Light in tiny holes
C. Genet1& T. W. Ebbesen1
The presence of tiny holes in an opaque metal film, with sizes smaller than the wavelength of incident light, leads to a wide
variety of unexpected optical properties such as strongly enhanced transmission of light through the holes and wavelength
filtering. These intriguing effects are now known to be due to the interaction of the light with electronic resonances in the
surface of the metal film, and they can be controlled by adjusting the size and geometry of the holes. This knowledge is
opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical
sensing and biophysics.
the Flemish painters in the sixteenth century to project an image
(albeit upside down) onto their canvases. It was in the middle of
than the wavelength of light, such apertures remained the object of
scientific study and debates for centuries thereafter, as an accurate
description and experimental characterization of their optics turned
out to be extremely difficult.
In the twentieth century, the interest naturally shifted to subwa-
velength holes as the technology evolved towards longer wavelengths
of the electromagnetic spectrum. With the rising importance of
microwave technology in the war effort of the 1940s, Bethe treated
His predictions, notably that the optical transmission would be very
weak, became the reference for issues associated with the miniatur-
ization of optical elements and the development of modern char-
acterization tools beyond the diffraction limit, such as the scanning
near-field optical microscope (SNOM), which typically has a small
aperture in the metal-coated tip as the probing element3.
In this context, the report of the extraordinary transmission phe-
metal screen4generated considerable interest because it showed that
orders of magnitude more light than Bethe’s prediction could be
transmitted through the holes. This has since stimulated much fun-
damental research and promoted subwavelength apertures as a core
element of new optical devices. Central to this phenomenon is the
role of surface waves such as surface plasmons (SP), which are essen-
tially electromagnetic waves trapped at a metallic surface through
their interaction with the free electrons of the metal5,6(Box 2). This
combination of surface waves and subwavelength apertures is
what distinguishes the enhanced transmission phenomenon from
the idealized Bethe treatment and gives rise to the enhancement.
Moreover, modern nanofabrication techniques allow us to tailor
the dynamics of this combination by structuring the surface at the
subwavelength scale. This opens up a wealth of possibilities and
applications from chemical sensors to atom optics.
Wewillreview here thepresent understanding ofthe transmission
through subwavelength apertures in metal screens, starting for the
hole in a screen is probably the simplest optical element
possible, and was an object of curiosity and technological
application long before it was scientifically analysed. A
pinhole was at the heart of the camera obscura used by
sake of clarity with simple isolated holes and ending with arrays. As
we will see, SPs play an essential role at optical wavelengths in all the
considered structures. Applications such as tracking single molecule
fluorescence in biology, enhanced vibrational spectroscopy of mole-
cular monolayers and ultrafast photodetectors for optoelectronics
illustrate the broad implications for science and technology.
Figure 1a shows a single hole milled in a free-standing Ag film,
characterized by both the diameter of the hole and its depth. When
Bethe considered such a system, he idealized the structure by assum-
ing that the film was infinitely thin and that the metal was a perfect
conductor. With these assumptions, he derived a very simple
wavelength l, and r is the radius of the hole. It is immediately appar-
ent that gBscales as (r/l)4and that therefore we would expect the
optical transmission to drop rapidly as l becomes larger than r, as
shown in Box 1. In addition, the transmission efficiency is further
attenuated exponentially if the real depth of the hole is taken into
account7. This exponential dependence reflects the fact that the light
cannot propagate through the hole if l.4r, whereupon the trans-
mission becomes a tunnelling process. The cutoff condition l.4r
is of course a first approximation and in real situations the cutoff
occurs at longer wavelengths when the finite conductivity is taken
into account8(see Box 1).
to the polarization of the incident light2. If the diffraction pattern is
scanned along the direction of the incoming polarization the intens-
ity should be constant (like a spherical wave in a plane) while in the
(the angular dependence is acos2h function, typical ofa dipole emis-
The increasing use of SNOM and interest in the extraordinary
transmission phenomenon have stimulated experimental9–11and
theoretical12–16studies, the results of which challenge Bethe’s predic-
tions. In particular, it has become possible to measure the transmis-
sion and diffraction from a single subwavelength aperture in a
metallic film at optical wavelengths9–11. Angular measurements at
1ISIS, Universite ´ Louis Pasteur and CNRS (UMR7006), 8 alle ´e G. Monge, 67000 Strasbourg, France.
