Leaky surface-plasmon theory for dramatically enhanced transmission through a subwavelength aperture, Part I: Basic features

About three years ago, several groups in the physics community working collaboratively found experimentally a very interesting and initially puzzling effect. For a couple of years before that they had been studying a periodic array of sub wavelength holes in a metal plate at optical wavelengths. They reported that under the right conditions, transmission through the holes increased dramatically. More recently, they have been investigating the transmission through a single subwavelength-sized hole in a metal plate. When this tiny hole is made in a smooth metal plate, transmission through the hole was extremely small, as expected from small-aperture theory. Then, they found experimentally that when a periodic array of dimples or grooves is imposed on the initially smooth surface surrounding the hole the transmission of power through the hole is enhanced by "up to several orders of magnitude" (H.J. Lezec et al, PIERS 2002, p. 649, 2002). This astonishing increase in the transmission was difficult to believe at first, but measurements at optical frequencies were made at several laboratories and they have been reported in many papers. There is general agreement that surface plasmons (a class of surface waves) play a key role in this enhancement in transmission, but, to quote one recent paper (T. Thio, ibid., p. 648, 2002), "Although surface plasmons clearly play an important role in the transmission enhancement, the details of the enhancement mechanism are not yet understood." We believe that this interesting effect can be fully explained in terms of leaky waves, and we present here, in Part 1, the main features of the appropriate leaky-wave theory.