Composite Orbital Angular Momentum for Super-resolution Ultrasound Imaging

Acoustic imaging is widely used in biomedicine and industrial nondestructive testing, yet its spatial resolution remains fundamentally limited by acoustic diffraction. Despite significant progress in nonlinear harmonic imaging, localization microscopy, and acoustic metamaterials, these approaches face intrinsic trade-offs between efficiency, invasiveness, structural complexity, and scalability. Here, we introduce a far-field super-resolution ultrasound imaging strategy based on composite orbital angular momentum (OAM). By synthesizing vortex beams spanning multiple topological charges, the collective angular spectrum converges toward a Dirac delta function, enabling the recovery of high spatial frequencies from far-field scattered fields. Numerical simulations and underwater experiments demonstrate robust subdiffraction imaging of metallic wires, complex steel structures, and blood vessels, achieving resolutions down to 0.2λ in simulation and 0.24λ in experiment at 1 MHz, well beyond the Rayleigh limit. Moreover, tailored OAM spectra enable functional imaging capabilities such as chirality discrimination, offering sensitivity to structural handedness. Our results establish composite OAM as a powerful new degree of freedom for ultrasound imaging.