Cloak
By Marco Wong
黃俊銘
This article may be useful as supplementary reading for physics classes, based
on the DSE syllabus.
根據物理科文憑試課程剛要，本文或可作為有用的補充讀物。
Real-life
If
only being invisible was as easy as hiding
under a nifty cloaking device bequeathed from
your father who happened to dabble in wizarding
mischief in his younger days. Unfortunately, us
‘muggles’ are just not blessed with the magical
gene, but the good news is that we can strive to
understand and invent technology inches away
from the supernatural, thanks to a little thing called
science. In fact, invisibility has been researched
for quite some time now, with several inventions
coming incredibly close to fruition.
What we cur rently under stand is that an
object appears to be ‘invisible’ when light is able
to pass through the space that the object in
question occupies, at least in the perspective of
the observer. However, most objects are opaque,
which block light from penetrating through. To
circumvent this, one way would be to bend light
with the aid of ‘smart’ metamaterials, which are
able to marginally alter the path of light.
In 2006, Duke University researchers successfully
hid an object from microwave detection by using
a material that can bend electrons. Composed of
copper and fibre glass, the material has a unique
interaction against the planar electromagnetic
field. The device was specifically tuned to minimise
distortion of microwaves caused by the object,
creating inter ference patterns which cancel
each other out [1]. Fast forwarding six years,
Korean researchers at Yonsei University created
a 10 mm thick elastic cloak with the claim that
any haphazardly shaped object can be hidden
from broadband microwave detection, bringing
the device closer to practical application. These
inventions are the initial step to concealing objects
in the visible light spectrum, as visible light has
a much shorter wavelength in comparison, thus
exhibiting a smaller degree of refraction [2].
To cloak an object under v i s ible l ight,
refraction is still utilised but on a much larger
scale. In 2014, researchers at the University
of Rochester const ructed a device made
of four lenses, the first of which can boast
successful cloaking of small objects in three-
dimensional space. The technology is surprisingly
straight forward - two diverging lenses are
arranged between two converging lenses,
whereby light is concentrated as a thin beam
between the diverging lenses, creating a
doughnut shaped region of invisibility. Sounds
fine and dandy, but the biggest limitation
of this device is that it only works with small
angles [3].
Researcher s wor ldwide are now
more focused on creating solid state
metamaterials for the purpose of cloaking. In
2014, a research team from Karlsruhe Institute
of Technology (KIT) in Germany constructed a
diffusive optical cloaking device using common
materials that hides objects placed within it [4].
The idea behind it is to allow light to enter through
the material and guide it around the object, and
then ultimately re-establishing the original path
of the light before disturbance. Instead of using
refraction, this ‘cloak’ adopts light scattering.
With the use of a medium, the propagation of
light is slowed down until it reaches a cylinder
containing the object to be hidden. Coated with
titanium dioxide doped polydimethylsiloxane, the
material is then able to scatter light and speed
