15
C U R I O S I T Y - D R I V E N
R E S E A R C H @ H K U S
It is commonly believed that light carries momentum and as
such, light always pushes objects away. “Tractor beams” of
light that pull objects toward them only exist in movies and
science fiction. However, in 2011, Prof Chan and his collaborators
solved the problem of making light attract an object, finally bring-
ing the futuristic possibilities envisioned in the popular 1960s
television series
Star Trek
closer to reality. The “tractor beam”
breakthrough was published as a cover story in
Nature Photonics
journal and drew substantial media interest. Prof Chan and his
student showed subsequently in an article in
Nature Communica-
tions
how light could push an object sideways. The tractor beam
concept is different from the so-called “optical tweezers” commonly
used to manipulate small particles. Optical tweezers, relatively
easy to realize with today’s technology, act to grab a particle while
the tractor beam pulls a particle, offering the possibility of mani-
pulation with zero contamination. The tractor beam is very selec-
tive in the properties of the particles it pulls, so it can be useful for
the optical sorting of micro-particles in an inexpensive device.
Tractor Beam
Physics textbooks teach that a vacuum has a refractive
index of one. Since a vacuum is already “empty space”,
does it mean its refractive index cannot be less than
one? The amazing versatility of man-made materials
means that some artificial materials can, in principle,
have refractive indices of less than one. Prof Chan’s
team recently designed and made photonic crystals
with a refractive index approaching zero, which is opti-
cally emptier than a vacuum. Such zero indexmaterials
have unusual and interesting properties, including an
infinite phase velocity. In addition, light travels inside
such materials without any phase change, as if space
does not exist. Interestingly, Prof Chan discovered a
hidden mathematical relationship between the “Dirac
cone physics” in graphene (a new electronic material)
and a metamaterial with a zero refractive index.
Top left: Some photonic
crystals can be specifically
designed so that they have a
conical frequency-momentum
relationship called a “Dirac
cone”. Theory shows that
these photonic crystals behave
as if the effective refractive
index is zero at some specific
frequency.
Top right: A “photonic crystal”
composed of an ordered array
of dielectric (e.g. Si) cylinders.
Bottom: A prism of “zero
index” photonic crystals, made
experimentally in Prof WY Tam’s
laboratory as a realization of
the blueprint shown in the top
right diagram.
Nature Materials, 10, 582 (2011)
Zero Refractive Index
Left: In some special optical beams, the projection of the photon momentum
on the beam axis is less than the total momentum. Some particles can have
multiple excitations that can interfere to form a forward directed beam. Under
such circumstances, the scattered light can have more momentum on the
beam axis than the incoming light, allowing for light to “pull” without violating
the conservation of momentum.
Far left: A schematic picture showing a beam of light pulling a spherical
particle toward the light source.
Nature Photonics 5, 531 (2011)
The Starship Enterprise using its tractor beam to pull an object without
touching it in an episode of
Star Trek: The Next Generation.
Bessel beam
K x
K y
K in
K ou
t
