Recently, a lot of effort has been devoted to creating materials or
devices that can effectively cloak objects (e.g. dielectric or metallic
cylinders or spheres) from the electromagnetic fields that exist in the
medium that surrounds the object. See e.g. recent works by the groups
of profs. John Pendry, David Smith, Nader Engheta, and Vladimir
Shalaev. Cloak hides an
arbitrary object (of a fitting size) from electromagnetic fields,
meaning that removing the object with the cloak does not change
electromagnetic fields outside the cloak volume. In other words,
ideally cloaked objects do not scatter in any direction (do not create
reflections, do not create shadows, and do not scatter in other
directions). Technically speaking, this means that their total
scattering cross section is zero for any direction of illumination and
any polarization of the incident field. There is also a notion of
invisible objects. An invisible
object also does not scatter, but with that technology one cannot hide
an arbitrary object inside.
Our recent overview of this field, inlcuding some historical aspects, can be found here. This lecture (as well as the lecture on the transmission-line cloak developed in our group, see below) was given at the European Doctoral School on Metamaterials (Rome, Italy, October 25, 2007), and, with some updates, at the European School of Antennas (Göteborg, Sweden, November 1, 2007). See also the review paper P. Alitalo and S. Tretyakov, Electromagnetic cloaking with metamaterials, Materials Today, vol. 12, no. 3, pp. 22-29, March 2009.
In this project we study an alternative method of cloaking in
the
microwave region. The method is based on using transmission-line
networks that mimic the wave propagation in the surrounding medium,
thus being left "undetected" by an incident electromagnetic wave. The
evident drawback is that the objects that can be cloaked with this
method have to be smaller than the period of the network, but this can
be e.g. a two-dimensional or three-dimensional interconnected mesh of
small objects, since the fields are confined in the transmission lines.
Also, this cloak operates only for one polarization if the antennas
work only for one polarization. See the figure below for the
illustration of the idea.
See a preprint of our recent paper on this subject here (published paper: P. Alitalo, O. Luukkonen, L. Jylhä, J. Venermo, and S.A. Tretyakov, Transmission-line networks cloaking objects from electromagnetic fields, IEEE Trans. Antennas Propagation, vol. 56, no. 2, pp. 416-424, 2008). In this paper we
consider loaded and unloaded transmission-line networks and the
possibility of using such networks in cloaking applications. If the
cloak (and the object that needs to be cloaked) are placed in free
space, ideal wave propagation in an unloaded network is impossible to
achieve with conventional materials. By loading the network with e.g.
capacitive loads, the correct propagation constant can be achieved in a
very narrow frequency band. Since the most currently available cloaking
methods are limited to cloaking of time-harmonic fields only when
placed in free space (a signal that is re-routed around the cloaked
object would have to travel faster than light), the most promising
feature of the cloak proposed here is the fact that for the unloaded
network, the group velicity is equal to the phase velocity. Although
they are both slightly different from those in free space, effective
cloaking can be obtained also from signals. In the figure below, we
present FDTD simulations of a reference object (left) and a cloak which
can encompass the reference object (right). The cloak diameter is 128
mm and the center frequency of the incoming pulse is 1 GHz. In the
figures the pulse has just passed the reference object and the cloak.
Another way to realize broadband cloaks is to use conically shaped metal sheets, as is explained in this paper: S. Tretyakov, P. Alitalo, O. Luukkonen, and C. Simovski, Broadband electromagnetic cloaking of long cylindrical objects, Physical Review Letters, vol. 103, p. 103905, 2009 (see also popular article).
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