Vol 445j4 January 2007jdoi:10.1038/nature05350
the exit of subwavelength apertures have revealed that the light dif-
fracts less than expected9,10. Similarly, the transmitted light can have
unexpected features10. The simple circular aperture of Fig. 1a has a
transmission spectrum with a peak as shown in Fig. 1b not predicted
by equation (1) or by other conventional theories2,7. Similar mea-
surements can be made on a rectangular hole (Fig. 1c) where the
spectrum becomes sensitive to the incident light polarization as
can be seen in Fig. 1d. The appearance of such resonant peaks can
type known as localized SP that has been confirmed by theoretical
long axis of the rectangular hole, we can selectively excite the corres-
ponding localized SPs (Fig. 1d). Such behaviour is very reminiscent
by localized SPs. Whereas the localized SP modes are defined by the
lateral dimensions of the aperture, theoretical studies have shown
that in addition to such SP modes14other resonant modes defined
along the depth of the hole might also be present and contribute to
the transmission signal12. Further experimental studies on this issue
at optical wavelengths are necessary.
Bethe’s theory describes the transmission as a smooth decreasing
function of the wavelength, as given by equation (1) and shown in
Box 1, whereas the experiments discussed above reveal the presence
of a resonance superimposed on a smooth background, thus provid-
presented in this review, it is always the presence of some type of
resonance that leads to transmission enhancement. This reveals yet
again that Bethe’s theory is too idealized to treat situations where
surface modes are involved and where propagating or evanescent
modes can additionally be excited inside the hollow aperture12,
thereby significantly underestimating the transmission efficiency.
We define the transmission as being extraordinary when it is so
enhanced that the transmission efficiency g is larger than 1, in other
sections, g can be much larger than one for certain aperture struc-
tures under appropriate conditions.
For experimental reasons, it is very difficult to quantify g for a
single hole. As was pointed out above, the emission pattern from a
singe aperture in a real metal is not isotropic and therefore the abso-
lute transmission can only be determined by measuring the absolute
intensity over all angles and then summing the data. This remains an
experimental challenge. As we shall see in the section on optimizing
Box 1jLight transmission through apertures
When light scatters through apertures, it diffracts at the edges. In the
of radius r,,l in the idealized situation of an infinitely thin and
perfect metal sheet. He has shown that the transmission T(l) scales
uniformly with the ratio of r to l to the power of four, as described in
equation (1) and schematically shown below in Box 1 Fig. 1.
Box 1 Figure 1 | Diffraction and typical transmission spectrum of visible
However, a real aperture is characterized by a depth and therefore
has waveguide properties. The transmission of light through such a
confined space of the waveguide essentially modifies the dispersion
relation of the electromagnetic field. The lateral dimensions of the
waveguide define the wavelength at which light can no longer
propagate through the aperture. This wavelength is known as the
cutoff wavelength lc. When the incident wavelength l.lcthe
transmission is exponentially small, characterizing the non-
propagating regime as shown in Box 1 Fig. 2. With real metals, the
cutoff wavelength cannot be sharply defined because one goes
continuously from propagative to evanescent regime as the
There isa straightforward relationship between the cross-section of
the waveguide and lc. However, one should take into account that lc
account, reflecting the penetration of the electromagnetic field inside
the walls of the metal waveguide. It is possible to control and even to
eliminate cutoff wavelengths even when the lateral dimensions are
much smaller than l,by playingwith more complex geometries. While
simple apertures are always characterized by the existence of cutoff
wavelengths, an annular hole, for example, which resembles a coaxial
cable, has no cutoff wavelength and is always propagating. The
polarization of the incident light is also an important parameter, and
with non-cylindrical waveguides, the transmission can be made
extremely polarization sensitive. A striking illustration is provided by a
slit. Here, for incident polarization parallel to the long axis, the
transmission can be made subwavelength, as soon as the short
dimension of the rectangle is smaller than l. However, for the
always propagates through it. This allows for many possibilities in the
choice of geometry depending on the application.
