This thesis reports the remarkable discovery that by arranging the dipoles in an ordered array
with particular spacings it is possible to greatly enhance the cross-section and achieve a
strong light-matter coupling (>98% of the incident light). It also discusses the broad
background to cooperative behaviour in atomic ensembles and analyses in detail effects in one-
and two-dimensional atomic arrays. In general when light interacts with matter it excites
electric dipoles and since the nineteenth century it has been known that if the amplitude of
these induced dipoles is sufficiently large and their distance apart is on the scale of the
wavelength of the light then their mutual interaction significantly modifies the light-matter
interaction. However it was not known how to exploit this effect to modify the light-matter
interaction in a desirable way for example in order to enhance the optical cross-section.
