Sub-Wavelength Focusing at the Multi-Wavelength Range Using Superoscillations: An Experimental Demonstration

We experimentally demonstrate the formation of a superoscillatory sub-wavelength focus at a multi-wavelength working distance. We first discuss and distinguish superlensing, superdirectivity and superoscillation as different methods which, in their respective ways, achieve sub-diffraction resolution. After establishing superoscillation as a potential way towards sub-wavelength focusing at the multi-wavelength range, we proceed to design, synthesize and demonstrate a superoscillatory sub-wavelength focus in a waveguide environment. Our measurements confirm the formation of a focus at 75% the spatial width of the diffraction limited sinc pulse, 4.8 wavelengths away from the source distributions. This working distance is an order of magnitude extended from those of superlenses and related evanescent-wave-based devices, and should pave way to various applications in high-resolution imaging.