Dual-frequency ultrasonic holography by binary aperture plates

Binary amplitude-based holograms offer an efficient approach for precise underwater ultrasonic wave control due to their structural simplicity. However, their limited information capacity restricts multi-frequency and/or multi-depth holographic projection when compared to phase-modulation techniques. Here, we present a binary aperture design based on a multi-frequency optimal accumulation algorithm (MFOAA) to realize dual-frequency ultrasonic holography (DFUH) in water. The MFOAA introduces an additional frequency degree of freedom, thereby enhancing the information encoding capacity of binary apertures. Both theoretical and simulation results have demonstrated that the well-designed binary aperture plates (BAPs) can realize DFUH projections, including the dual-frequency geometric shapes, English letters, and Arabic numerals. Finally, the proposed DFUH is experimentally confirmed to be possible with the BAP, which is made by drilling holes in a stainless-steel plate. Our work establishes a foundation for multi-frequency ultrasonic wave engineering, with potential applications in biomedical imaging, particle manipulation, and ultrasound haptics.