  Continuous films of metals like gold and silver with a thickness of a few
tens of nm have poor optical transmission in the visible and infrared. However,
the same films become largely transparent when the transmission is mediated by
coupled surface plasmon polaritons on the two surfaces of the film. Likewise,
it is expected that optical transmission through a hole in an opaque metal film
would be negligible if the size of the hole is much smaller than the wavelength
of the incident radiation. Contrary to this, it was found a few years ago that
opaque metal films perforated with a periodic array of sub wavelength holes
exhibit large transmission at certain wavelengths. Such extraordinary optical
properties of the metal films have attracted much attention in recent years.
They have led to several proposals and demonstrations of nanophotonic
applications in a wide range of areas, such as microscopy, spectroscopy,
optoelectronics, optical data storage, bio-chemical sensing and so on. In
parallel, extensive effort has been made to elucidate the mechanisms of
efficient light energy transfer across metal films with arrays of sub
wavelength apertures like holes and slits. Despite this effort, not all aspects
of the underlying physics of the phenomena are fully understood yet. Here, we
provide a short review of the various concepts put forth to explicate the large
transmission of light across continuous metal films and films with sub
wavelength apertures.
