Crystal bends light back
Technology Research News
When light bends as it passes from one
material to another, scientists assume two things will happen: some of
the light will be reflected at the junction between the materials and
the light that goes through will only bend at a positive angle.
Researchers at the National Renewable Energy Laboratory have shown
that this is not always the case. They demonstrated that a junction in
yttrium orthovanadate -- a cheap, readily available crystal -- transmits
light without reflecting any of it, and is capable of guiding light and
beams of electrons through a wide range of angles, both positive and negative.
In principle, the crystal can guide all types of electromagnetic
radiation, including microwaves and electron beams. "Our method is applicable
to any frequency of light, even electrons," said Yong Zhang, a senior
scientist at the National Renewable Energy Laboratory.
The material is potentially valuable for channeling light in high-power
lasers and channeling electron beams in nanoscale electronic devices.
Refraction, or the bending of light, creates the illusion that
a drinking straw bends where air and water meet. Imagine a plane perpendicular
to the surface of the water that intersects the surface at the point where
the straw, resting at an angle, enters the water. If there were no water,
the straw would appear to pass through the imaginary plane in a straight
line. With water in the glass the straw still passes through the imaginary
plane even though it appears to bend.
In a junction with a negative index of refraction, however, light
beams bend so sharply that they do not pass through the imaginary plane,
but instead appear as if they are bouncing off of an invisible mirror.
To date, only a few artificial metamaterials, for example small
copper loops embedded in fiberglass boards, have shown a negative index
of refraction, and then only for microwaves. Unlike the researchers crystal
structure, these materials do not also show positive refraction.
Materials with a negative index of refraction are the subject
of intense research because they have the potential to form super lenses
that focus light more narrowly than ordinary lenses. Super lenses could
be used in chip manufacturing to enable existing equipment to make smaller
What sets the researchers's type of crystal apart is that it has
a twinning structure, which means that the orientation of the crystal's
atoms on one side of the junction is the mirror image of the atomic structure
on the other side. The researchers fired a laser beam through a crystal
to demonstrate that the junction will refract light at both positive and
negative angles, depending on the angle at which the light hits the junction.
The twinning structure exists in natural crystals, and can also
be made artificially. "We use real crystals, which are readily available,"
said Zhang. The junctions can also be made using metamaterials constructed
from rods or split rings of material.
One reason materials with negative indexes of refraction have
eluded researchers is that scientists had thought that such materials
could only exist if permittivity and/or permeability were negative for
the material on one side of the junction. Permittivity is the ratio of
the strength of an external electric field to the flow of electricity
in a material. Permeability is the ratio of the strength of an external
magnetic field to the magnetic force penetrating a material. The combination
determines the speed at which electromagnetic energy like light passes
through a material.
Neither permittivity or permeability are negative in naturally
formed materials, which is why research into negative refraction has focused
on developing artificial metamaterials. The permittivity and permeability
of the twinning crystal structure are positive.
In most cases, negative permittivity or permeability is required,
but not in all cases, as it turns out. "There are an infinite number of
ways to orient the symmetry axes [of crystals]," said Zhang. "People simply
couldn't search them all. Our goal was not to search for negative refraction,
but to study the transmission of electrons through the twin boundary in
semiconductors," he said.
The twin crystal structure could be used to channel light now,
according to Zhang. It could take 10 years to develop methods of controlling
electron beams, he said.
Zhang's research colleagues were Brian Fluegel and Angelo Mascarenhas.
The work appeared in the October 10, 2003 issue of Physical Review Letters.
The research was funded by the Department of Energy (DOE).
Timeline: Now; 10 years
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Materials Science and Engineering; Physics
Story Type: News
Related Elements: Technical paper, "Total Negative Refraction
in Real Crystals for Ballistic Electronics and Light," Physical Review
Letters, October 10, 2003
November 5/12, 2003
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