Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Upholding the diffraction limit in the focusing of light and sound   The concept of the diffraction limit put forth by Ernst Abbe and others has
been an important guiding principle limiting our ability to tightly focus
classical 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 "below
the diffraction limit". We argue that the diffraction limit defined in a
reasonable way, for example in terms of the upper bound on the wave numbers
corresponding to the spatial Fourier components of the intensity profile, or in
terms of the spot size into which at least 50% of the incident power can be
focused, still stands unbroken to this day. We review experimental observations
of "subwavelength" or "sub-diffraction-limit" focusing, which can be
principally broken down into three broad categories: (i) "super-resolution",
i.e. the technique based on the modification of the pupil of the optical system
to reduce the width of the central maximum in the intensity distribution at the
expense of increasing side bands; (ii) solid immersion lenses, making use of
metamaterials with a high effective index; (iii) concentration of intensity by
a subwavelength structure such as an antenna. Even though a lot of interesting
work has been done along these lines, none of the hitherto performed
experiments violated the sensibly defined diffraction limit.

2
The categories of this paper are: math
36
The categories of this paper are: cmp-lg
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
5
The categories of this paper are: gr-qc
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
18
The categories of this paper are: econ
26
The categories of this paper are: bayes-an
11
The categories of this paper are: hep-lat
13
The categories of this paper are: quant-ph
23
The categories of this paper are: solv-int
29
The categories of this paper are: funct-an
6
The categories of this paper are: astro-ph
24
The categories of this paper are: dg-ga
8
The categories of this paper are: hep-ex