Optimization of superoscillatory sub-diffraction limit focusing via engineering of a secondary aperture

The ability to focus light into a small spot is essential for high-resolution imaging, optical data storage, lithography, and optical trapping. A tighter focus directly improves resolution, precision, and efficiency, defining how finely structures can be imaged, written, or manipulated. However, in classical optics, the focal spot size is ultimately limited by diffraction, typically to about 0.5λ/NA, where λ is the wavelength and NA the numerical aperture. Superoscillatory light fields and lenses can overcome this limit, enabling sub-diffraction focusing but producing a weak central spot with strong sidelobes. In this work, we study the propagation of superoscillatory light generated by a binary aperture and design a secondary aperture to preserve the sub-diffraction spot while suppressing sidelobes. Building on Hu et al. ( Adv. Photonics 3, 045002, 2021), we employ the FDTD method to simulate a superoscillatory image free of off-center maxima.