Box 1 Figure 2 | A cylindrical waveguide with a radius r much smaller
than the wavelength l of the incident electromagnetic field milled in a
attenuation of the subwavelength regime. A transmission spectrum can
reveal the different propagating and evanescent regimes.
Intensity (arbitrary units)
Wavelength (nm)Wavelength (nm)
Intensity (arbitrary units)
Figure 1 | Optical transmission properties of single holes in metal films.
white light. a, A circular aperture and b, its transmission spectrum for a
270nm diameter in a 200-nm-thick film. c, A rectangular apertureand d, its
transmission spectrum as a function of the polarization angle h for the
Figure adapted from ref. 10, with permission.
NATUREjVol 445j4 January 2007
subwavelength apertures below, the best transmission signals are
obtained in noble metals such Au and Ag. To obtain detectable reso-
nances at visible wavelengths, the dimensions of the holes should
be of the order of 150 to 300nm and the films not much thicker than
200 to 300nm.
Smallapertures are routinely used in SNOM tipsto explore and to
map with subwavelength resolution the electromagnetic field in the
immediate vicinity of a surface3. More recently, tiny apertures have
been implemented in fluorescence correlation spectroscopy17–19, a
powerful technique for the study of the diffusion and reaction of
single fluorescent biomolecules in which the information is derived
from the analysis of the statistical fluctuations of individual mole-
is defined by the focal point of a laser beam, that is, about 1mm3,
which puts alimit on the upper concentration that can be used while
still observing statistical fluctuations. By using small apertures in
metal (see Fig. 2)18,19, the analysed volume has been reduced by a
factor of 1,000, allowing one to study molecular events at nearly
millimolar concentrations—closer to biological conditions. In addi-
tion, such structures give rise to other benefits: the localized SP fields
increase the excitation rate of the molecules in its vicinity10,14,19, the
from the fluorescent state are affected19. All these can lead to an
increase in the detected signal, rendering fluorescence correlation
spectroscopy ever more useful as a tool for biology.
Single apertures surrounded by periodic corrugations
With modern nanofabrication techniques it is possible to modify the
optical properties of a single aperture by sculpting the surrounding
material at the scale of the wavelength20–28. Such modifications give
rise to much higher transmission than single holes at selected wave-
lengths and in addition, novel lensing effects including beaming can
be induced by texturing the output surface of the aperture—as dis-
When a single aperture is surrounded by circular corrugations as
the incident light into SPs at a given l. As a consequence the electro-
magnetic fields at the surface become intense above the aperture,
resulting in very high transmission efficiencies and a well-defined
spectrum (Fig. 3a). Here the resonant wavelength is mainly deter-
The resonance is, however, slightly more red-shifted than the period
owing to the interaction with the light directly transmitted through
the hole. This should be considered in tuning the structure to be
bright at a desired wavelength. When such a structure is milled in a
metal like Ag, the value of g can be much larger than one21. Again,
of the same dimensions the transmission gain can be an order of
magnitude at resonant wavelengths20. This, as we shall see below,
has important applications.
Box 2jCoupling to SPs
At the interface separating a dielectric with a permittivity edand a
metal with a permittivity em, SPs can be resonantly excited by the
coupling between free surface charges of the metal and the incident
electromagnetic field. Such a mode is characterized by a surface wave
vector that obeys the following dispersion relation:
Box 2 Figure 1 | SP dispersion relation. The dotted line corresponds to
the light line. The hatched sector of propagating waves does not overlap
with the evanescent sector below the light line that fully contains the SP
dispersion relation. kincis the transverse component of the incident wave
vector and G corresponds to the momentum needed to couple to the SP
mode in the evanescent sector.
