Abstract
1 Introduction
Owing to the wave nature of optical diffraction, it is extremely challenging to achieve a highly localized wave packet in free space, which essentially limits the imaging performance of an optical system. In 1873, Erns Abbe first introduced the concept of optical diffraction limit,1 which is a constraint on the smallest light spot with a spatial size of about , where is the optical wavelength. Essentially, the diffractive light wave with high spatial frequency, named an evanescent wave, tends to exist on the near-field surface of an object and decays exponentially with distance. As a result, delicate information of the object carried by the evanescent wave cannot be delivered to the far-field region. In the past, substantial effort was made to break such a diffraction barrier2
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