New research provides
a detailed explanation for a baffling effect
in which much larger-than-expected amounts of light passed
through a silver-coated quartz barrier with tiny openings: namely, a
periodic array of 150-nm holes up to 10 times smaller than the
wavelength of the light sent through. This unexpected experimental effect
bodes well for scaling
down optical devices to nanometer dimensions.
Light can pass
through such tiny holes due to the actions of
surface plasmons (SPs), collective oscillations of electrons at
the boundary between conductors and insulators.
According to one of the research collaborations investigating this
effect, the light gets through the holes in the form of an SP "molecule,"
consisting of two polaritons, one on each side of the metal film, that
interact with one another with exponentially decaying electromagnetic
fields, forming "molecular" levels in very much the same way
that atomic electron wavefunctions interact to form molecular levels
in a diatomic molecule.
The plot illustrates
the effect of the SP molecules.
The x axis depicts the propagation of light. The y-axis runs
along a cut of the periodic array of holes (the cut considered
is represented schematically in the upper left panel).
To show more clearly
the formation of the SP molecule, this plot
neglects the effects of light absorption by the metal. The upper
right panel shows the wavelengths (780 and 788 nm) at which
light is transmitted through the metal in this case.
As researchers
discovered, the mathematics of the SP molecule's
electromagnetic field are essentially the same as the ones
describing the formation of molecular electronic levels from
the levels of (otherwise) isolated atoms. Suppose there are two
atoms that, when very far apart, have their own sets of energy levels.
When the atoms come closer, these separate sets of energy levels
combine to form a set of molecular levels.
In the plasmon
molecule something analogous occurs: if the two
metal surfaces were very far apart, there would be two isolated
surface plasmons . If the metal is not too thick, these two surface
plasmons "talk" to each other, and form a set of combined
levels.
Two separate cases
are shown in (a) and (b), corresponding to the two
different plasmon molecule levels: the symmetric (b) and antisymmetric
(a) linear combination of surface plasmons at both interfaces. Note
that, as expected, in the antisymmetric case the electric field
intensity at the middle of the hole is much smaller than in the
symmetric case.
It is also worth
noticing the huge enhancement of the fields at the
surfaces, by a factor of order 500 in intensity, due to the
plasmons. In this scale the field of the incoming and outgoing
wave cannot be resolved, so large are the fields close to the metal!
(Thanks to Luis
Martin Moreno and Francisco Jose Garcia Vidal for providing the figure
and the explanation.)
Reported by:
L. Martín-Moreno, F. J. García-Vidal, H. J. Lezec, K.
M. Pellerin, T. Thio, J. B. Pendry, and T. W. Ebbesen, in Physical
Review Letters 86, 1114 (5 February 2001).
Associated
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