From a Flat Mirror, Designer Light: Bizarre Optical Phenomena Defies Laws of Reflection and Refraction

From a Flat Mirror, Designer Light: Bizarre Optical Phenomena Defies Laws of Reflection and Refraction

  4:34:00 PM    Science Lover

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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.

Top, clockwise from left: Patrice Genevet, Nanfang Yu, Federico Capasso, Zeno Gaburro, and Mikhail A. Kats. Bottom: A simulation of the image that would appear in a large mirror patterned with the team's new phase mirror technology. (Credit: Photos by Eliza Grinnell and Nanfang Yu)

The discovery, published this week in 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," says 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 wavefront 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," explains 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," says 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.

Yu, Capasso, and Gaburro's co-authors included Patrice Genevet, Mikhail A. Kats, Francesco Aieta, and Jean-Philippe Tetienne.

Story Source: The above story is reprinted (with editorial adaptations) from materials provided by Harvard University, via EurekAlert!, a service of AAAS.

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Novel Negative-Index Metamaterial Bends Light 'Wrong' Direction

Novel Negative-Index Metamaterial Bends Light 'Wrong' Direction

  7:48:00 PM    Science Lover

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A group of scientists led by researchers from the California Institute of Technology (Caltech) has engineered a type of artificial optical material -- a metamaterial -- with a particular three-dimensional structure such that light exhibits a negative index of refraction upon entering the material. In other words, this material bends light in the "wrong" direction from what normally would be expected, irrespective of the angle of the approaching light.

Arrays of coupled plasmonic coaxial waveguides offer 
a new approach by which to realize negative-index 
metamaterials that are remarkably insensitive to angle 
of incidence and polarization in the visible range. 
(Credit: Caltech/Stanley Burgos)

This new type of negative-index metamaterial (NIM), described in an advance online publication in the journal Nature Materials, is simpler than previous NIMs -- requiring only a single functional layer -- and yet more versatile, in that it can handle light with any polarization over a broad range of incident angles. And it can do all of this in the blue part of the visible spectrum, making it "the first negative index metamaterial to operate at visible frequencies," says graduate student Stanley Burgos, a researcher at the Light-Material Interactions in Energy Conversion Energy Frontier Research Center at Caltech and the paper's first author.

"By engineering a metamaterial with such properties, we are opening the door to such unusual -- but potentially useful -- phenomena as superlensing (high-resolution imaging past the diffraction limit), invisibility cloaking, and the synthesis of materials index-matched to air, for potential enhancement of light collection in solar cells," says Harry Atwater, Howard Hughes Professor and professor of applied physics and materials science, director of Caltech's Resnick Institute, founding member of the Kavli Nanoscience Institute, and leader of the research team

What makes this NIM unique, says Burgos, is its engineering. "The source of the negative-index response is fundamentally different from that of previous NIM designs," he explains. Those previous efforts used multiple layers of "resonant elements" to refract the light in this unusual way, while this version is composed of a single layer of silver permeated with "coupled plasmonic waveguide elements."

Surface plasmons are light waves coupled to waves of electrons at the interface between a metal and a dielectric (a non-conducting material like air). Plasmonic waveguide elements route these coupled waves through the material. Not only is this material more feasible to fabricate than those previously used, Burgos says, it also allows for simple "tuning" of the negative-index response; by changing the materials used, or the geometry of the waveguide, the NIM can be tuned to respond to a different wavelength of light coming from nearly any angle with any polarization. "By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with a nearly isotopic refractive index tuned to operate at visible frequencies."

This sort of functional flexibility is critical if the material is to be used in a wide variety of ways, says Atwater. "For practical applications, it is very important for a material's response to be insensitive to both incidence angle and polarization," he says. "Take eyeglasses, for example. In order for them to properly focus light reflected off an object on the back of your eye, they must be able to accept and focus light coming from a broad range of angles, independent of polarization. Said another way, their response must be nearly isotropic. Our metamaterial has the same capabilities in terms of its response to incident light."

This means the new metamaterial is particularly well suited to use in solar cells, Atwater adds. "The fact that our NIM design is tunable means we could potentially tune its index response to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells," he explains. "And the fact that the metamaterial has a wide-angle response is important because it means that it can 'accept' light from a broad range of angles. In the case of solar cells, this means more light collection and less reflected or 'wasted' light."

"This work stands out because, through careful engineering, greater simplicity has been achieved," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

Their work was supported by the Energy Frontier Research Centers program of the Office of Science of the Department of Energy, the National Science Foundation, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek, and "NanoNed," a nanotechnology program funded by the Dutch Ministry of Economic Affairs.

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