaterials in many applications

Recent developments in monolithic acoustic holograms, which is equivalent to the optical kinoform in acoustics, enabled a new level of acoustic manipulation. Their high-resolution control capabilities have been demonstrated underwater in frequencies up to the megahertz range and have been investigated in applications such as high-resolution acoustic holography, particle levitation, cell patterning, and contactless energy transfer. Additionally, they can also be easily fabricated using conventional 3D printing techniques. The majority of the work in monolithic passive acoustic hologram has been in manipulating the acoustic pressure field using transmission holograms. In this work, we introduce a monolithic reflective hologram with the capability of constructing arbitrary wavefronts or pressure patterns in the desired location in space. Such an effect can be achieved using a spatial thickness map that creates a relative phase difference in the wavefront. The thickness map will depend on the desired pattern and the speed of the sound of the hologram material and the medium of propagation. This unique thickness map is obtained using an iterative version of the Angular Spectrum Approach (ASA), where multiple front and back propagations between the pattern plane and the hologram plane are carried out until a satisfactory thickness map is achieved. This type of patterning can develop new capabilities in beam steering, medical imaging, and many ultrasonic applications. The results are verified using theoretical models, multiphysics simulations in COMSOL, and experimental investigations underwater.

This project is funded by NSF  ECCS  Program with Dr. Radhakisan Baheti, ECCS-1711139.

Principal Investigator and project adviser Dr. Shima Shahab