Here, v is the pulsation of the electromagnetic field and c the velocity
of lightin vacuum. Provided that the realpartof emissmaller than2ed,
this wave vector has positive real and imaginary parts. The latter
corresponds to the propagation length of the surface wave before it is
damped inside the metal, and can be tens of micrometres at the
smooth surface of noble metals, such as Au or Ag, at optical
wavelengths. The real part of kSPis plotted in Box 2 Fig. 1. It is always
ones. This implies immediately that such a mode is evanescent and
additional momentum G is needed to go from the propagating sector
where the wave vector kincof the incident light falls to the evanescent
one where SP modes exist. This is expressed in the simple resonance
condition kSP5kinc1G, which is a function of the incident pulsation
and incident angle h.
One way to provide the missing momentum G necessary for
coupling incoming light to SPs is to use a periodic array. In one
2p/a0where a0is the period of the structure. This is the origin of the
optical resonant behaviour of the array, because only when:
does light couple to SPs (i is an integer). The electromagnetic wave is
then trapped momentarily on the surface, giving rise to the
transmission peaks. The array generates a complex band structure, as
zone edges), SPs are back-reflected so strongly that they cannot
propagate any more. Bandgaps appear in the SP dispersion relation,
corresponding to stationary waves and high field enhancements.
It should be noted that, when illuminated, non-periodic structures
such as single holes, sharp edges, particles and so on can generate
localized SP modes. This is possible when the dimensions of the
defects are smaller than the wavelength of the incident field,
generating a broad spectrum of G vectors (stemming from the spatial
Fourier spectrum of the particular defect) in which a solution to the
is dependent on the particular profile of the defect.
Box 2 Figure 2 | SP band structure on a periodic array.
Approximate observation volume
Figure 2 | Schematicdiagramof the fluorescence correlation spectroscopy
The fluorescence is collected from the same side as the incident excitation.
Courtesy of J. Wenger.
NATUREjVol 445j4 January 2007
surprisingly narrow beam can be generated, having a divergence of
less than a few degrees20, which is far smaller than that of the single
apertures discussed earlier. This is because the light emerging from
modes existing in the grooves—which in turn scatter the surface
waves into freely propagating light22–24. This then interferes with
the light that has travelled directly through the hole generating the
focused beam. A variant of the double-sided bull’s-eye structure is a
slit with parallel grooves on both sides of the film, which in addition
disperses light spatially according to wavelength20,28. Such double-
have a focal plane like a lens but at the same time have other unusual
features. For instance, by having grooves with different periods on
either side of the film next to a slit, the direction of the output beam
can be made independent of the input beam, unlike conventional
lenses or gratings, suggesting many practical applications (Fig. 3b).
The antenna capacity of the corrugations to concentrate the
possibilities such as a bright subwavelength spot for ultradense
optical-data storage and nonlinear phenomena29–31. Of paramount
importance to modern optical telecommunication are photodetec-
tors that can translate an optical signal into an electrical one and
thereby convert the flow of information being carried through the
telecom network into a displayable signal on the screen. Such photo-
detectors must therefore be as fast as possible to handle the large
amount of data flow. Typically the operating speed of a photodetec-
enough photons. To circumvent this problem, an ultrafast photode-
tector has been realized that elegantly combines a very small photo-
electrical element with a bull’s-eye antenna structure (shown in
Fig. 4) that collects and concentrates the incoming photons32. This
combination illustrates well the potential benefit of plasmonic
devices for optoelectronics.
Periodic arrays of holes in an opaque metal film have so far been the
structures that have found the most applications owing to the sim-
plicity with which their spectral properties can be tuned and scaled.
