LETTERS TO THE EDITORS
To the Editors:
In the article "A Bright Future for Subwavelength Light
Sources" (January-February), Tineke Thio considered a model
involving nonresonant evanescent waves diffraction and interferences
to explain the typical transmission spectra through hole arrays,
which is observed either with metallic or dielectric films.
However, our own work makes clear that the existence of eigenmodes
(bound modes) are required to observe the general features of the
hole arrays' transmission, although a strong hypothesis on the
origin of the eigenmodes was not required.
The eigenmodes related to the film, whatever the material, are
resonantly excited by the incident light and converted into
transmitted waves (thanks to the film corrugation). These waves
interfere with nonresonantly transmitted waves (due to
"Bethe" transmission, for instance). As a consequence, the
transmission spectra are well depicted as a series of asymmetrical
According to the contributions of resonant and nonresonant
processes, eigenmode resonance wavelengths can be associated either
with transmission peaks or dips. This is likely to be the reason for
the transmission dependence against the grooves locations
(maintaining the same spacing between them) from the central
aperture in the circular arrays, as the nonresonant contribution changes.
According to the material properties, one observes various
eigenmodes: surface plasmons (metallic films), guided slab modes or
Brewster-Zennek surface modes (dielectric films), surface
phonon-polaritons (ionic materials in the restrahlen band),
"Pendry" surface modes (perfectly conducting materials).
Other authors have also confirmed experimentally or theoretically
many of these results.
University of Namur, Belgium
Dr. Thio responds:
Many researchers share Dr. Sarrazin's view that a resonance must be
implicated in the transmission properties of subwavelength hole
arrays. This is entirely reasonable if a hole in an array indeed
transmits 1,000 times more light than a solitary hole. However,
evidence is accumulating that the enhancement is really at most a
factor of 10, and a resonance is not necessary to account for such a
modest enhancement. The transmission spectra of hole arrays exhibit
a stunning similarity across a large range of geometrical and
materials parameters. So instead of indulging in a proliferation of
possible resonance modes, let us use Ockham's razor, or the
principle of adopting the simplest theory that explains all the
observations. The model of diffraction and interference of
evanescent waves does the job.