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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Solar Power Without Solar Cells: A Hidden Magnetic Effect of Light Could Make It Possible

Solar Power Without Solar Cells: A Hidden Magnetic Effect of Light Could Make It Possible

  9:05:00 AM    Science Lover

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A dramatic and surprising magnetic effect of light discovered by University of Michigan researchers could lead to solar power without traditional semiconductor-based solar cells.

Researchers have discovered a dramatic and surprising magnetic effect of light that could lead to solar power without traditional semiconductor-based solar cells. (Credit: © Sean Gladwell / Fotolia)

The researchers found a way to make an "optical battery," said Stephen Rand, a professor in the departments of Electrical Engineering and Computer Science, Physics and Applied Physics.

In the process, they overturned a century-old tenet of physics.

"You could stare at the equations of motion all day and you will not see this possibility. We've all been taught that this doesn't happen," said Rand, an author of a paper on the work published in the Journal of Applied Physics. "It's a very odd interaction. That's why it's been overlooked for more than 100 years."

Light has electric and magnetic components. Until now, scientists thought the effects of the magnetic field were so weak that they could be ignored. What Rand and his colleagues found is that at the right intensity, when light is traveling through a material that does not conduct electricity, the light field can generate magnetic effects that are 100 million times stronger than previously expected. Under these circumstances, the magnetic effects develop strength equivalent to a strong electric effect.

"This could lead to a new kind of solar cell without semiconductors and without absorption to produce charge separation," Rand said. "In solar cells, the light goes into a material, gets absorbed and creates heat. Here, we expect to have a very low heat load. Instead of the light being absorbed, energy is stored in the magnetic moment. Intense magnetization can be induced by intense light and then it is ultimately capable of providing a capacitive power source."

What makes this possible is a previously undetected brand of "optical rectification," says William Fisher, a doctoral student in applied physics. In traditional optical rectification, light's electric field causes a charge separation, or a pulling apart of the positive and negative charges in a material. This sets up a voltage, similar to that in a battery. This electric effect had previously been detected only in crystalline materials that possessed a certain symmetry.

Rand and Fisher found that under the right circumstances and in other types of materials, the light's magnetic field can also create optical rectification.

"It turns out that the magnetic field starts curving the electrons into a C-shape and they move forward a little each time," Fisher said. "That C-shape of charge motion generates both an electric dipole and a magnetic dipole. If we can set up many of these in a row in a long fiber, we can make a huge voltage and by extracting that voltage, we can use it as a power source."

The light must be shone through a material that does not conduct electricity, such as glass. And it must be focused to an intensity of 10 million watts per square centimeter. Sunlight isn't this intense on its own, but new materials are being sought that would work at lower intensities, Fisher said.

"In our most recent paper, we show that incoherent light like sunlight is theoretically almost as effective in producing charge separation as laser light is," Fisher said.

This new technique could make solar power cheaper, the researchers say. They predict that with improved materials they could achieve 10 percent efficiency in converting solar power to useable energy. That's equivalent to today's commercial-grade solar cells.

"To manufacture modern solar cells, you have to do extensive semiconductor processing," Fisher said. "All we would need are lenses to focus the light and a fiber to guide it. Glass works for both. It's already made in bulk, and it doesn't require as much processing. Transparent ceramics might be even better."

In experiments this summer, the researchers will work on harnessing this power with laser light, and then with sunlight.

The paper is titled "Optically-induced charge separation and terahertz emission in unbiased dielectrics." The university is pursuing patent protection for the intellectual property.

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