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Left and far left: Light-scattering patterns of a cup
and a spoon. The scattering patterns are different,
and the different patterns allow our eyes to detect
different objects.
Center: By using a negative index metamaterial,
the scattering pattern of a spoon can be changed
to that of the cup. An observer in the far field will
see a cup, although the object is actually a spoon.
Physical Review Letters, 102, 253902 (2009)
Platform 9¾ at King’s Cross Station where students travel to Hogwarts in the world
of Harry Potter appears to the eye to be a wall. In the fictional story, it is actually an
open channel that the body can pass through. Using the novel notion of “illusion
optics”, HKUST researchers have demonstrated for the first time that such a phenomenon
is possible and, in principle, an apple can be made to look like an orange, or a spoon like
a cup. This is achieved by changing the light-scattering pattern of an object to that of
another object, making it appear like the second object, with the assistance of “negative
index” metamaterials specially designed at HKUST. The illusion device requires very
complex man-made materials. Prof Chan’s team is now working to realize such novel
effects with less complex materials.
Illusion Optics
Cloaking Device
In science fiction, such as HGWells’
The Invisible Man
, a person
could become invisible by drinking a chemical. In reality, no
such substance has been made. How about wrapping a material
around an object to make it invisible to the human eye? Yes,
theoretically, now it could be done, thanks to advanced
physics techniques that change the path of light, not the
object. In 2006, UK scientists designedman-made materials
(“metamaterials”) that could provide invisibility by
guiding light around an object, and the concept was
soon proved experimentally in the US. Instead of
creating a cloak to hide an object embedded inside,
Prof Chan’s group has designed metamaterials with opti-
cal properties that are the exact opposite of the material to
be cloaked so that light scattered by an object is compensated
by the cloak. An observer thus sees neither the object nor the
cloak. This method of invisibility has the advantage that the
hidden objects can still see the outside world. In conven-
tional cloaks, the objects are blinded because no light
enters the cloaked region. Such “remote cloaking”
currently works for a narrowband of frequencies.
Whether such cloaking strategies can
work for a broad range of frequencies
remains an open question.
Light coming from the
left is scattered by an
object, therefore we
see the object.
Dielectric
ɛ
= 2
Remote
cloaking
device
“Anti-object”
A “remote cloaking”
device (the circle) is
put next to the object.
The device is designed
to cancel the light
scattering by the object
so that the light wave
passes through as if
nothing is there.
Both the cloaking
device and the object
become invisible to
outside observers. The
principle behind is to
use a metamaterial
“anti-object” to cancel
the light scattering by
the object you want
to hide.
Physical Review
Letters, 093901 (2009)
A metallic cup
A dielectric spoon
Illusion device
