Exploiting a novel technique called phase discontinuity, researchers at
the Harvard School of Engineering and Applied Sciences (SEAS) have
induced light rays to behave in a way that defies the centuries-old
laws of reflection and refraction.
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| A simulation of the image that would appear in a large mirror patterned with the team's new phase mirror technology. |
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Nanfang Yu, Zeno Gaburro, Federico Capasso, and
colleagues at SEAS have created strange optical effects, including
corkscrew-like vortex beams, by reflecting light off a flat, nanostructured
surface. Image courtesy of Nanfang Yu/SEAS |
The discovery, published Sept. 2 in the journal Science, has led to a
reformulation of the mathematical laws that predict the path of a ray
of light bouncing off a surface or traveling from one medium into
another — for example, from air into glass.
“Using designer surfaces, we’ve created the effects of a
fun-house mirror on a flat plane,” said co-principal investigator
Federico Capasso, Robert L. Wallace Professor of Applied Physics and
Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS.
“Our discovery carries optics into new territory and opens the
door to exciting developments in photonics technology.”
It has been recognized since ancient times that light travels at
different speeds through different media. Reflection and refraction
occur whenever light encounters a material at an angle, because one
side of the beam is able to race ahead of the other. As a result, the
wave front changes direction.
The conventional laws, taught in physics classrooms worldwide, predict
the angles of reflection and refraction based only on the incident
(incoming) angle and the properties of the two media.
While studying the behavior of light impinging on surfaces patterned
with metallic nanostructures, the researchers realized that the usual
equations were insufficient to describe the bizarre phenomena observed
in the lab.
The new generalized laws, derived and experimentally demonstrated at
Harvard, take into account the Capasso group’s discovery that the
boundary between two media, if specially patterned, can itself behave
like a third medium.
“Ordinarily, a surface like the surface of a pond is simply a
geometric boundary between two media, air and water,” said lead
author Nanfang Yu, Ph.D. ’09, a research associate in
Capasso’s lab at SEAS. “But now, in this special case, the
boundary becomes an active interface that can bend the light by
itself.”
The key component is an array of tiny gold antennas etched into the
surface of the silicon used in Capasso’s lab. The array is
structured on a scale much thinner than the wavelength of the light
hitting it. This means that, unlike in a conventional optical system,
the engineered boundary between the air and the silicon imparts an
abrupt phase shift — dubbed “phase discontinuity”
— to the crests of the light wave crossing it.
Each antenna in the array is a tiny resonator that can trap the light,
holding its energy for a given amount of time before releasing it. A
gradient of different types of nanoscale resonators across the surface
of the silicon can effectively bend the light before it even begins to
propagate through the new medium.
The resulting phenomenon breaks the old rules, creating beams of light
that reflect and refract in arbitrary ways, depending on the surface
pattern.
In order to generalize the textbook laws of reflection and refraction,
the Harvard researchers added a new term to the equations, representing
the gradient of phase shifts imparted at the boundary. Importantly, in
the absence of a surface gradient, the new laws reduce to the
well-known ones.
“By incorporating a gradient of phase discontinuities across the
interface, the laws of reflection and refraction become designer laws,
and a panoply of new phenomena appear,” said Zeno Gaburro, a
visiting scholar in Capasso’s group who was co-principal
investigator for this work. “The reflected beam can bounce
backward instead of forward. You can create negative refraction. There
is a new angle of total internal reflection.”
Moreover, the frequency (color), amplitude (brightness), and
polarization of the light can also be controlled, meaning that the
output is in essence a designer beam.
The researchers have already succeeded at producing a vortex beam (a
helical, corkscrew-shaped stream of light) from a flat surface. They
also envision flat lenses that could focus an image without
aberrations.
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