enhanced optical transmission through nano-apertures
The optical transmission of an aperture punched in an opaque metal film
is extremely small when the aperture diameter d is much smaller than the
optical wavelength, d<<l.
The graph below shows the total transmitted light (when the incident
light is focused onto a spot with diameter D=5l)
as a function of d/l. The solid black line
is the geometric limit, T=(d/D)2, appropriate for large d.
The green dot-dashed line is Bethe's prediction for the case of an ideal
metal, which follows T~(d/l)6. The
graph shows that the throughput decreases precipitously as d/l
goes into the sub-wavelength limit. For a real metal film with finite
thickness the transmission follows that of a waveguide beyond cutoff,
which drops even more rapidly (blue dashed line).
We have shown that the optical transmission can be enhanced considerably
(red solid squares) when the metal surface around the aperture has a periodic
surface corrugation. The peak wavelength depends on the lattice constant
of the corrugation, and is thus tunable. The transmission can exceed the
geometric limit (T>(d/D)2), even when d<<l.
Refs.: Ebbesen, Lezec, Ghaemi, Thio, Wolff,
Nature 391, 667 (1998);
Thio, Pellerin, Linke, Ebbesen, Lezec,
Lett. 26, 1972 (2001).
Periodic arrays of apertures
A periodic aperture array constitutes an extreme case of surface corrugation.
The optical transmission spectra have both high transmission peaks as
well as deep transmission minima.
The transmission peaks are widely held to be cause by a resonance with surface
plasmon polarition (SP) modes on the surface of the metal film, which cause
the oscillating electric field to be strongly enhanced at the aperture entrances.
We have recently proposed a radically different model for the transmission
enhancement. In the new model the incident light scatters off the subwavelength
surface structure and generates an evanescent wave, labeled a composite
diffracted evanescent wave (CDEW), which travels on the surface and
interferes with the light that is directly impinging on the neighbouring
holes. Transmission enhancement or suppression is obtained when
the interference is constructive or destructive, respectively. The CDEW
model thus naturally includes both the transmission maxima and minima;
furthermore, it successfully explains certain aspects of the enhanced
transmission that, in the framework of the SP model, have remained mysteries,
including the occurrence of enhanced transmission in non-metallic systems
(which do not support SPs), efficient beaming effects, and discrepancies
between experiments and SP theory of the spectral positions as well as
widths of the transmission maxima.
Ref: Lezec and Thio, Optics
Express 12, 3629 (2004)
Details and FAQ (frequently asked questions) of
Contrary to what is widely claimed in the literature, the transmission
enhancement of aperture arrays is G<7 (when compared to a real
single, isolated hole of the same dimensions). However, for a single hole
surrounded by circular surface corrugation, the axial symmetry gives significantly
higher enhancement: In an optimised device the transmission enhancement
can be G>100. In this case also the peak wavelength is tunable
by an appropriate choice of the periodicity.
A systematic study of the corrugation geometry shows that the optical
geometry is a set of ring grooves, concentric around the central aperture,
with a depth h=100nm, corresponding to about three times the skin depth
of the metal, usually silver. The peak tranmission of such an optimised
device as be as high as T/f~3: three times more light is coming
through the aperture than is directly impinging on it. The enhanced transmission
can therefore exceed the geometric limit by a factor of 3, even well into
the subwavelength limit (see top figure of this page).
The wavelength at which the transmission maxima and minima occur can
be tuned by varying the lattice constant of the surface structure, the
refractive index of the dielectric medium adjacent to the metal, or the
incident angle. Such a tunability, together with the high transmission
even at subwavelength scale, make enhanced-transmission devices very attractive
in a number of applications, including high-density optical data storage,
near-field scanning optical microscopy (NSOM), optical switches
and modulators, and high efficiency display devices.