Abstract-Nonreciprocal Metasurface with Space-Time Phase Modulation

Xuexue Guo, Yimin Ding, Yao Duan, Xingjie Ni

https://arxiv.org/abs/1905.10316

Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations to wave propagation. However, it is challenging to realize efficient and ultrafast temporal modulation in a photonic system. Here, leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface, we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The metasurface, with traveling-wave modulation upon nonlinear Kerr building blocks, creates spatial phase gradient and multi-terahertz temporal phase wobbling, which leads to unidirectional photonic transitions in both momentum and energy spaces. We observed completely asymmetric reflections in forward and backward light propagations within a sub-wavelength interaction length of 150 nm. Our approach pointed out a potential means for creating miniaturized and integratable nonreciprocal optical components.

SpectroscopyNOW-Last Month's Most Accessed Feature: Invisibility cloak: Hiding the microscopic

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http://www.spectroscopynow.com/ir/details/highlight/14de329bf14/Last-Months-Most-Accessed-Feature-Invisibility-cloak-Hiding-the-microscopic.html

Cloaking device

A microscopic invisibility cloak based on brick-like blocks of gold nanoantennae could pave the way to a flexible device for making an object invisible in visible light.

Invisibility cloaks have been a staple of science fiction and fantasy for decade, who, after all, has not mused on what they might get up to if they could make themselves invisible? For several years camouflage covers that resemble the background environment to help hide a person were the best option, leafy greens allowing a soldier to crawl through undergrowth undetected perhaps, except when night-vision or thermal imaging is in place. Aircraft can be made "stealth" to hide them from the reflective waves of a radar tower but they can still be spotted as they fly by at altitude with a decent set of binoculars and a good eye.

In recent years, however, meta materials have emerged that might take invisibility to the next level. There have been demonstrations of infrared and other forms of invisibility but now, researchers at the US Department of Energy's Lawrence Berkeley National Laboratory and the University of California Berkeley have come up with a microscopic invisibility cloak that can hide a three dimensional object from the microscope's viewfinder. The team suggest that the principle should work on the macroscopic level to once it is scaled up.

Concealing to appeal

The team has worked with gold nanoantennae to fabricate a flexible skin a mere 80 nanometres in thickness that can be wrapped around a three-dimensional object of arbitrary shape the size of a clump of biological cells. Adopting the underlying lumps and bumps of the object. The meta-engineered surface of the skin cloak allows it to re-route reflected light waves so impinge on it so that the object's lumps and bumps are rendered invisible to optical detection when the cloak is activated.

"This is the first time a 3D object of arbitrary shape has been cloaked from visible light," explains meta materials expert Xiang Zhang, director of Berkeley Lab's Materials Sciences Division. "Our ultra-thin cloak now looks like a coat. It is easy to design and implement, and is potentially scalable for hiding macroscopic objects," he adds.

Zhang, working with Xingjie Ni, Zi Jing Wong, Michael Mrejen and Yuan Wang, point out that that it is the scattering of electromagnetic radiation, whether that is visible light, infrared, X-ray, or another band in the spectrum, and its interaction with matter that enables us to detect and observe objects. The researchers explains that the rules that govern these interactions in natural materials can be circumvented using meta materials whose optical properties arise from their physical structure rather than their chemical composition. The surface of a butterfly's wing has no coloured pigment, it's surface texture interacts with visible light to cause iridescence that gives rise to its beautiful colours and patterns and so might be thought of as a natural meta material.

For the past ten years, Zhang and his research group have been pushing the boundaries of how light interacts with fabricated meta materials. They have managed to curve the path of light or bend it backwards, creating negative refractive index meta materials, a phenomenon not seen before in nature, and to render objects optically undetectable. In the past, their meta material-based optical carpet cloaks were bulky and hard to scale-up, and entailed a phase difference between the cloaked region and the surrounding background that made the cloak itself detectable, although what it was concealing could not be detected, which defeats the object of being invisible one has to say.

Skin to carpet

"Creating a carpet cloak that works in air was so difficult we had to embed it in a dielectric prism that introduced an additional phase in the reflected light, which made the cloak visible by phase-sensitive detection," explains team member and co-lead author Ni, who has recently moved to Pennsylvania State University. "Recent developments in meta surfaces, however, allow us to manipulate the phase of a propagating wave directly through the use of sub-wavelength sized elements that locally tailor the electromagnetic response at the nanoscale, a response that is accompanied by dramatic light confinement."

In their experiments, the team shone red light struck on a sample object with an area of about 1300 square micrometres. When it was sheathed in the gold nanoantennae skin cloak, light reflected from the object's surface produced the same effect as if the light were simply reflecting from a plane mirror. The 3D object cloaked in this way is thus invisible even by phase-sensitive detection. The team points out that their cloaking device can be turned on or off simply by switching the polarization of the nanoantennae.

"A phase shift provided by each individual nanoantenna fully restores both the wavefront and the phase of the scattered light so that the object remains perfectly hidden," explains Wong. Ironically, this ability to manipulate the interactions of light and a meta material for invisibility hints at a future of high resolution optical microscopes and superfast optical computers and as a component of a future 3D display technology. Conversely, such a device could be used for security through obscurity applications allowing microscopic components to be hidden for privacy or security applications. purposes.

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Science 2015, 349, 1310-1314: "An ultrathin invisibility skin cloak for visible light"

Article by David Bradley

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The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

'Nanoantennas' Show Promise in Optical Innovations

The image in the upper left shows a schematic for an array of gold "plasmonic nanoantennas" able to precisely manipulate light in new ways, a technology that could make possible a range of optical innovations such as more powerful microscopes, telecommunications and computers. At upper right is a scanning electron microscope image of the structures. The figure below shows the experimentally measured refraction angle versus incidence angle for light, demonstrating how the nanoantennas alter the refraction. (Credit: Purdue University Birck Nanotechnology Center image)

http://www.sciencedaily.com/releases/2011/12/111222142459.htm

ScienceDaily (Dec. 22, 2011) — 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.

The researchers at Purdue University used the nanoantennas to abruptly change a property of light called its 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 we can dramatically modify how light propagates, and that opens up the possibility of many potential applications,"said Vladimir Shalaev, scientific director of nanophotonics at Purdue's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering.
Findings are described in a paper to be published online on Dec. 22 in the journal Science.
The new work at Purdue extends findings by researchers led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at the Harvard School of Engineering and Applied Sciences. In that work, described in an October Science paper, 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.
"What they pointed out was revolutionary," Shalaev said.
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 Purdue researchers took the work a step further, creating arrays of nanoantennas and changing the phase and propagation direction of light over a broad range of near-infrared light. The paper was written by doctoral students Xingjie Ni and Naresh K. Emani, principal research scientist Alexander V. Kildishev, assistant professor Alexandra Boltasseva, and Shalaev.
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," Shalaev said. "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.
"We have extended the Harvard team's applications to the near infrared, which is important, and we also showed that it's not a single frequency effect, it's a very broadband effect," Shalaev said. "Having a broadband effect potentially offers a range of technological applications."
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" behavior: 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," Shalaev said. "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.
"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," Shalaev said.
The work has been funded by the U.S. Air Force Office of Scientific Research and the National Science Foundation's Division of Materials Research.