Mode couplings in multiplex holographic acoustic lenses

Multiplex holographic acoustic lenses, composed of stacked elastic layers, offer a pathway toward practical and versatile acoustic holography. The layered architecture regularizes the surface profile for improved integration and expands operational flexibility across broad frequency ranges. However, multiple layers introduce complex inter-layer mode couplings that distort the transmitted wavefront and degrade hologram performance. We investigate these coupling mechanisms through full-field elastic wave simulations and experimental validation, showing that longitudinal resonances across layers produce spatial phase distortions that compromise field reconstruction. The conformal interface enhances acoustic coupling with soft media, broadening the design's applicability to biomedical settings. A conformal flat elastic layer is incorporated atop the multiplex structure, compensating for surface unevenness. This additionally requires identification of spurious mode interactions and thus restoring hologram fidelity. Additionally, these results provide fundamental insights into structural–acoustic interactions in layered holographic systems and establish general design principles for next-generation multifunctional acoustic lenses with applications in targeted therapy, ultrasound imaging, and noninvasive neuromodulation.