The physics for beyond-limit focusing has great academic importance, particularly for light propagation with control over focal point location and spot size. Asymmetric boundary confinement for a beyond-limit focusing lens in a metallic subwavelength structure redirects the surface current at the inner edges of the interface between an upstream double slit with width much smaller than the wavelength and a downstream slit with width close to one wavelength. This induces intense localized fields and tightly squeezes spatial distributions for the wave sources and corresponding images that are critical for focusing beyond the diffraction limit, as opposed to separated sources in super-oscillatory lenses and metalenses for optically large areas. This squeezing simultaneously increases the distance between source and image peaks and reduces their distribution widths one order less than the wavelength. The corresponding spatial spectrum alters to excite an inward-propagation wave mode with strong lateral wave momentum for focusing. The radiated inward propagation wave mode can be manipulated to help the focused field tunnel through the downstream slit, such that the focal point can reach the intermediate zone. Source and image radiation confirmed that focused field propagation had good agreement with simulation results. The proposed analytical Green’s method opens new physics applications for this fundamental focusing process, offering new opportunities to further reduce spot size and tailoring the focal length for important potential plasmonic applications.
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