Among other things, they can act as filters for which the transmitted
colour can be selected by merely adjusting the period. As we saw in
the previous section (and Box 2), periodic metallic structures can
convert light into SPs by providing the necessary momentum con-
servation for the coupling process. It is therefore not surprising that
periodic arrays of holes such as those shown in Fig. 5 can give rise to
the extraordinary transmission phenomenon4where the transmis-
sion spectrum contains a set of peaks with g larger than one even
when the individual holes are so small that they do not allow pro-
detail both theoretically33–59and experimentally58–79. As in the case of
single holes surrounded by periodic grooves22, the process can be
divided into three steps: the coupling of light to SPs on the incident
surface, transmission through the holes to the second surface and
then re-emission fromthesecond surface. Atthepeak transmissions,
wise inefficient transmission through each individual hole (Box 1).
If we apply the momentum-matching conditions discussed in Box
2 toatwo-dimensional triangulararray shown inFig. 5, wecanshow
that the peak positions lmaxat normal incidence are given in a first
where P is the period of the array, emand edare respectively the
dielectric constantsofthemetalandthedielectric materialincontact
with the metal and i, j are the scattering orders of the array. Because
equation (2)doesnot takeinto account thepresenceoftheholesand
the associated scattering losses, it neglects the interference that gives
rise to a resonance shift42,43. As a consequence, it predicts peak posi-
ally, as can be seen in Fig. 5.
Surface plasmon antenna
Figure 4 | Ultrafastminiaturephotodetector. Thisdeviceconsistsofasmall
Si photoelectric element and a SP antenna (reproduced from ref. 32 with
permission). The incoming light is harvested by the periodic structure
surrounding the central hole, which then transmits it to the underlying
500600 700800 900
Figure 5 | Transmission spectrumof hole arrays. The triangular hole array
was milled in a 225-nm-thick Au film on a glass substrate with an index-
matching liquid on the air side (hole diameter 170nm, period 520nm). The
transmission spectrum is measured at normal incidence using collimated
white light.The insetshowstheimageof theactual array.I/Ioistheabsolute
transmission of the array and g is the same transmission but normalized to
the area occupied by the holes.
500 600 700 800
Intensity (arbitrary units)
Figure 3 | Optical properties of single apertures surrounded by periodic
corrugations. a, Transmission spectrum of a single hole surrounded by
periodic corrugations (left) prepared by focused ion beam (hole diameter
300nm, period 650nm). b, Schematic illustration of redirecting beam by
single-slit aperture surrounded by grooves of different periodicity on the
input (P1) and output (P2) surfaces.
NATUREjVol 445j4 January 2007
Implicit in the resonance conditions defined by equations such as
equation (2) are the symmetry relations of the array. Therefore the
SPs generated in the array will propagate along defined symmetry
axes with their own polarization depending on the (i, j) number of
the mode. This results in a rich polarization behaviour that can be
revealed in particular under focused light illumination79.
We emphasize that both surfaces on either side of the holes can
sustain SP modes offset from each other by the difference in edof the
material in immediate contact with the metal surface (typically glass
and air), as predicted by equation (2). Hence, the transmission spec-
trum of asymmetric structures contains two sets of peaks, each set
belonging to one of the surfaces. In many applications, hole arrays of
a finite size are used for practical reasons. If the arrays contain small
numbers of holes then the periodicity is not well defined and the
contribution from the edges becomes significant, changing the spec-
trum and leading to unusual re-emission patterns33.
One interesting feature of hole arrays is the fact that each hole
on the output surface acts like a new point source for the light.
Therefore, if a plane wave (that is, a collimated beam) impinges on
interference as the light travels away from the output surface.
Naturally, because the array is also a grating, the transmission gives
rise to different diffraction orders depending on the wavelength to
period ratio. For the longest-wavelength (1,0) peak shown in Fig. 5,
only the 0th order diffraction is formed, because l.P. When l,P,
The shape and dimensions of the holes in an array do influence its
ive apertures, switching from circular to rectangular holes changes
the spectrum as a result of the simultaneous change in both the
wavelength (the wavelength above which the aperture no longer
allows light propagation; see Box 1). Nevertheless, the spectrum is
dominated by the SP modes because of the periodicity of the array65.
