Sunday, January 8, 2012
Sunday, January 1, 2012
Miracle Antennas for Optical Innovations
Researchers have shown how arrays of tiny plasmonic nanoantennas are
able to precisely manipulate light in new ways that could make possible a range
of optical innovations such as more powerful microscopes, telecommunications
and computers.
This ultrathin layer of plasmonic nanoantennas makes the phase of light
change strongly and abruptly, causing light to change its propagation
direction, as required by the momentum conservation for light passing through the
interface between materials.
The researchers at Purdue University used the nanoantennas
to abruptly change the phase of light phase. Light is transmitted as waves
analogous to waves of water, which have high and low points. The phase defines
these high and low points of light.
By abruptly changing the phase light propagates opens up the possibility
of many potential applications. , Harvard researchers modified Snell's law, a
long-held formula used to describe how light reflects and refracts, or bends,
while passing from one material into another.
Until now, Snell's law has implied that when light passes from one
material to another there are no abrupt phase changes along the interface between
the materials. Harvard researchers, however, conducted experiments showing that
the phase of light and the propagation direction can be changed dramatically by
using new types of structures called metamaterials, which in this case were
based on an array of antennas.
The wavelength size manipulated by the antennas in the Purdue experiment
ranges from 1 to 1.9 microns.
The near infrared, specifically a wavelength of 1.5 microns, is
essential for telecommunications. Information is
transmitted across optical fibers using this wavelength, which makes this
innovation potentially practical for advances in telecommunications.
The Harvard researchers predicted how to modify Snell's law and
demonstrated the principle at one wavelength.
The innovation could bring technologies for steering and shaping laser
beams for military and communications applications, nanocircuits for computers
that use light to process information, and new types of powerful lenses for
microscopes.
Critical to the advance is the ability to alter light so that it
exhibits anomalous behaviour: notably, it bends in ways not possible using conventional
materials by radically altering its refraction, a process that occurs as
electromagnetic waves, including light, bend when passing from one material
into another.
Scientists measure this bending of radiation by its index of refraction.
Refraction causes the bent-stick-in-water effect, which occurs when a stick
placed in a glass of water appears bent when viewed from the outside. Each
material has its own refraction index, which describes how much light will bend
in that particular material. All natural materials, such as glass, air and
water, have positive refractive indices. However, the nanoantenna arrays can
cause light to bend in a wide range of angles including negative angles of
refraction.
Importantly, such dramatic deviation from the conventional Snell's law
governing reflection and refraction occurs when light passes through structures
that are actually much thinner than the width of the light's wavelengths, which
is not possible using natural materials. Also, not only the bending effect, refraction,
but also the reflection of light can be dramatically modified by the antenna
arrays on the interface, as the experiments showed.
The nanoantennas are V-shaped structures made of gold and formed on top
of a silicon layer. They are an example of metamaterials, which typically
include so-called plasmonic structures that conduct clouds of electrons called
plasmons. The antennas themselves have a width of 40 nanometers, or billionths
of a meter, and researchers have demonstrated they are able to transmit light
through an ultrathin plasmonic nanoantenna layer about 50 times smaller than
the wavelength of light it is transmitting.
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