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The concept of the diffraction limit put forth by Ernst Abbe and others hasbeen an important guiding principle limiting our ability to tightly focusclassical waves, such as light and sound, in the far field. In the past decade,numerous reports have described focusing or imaging with light and sound "belowthe diffraction limit". We argue that the diffraction limit defined in areasonable way, for example in terms of the upper bound on the wave numberscorresponding to the spatial Fourier components of the intensity profile, or interms of the spot size into which at least 50% of the incident power can befocused, still stands unbroken to this day. We review experimental observationsof "subwavelength" or "sub-diffraction-limit" focusing, which can beprincipally broken down into three broad categories: (i) "super-resolution",i.e. the technique based on the modification of the pupil of the optical systemto reduce the width of the central maximum in the intensity distribution at theexpense of increasing side bands; (ii) solid immersion lenses, making use ofmetamaterials with a high effective index; (iii) concentration of intensity bya subwavelength structure such as an antenna. Even though a lot of interestingwork has been done along these lines, none of the hitherto performedexperiments violated the sensibly defined diffraction limit.

Upholding the diffraction limit in the focusing of light and sound