If the transmission peak falls below the cutoff, its intensity drops
exponentially withthe depth ofthe hole and hence the film thickness
(or hole depth) is a critical parameter in these structures33. It should
be noted that arrays of slits have more complex transmission spectra
than do hole arrays because the slits can be made propagative under
the appropriate polarization (Box 1). As a consequence, the trans-
mission spectra typically contain the signature of both cavity modes
in the slits (often at wavelengths that equal twice the slit depth
divided by an integer) and SP modes, owing to the slit peri-
Hole arrays have many applications, from optical elements to
sensors for chemistry and biology. For instance, the array acts like
a tunable filter because the wavelength selectivity of the array trans-
mission can be adjusted simply by changing the period, as predicted
by equation (2) and illustrated in Fig. 6. The letters ‘hn’ are obtained
by fabricating a periodic dimple array in which some of the dimples
are milled through to form holes, which in turn reveal the spectral
signature of the array.
The combination of the large electromagnetic fields generated by
contact with the surface (equation (2)) and the simplicity of the
arrays have spurred efforts to use them to detect molecules and to
enhance spectroscopic signals (fluorescence, Raman and so on)80–89.
In this perspective, the enhanced infrared molecular vibrational
spectroscopy exemplifies well the usefulness of the hole arrays for
chemistry and biology80. Arrays of square holes in a Ni film with a
periodicity tuned to the infrared were prepared and modified with
aldehyde on the surface. When such an array with a single adsorbed
molecular layer on surface is placed in a Fourier-transform infrared
apparatus, the infrared vibrational absorption spectrum (Fig. 7) that
is extracted is at least 100 times stronger than if the apparatus had
signal enhancement is due to the fact that when the light is trapped
momentarily on the surface in terms of SPs, its interaction time with
the molecules increases and therefore so does the probability of the
absorption. Note that absorption enhancement of electronic transi-
enhancement factor is then only a factor of ten, owing to the shorter
SP lifetime on the surface at optical wavelengths90. Needless to say,
such results are extremely promising for studying molecular mono-
layers and surface reactions by static or time-resolved spectroscopy
is relatively easy to implement.
Other considerations for optimizing apertures
So far we have discussed mainly the broad features associated with
being related to the presence of SPs, they are in turn very much
dependent on geometric factors and the properties of the metal.
Figure 6 | Holes in a dimple array generating the letters ‘hn’ in
transmission. An arrayof dimplesis preparedbyfocused-ion-beam milling
an Ag film. Some of the dimples are milled through to the other side so that
the transmitted colour is determined by the period of the array. In this case
and green colours.
10 µ µm
Figure 7 | Infrared enhanced vibrational spectra. Vibrational absorption
spectrum of formaldehyde (CH2O) monolayer adsorbed on a Ni hole array
covered with Cu oxide (adapted from ref. 81 with permission). Note that
absorbance of 0.8 implies that 84% of the incident light is absorbed by the
molecularmonolayer. The Ni hole array here acts like the antenna,trapping
the light momentarily on the surfaceand therefore increasingthe likelihood
of absorption by the CH2O.
NATUREjVol 445j4 January 2007
The choice of the metal depends on the wavelength to be used
because the dielectric constants of metals are wavelength dependent.
Ideally, the dielectric constant of the metal should have a high abso-
the absorption into the metal. Thiscombination gives rise to high SP
fields at the surface and minimizes the losses. Therefore Ag is ideally
while above 600nm Au is even better because it suffers little oxida-
tion. In the infrared, metals such as Ni or Cu can also be used.
Interestingly, high transmittivity through structures similar to
those described above but scaled to the microwave region of the
spectrum have also been reported where SPs are not considered to
exist91–95. This is explained by the fact that when a metal surface is
corrugated, it is the effective dielectric constant that is modified
rather than the bulk metal37(this is similar to the way the wetting
properties of a material are changed by nanostructuring). As a con-
formed, which enhances the transmittivity94,95. More generally, it is
essential to trap the electromagnetic wave in the vicinity of the aper-
ture to observe enhanced transmission and its related phenomena.
itons can also be used. There has been some discussion on whether
a recent analysis has confirmed the key role of SPs96,97, in agreement
with the vast majority of studies.
