Enhancement of
evanescent fields and near-field sub-wavelength imaging in simple
passive structures
It is known that a slab of backward-wave
medium (material with negative permittivity and permeability) can
work as a "perfect lens", amplifying the evanescent part of the
incident field. This phenomenon suggests many important applications in
(nano-)imaging and detection. However, it is very difficult to
practically realize such materials.
In this project (funded by the Finnish
Academy and TEKES) we studied alternative possibilities to realize such
devices.
First, it is important to understand that a perfect lens makes two
very different operations. It focuses PROPAGATING waves, as a usual
lens, and it enhances (amplifies) EVANESCENT waves. The last operation
is very unique for this device. No other passive optical device can
amplify evanescent field components: Evanescent fields exponentially
decay in space and are lost forever. Amplification of evanescent fields
is a very important function, while propagating modes can be focused by
a simple lens.
We have shown that a system of two coupled
planar sheets possessing surface mode (polariton) resonances can be
used for the purpose of evanescent field restoration and, thus, for the
sub-wavelength near-field imaging. The sheets are placed in free space
so that they are parallel and separated by a certain distance. Due to
interaction of the resonating surface modes (polaritons) of the sheets,
exponential growth in the amplitude of an evanescent plane wave
coming through the system can be achieved.
This photo shows our simple expermental set-up. Evanescent fields are
excited by a small dipole source (shown above the device) inside a
below-cut-off plane waveguide. These fields are amplified by a system
of two
small resonant particles positioned on two foam holders (light blue).
The field distribution is measured by a small probe scanning over the
dense metal mesh.
Measured field distribution. The field in the image plane is as strong
as the field of the original source.
For details, see S. Maslovski,
S. Tretyakov, P. Alitalo, Near-field enhancement and imaging in double
planar polariton-resonant structures, J.
Applied Physics, vol. 96, no. 3, pp. 1293-1300, 2004.
A preprint is available here.
Furthemore, this principle can be extended to superlenses that not only
enhance evanescent fields and restore their distribution in the image
plane, but also ENLARGE the image. This allows us to link a superlens
that senses and amplifies evanescent fields with the usual
propagating-wave optics, because after enlarging the fields can be
coupled to propagating modes of free space. This principle was
demonstrated in P.
Alitalo, S. Maslovski,
S. Tretyakov, Near-field enhancement and imaging in
double cylindrical polariton-resonant structures: Enlarging superlens, Phys. Lett. A, vol. 357, no. 4-5,
pp. 397-400, 2006.
The picture above shows an experimental set-up with two small antennas
(acting as two sub-wavelength objects to be imaged) inside a
cylindrical structure with two grdis of small resonant particles. The
measured electric field distibution shows amplificiation of evanescent
fields and enlarging of the image.
The concept of superlenses based on resonant particles can be extended
into visible frequency range if metal nanoparticles are used as
resonant inclusions, see P.
Alitalo, C. Simovski, A. Viitanen, and S.
Tretyakov, Near-field enhancement and subwavelength imaging in the
optical
region using a pair of two-dimensional arrays of metal nanospheres, Physical Review B, vol. 74, p. 235425,
2006.
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