Thegeometrical factors that influence the optical properties of the
holes are numerous: symmetry of the structure, the aspect ratios and
shape of the holes, aperture area, profile of the corrugations and so
forth. These variables determine the electromagnetic field distri-
and their scattering efficiencies, and, the in-plane and out-of-plane
coupling to light. The depth of the holes (or thickness of the metal
film) is important for several reasons. If the films are too thin, they
are partially transparent to the incident light and no holes are neces-
sary to achieve significant transmission, especially if the surfaces are
in addition resonantly corrugated98,99. To obtain a large contrast
between the aperture brightness and the surrounding metal, the
metal must be opaque (optically thick), which implies that the film
depth is the penetration depth in the metal at which incoming light
of 20nm for noble metals in the visible spectrum, so film thicknesses
such thick films, the surface modes on either side of a metal film can
also interact via the holes, split and give rise to modes with new
energies. Such an effect is especially visible in hole arrays and dis-
appears as the film thickness increases and the modes on either side
There are many ways to fabricate aperture structures, depending
on the scale of interest. For the optical regime, the techniques of
choice are: focused ion beam, electron beam lithography and photo-
lithography. The latter two involve several steps but are particularly
useful for large-scale structures. The focused ion beam technique, in
which the sample is milled by focused ion bombardment, is ideally
suited for texturing the metal surface, for instance when preparing
grooves around an aperture. Finally, care is needed in preparing the
metal films because their quality is an important parameter in the
optical properties of the structure.
Surface-wave-activated holes in metal films are finding applications
well beyond the illustrations given in the above paragraphs. In the
field of opto-electronics for instance, studies are being carried out to
extract more light from light-emitting devices100. The metal electro-
des of such devices, which are normally a source of loss, can be
structured with holes to help extract the light from the diode. The
need for ever-smaller features on electronic chips is pushing photo-
lithography touse shorter wavelengths, withtheassociated increased
costs and complications. The use of extraordinary optical transmis-
sion could perhaps circumvent this problem by using SP-activated
lithography masks, which allow subwavelength features in the near-
field and high throughput101–103.
of application, whether for the realization of devices or for the spec-
activated holes,their small sizes andtheir simplicity make themideal
candidates for integration on biochips as sensing elements. As in all
SP-enhanced phenomena, both the input and output optical fields
potentially focus the signal towards a detector. For the purpose of
making SP-active devices, the transmission of hole arrays can be
switched by controlling the refractive index of molecular materials
either electrically104or optically up to terahertz speeds105.
Finally, subwavelength holes might find use inquantum and atom
optics. For instance, hole arrays are promising tools in the study of
the physical nature—quantum versus classical—of SPs as collective
excitations when implemented in quantum entanglement experi-
ments106,107. It has been shown theoretically that the extraordinary
transmission phenomenon can also be expected for matter waves
involving ultracold atoms108such as those used in Bose–Einstein
condensates. This presents opportunities to create optical elements
to manipulate atoms and control their direction.
The potential of the optics of tiny holes in metal screens lies in the
contrast between the strong opacity of the metal and the aperture,
combined with the fact that the metal allows for high local field
enhancements. In addition, the properties of these apertures can be
tailored by structuring the metal with modern nanofabrication tech-
niques. The simplicity of the structures and their ease of use should
further expand their application in a variety of areas and lead to
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Acknowledgements Our research was supported by the European Community,
Network of Excellence PLASMO-NANO-DEVICES, STREP SPP, the ANR grant
COEXUS, the CNRS, and the French Ministry of Higher Education and Research.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
Correspondence should be addressed to T.W.E. (firstname.lastname@example.org).
NATUREjVol 445j4 January 2007