Abstract-Infrared Topological Plasmons in Graphene

Dafei Jin, Thomas Christensen, Marin Soljačić, Nicholas X. Fang, Ling Lu, and Xiang Zhang

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.245301

We propose a two-dimensional plasmonic platform—periodically patterned monolayer graphene—which hosts topological one-way edge states operable up to infrared frequencies. We classify the band topology of this plasmonic system under time-reversal-symmetry breaking induced by a static magnetic field. At finite doping, the system supports topologically nontrivial band gaps with mid-gap frequencies up to tens of terahertz. By the bulk-edge correspondence, these band gaps host topologically protected one-way edge plasmons, which are immune to backscattering from structural defects and subject only to intrinsic material and radiation loss. Our findings reveal a promising approach to engineer topologically robust chiral plasmonic devices and demonstrate a realistic example of high-frequency topological edge states.

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Abstract-Infrared Topological Plasmons in Graphene

Dafei Jin, Thomas Christensen, Marin Soljačić, Nicholas X. Fang, Ling Lu, and Xiang Zhang

Phys. Rev. Lett. 118, 245301 – Published 16 June 2017

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.245301

We propose a two-dimensional plasmonic platform—periodically patterned monolayer graphene—which hosts topological one-way edge states operable up to infrared frequencies. We classify the band topology of this plasmonic system under time-reversal-symmetry breaking induced by a static magnetic field. At finite doping, the system supports topologically nontrivial band gaps with mid-gap frequencies up to tens of terahertz. By the bulk-edge correspondence, these band gaps host topologically protected one-way edge plasmons, which are immune to backscattering from structural defects and subject only to intrinsic material and radiation loss. Our findings reveal a promising approach to engineer topologically robust chiral plasmonic devices and demonstrate a realistic example of high-frequency topological edge states.

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Metamolecules That Switch Handedness at Light-Speed

http://newscenter.lbl.gov/2012/07/10/metamolecules-that-switch-handedness-at-light-speed/

(Top) Scanning electron microscopy image of optically switchable chiral THz metamolecules, (Bottom) The purple, blue and tan colors represent the gold meta-atom structures at different layers, with the two silicon pads shown in green. (courtesy of Zhang, et. al)

A multi-institutional team of researchers that included scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has created the first artificial molecules whose chirality can be rapidly switched from a right-handed to a left-handed orientation with a  beam of light. This holds potentially important possibilities for the application of terahertz technologies across a wide range of fields, including reduced energy use for data-processing, homeland security and ultrahigh-speed communications.

Chirality is the distinct left/right orientation or “handedness” of some types of molecules, meaning the molecule can take one of two mirror image forms. The right-handed and left-handed forms of such molecules, called “enantiomers,” can exhibit strikingly different properties. For example, one enantiomer of the chiral molecule limonene smells of lemon, the other smells of orange. The ability to observe or even switch the chirality of molecules using terahertz (trillion-cycles-per-second) electromagnetic radiation is a much coveted asset in the world of high technology.

“Natural materials can be induced to change their chirality but the process, which involves structural changes to the material, is weak and slow. With our artificial molecules, we’ve demonstrated strong dynamic chirality switching at light-speed,” says Xiang Zhang, one of the leaders of this research and a principal investigator with Berkeley Lab’s Materials Sciences Division.

Working with terahertz (THz) metamaterials engineered from nanometer-sized gold strips with air as the dielectric – Zhang and his colleagues fashioned a delicate artificial chiral molecule which they then incorporated with a photoactive silicon medium. Through photoexcitation of their metamolecules with an external beam of light, the researchers observed handedness flipping in the form of circularly polarized emitted THz light. Furthermore, the photoexcitation enabled this chirality flipping and the circular polarization of THz light to be dynamically controlled.

“In contrast to previous demonstrations where chirality was merely switched on or off in metamaterials using photoelectric stimulation, we used an optical switch to actually reverse the chirality of our THz metamolecules,” Zhang says.

Zhang, who holds the Ernest S. Kuh Endowed Chair Professor of Mechanical Engineering at the University of California (UC)  Berkeley, where he also directs the Nano-scale Science and Engineering Center, is one of three corresponding authors of a paper describing this work in Nature Communications. The paper is titled “Photoinduced handedness switching in terahertz chiral metamolecules.” The other corresponding authors are Shuang Zhang of the University of Birmingham in the United Kingdom, and Antoinette Taylor of DOE’s Los Alamos National Laboratory.

The optically switchable chiral THz metamolecules consisted of  a pair of 3D meta-atoms of opposite chirality made from precisely structured gold strips. Each meta-atom serves as a resonator with a coupling between electric and magnetic responses that produces strong chirality and large circular dichroism at the resonance frequency.

“When two chiral meta-atoms of the same shape but opposite chirality are assembled to form a metamolecule, the mirror symmetry is preserved, resulting in the vanishing of optical activity,” Zhang says. “From a different point of view, the optical activity arising from these two meta-atoms of opposite chirality cancels out each other.”

Schematic shows the chirality switching metamolecule consists of four chiral resonators with fourfold rotational symmetry. An external beam of light instantly reverses the metamolecule’s chirality from right-handed to left-handed. (courtesy of Zhang, et. al)

Silicon pads were introduced to each chiral meta-atom in the metamolecule but at different locations. In one meta-atom, the silicon pad bridged two gold strips, and in the other meta-atom, the silicon pad replaced part of a gold strip. The silicon pads broke the mirror symmetry and induced chirality for the combined metamolecule. The pads also functioned as the optoelectronic switches that flipped the chirality of the metamolecule under  photoexcitation.

Says corresponding author Shuang Zhang, “Our scheme relies on the combination of two meta-atoms with opposite properties, in which one is functional while the other is inactive within the frequency range of interest. With suitable design, the two meta-atoms respond oppositely to an external stimulus, that is, the inactive one becomes functional and vice versa.”

THz electromagnetic radiation – also known as T-rays – falls within the frequency range of molecular vibrations, making it an ideal none-invasive tool for analyzing the chemical constituents of organic and non-organic materials. Being able to flip the handedness of chiral metamolecules and control the circular polarization of THz light could be used to detect toxic and explosive chemicals, or for wireless communication and high-speed data processing systems. As most biological molecules are chiral, including DNA, RNA and proteins, THz-based polarimetric devices should also benefit medical researchers and developers of pharmaceutical drugs among others.

“The switchable chirality we can engineer into our metamaterials provides a viable approach towards creating high performance polarimetric devices that are largely not available at terahertz frequencies,” says corresponding author Antoinette Taylor. “This frequency range is particularly interesting because it uniquely reveals information about physical phenomena such as the interactions between or within biologically relevant molecules. It may enable control of electronic states in novel material systems, such as cyclotron resonances in graphene and topological insulators.”

Taylor and her co-authors say that the general design principle of their optically switchable chiral THz metamolecules is not limited to handedness switching but could also be applied to the dynamic reversing of other electromagnetic properties.

In addition to the corresponding authors, other authors of the Nature Communications paper were Jiangfeng Zhou, Yong-Shik Park, Junsuk Rho, Ranjan Singh, Sunghyun Nam, Abul Azad, Hou-Tong Chen and Xiaobo Yin.

This research was primarily supported by the DOE Office of Science.

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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and URS for the Department of Energy’s National Nuclear Security Administration. Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns. For more information visit http://www.lanl.gov/

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.  For more information, please visit science.energy.gov.

Additional Information

For more information about the research of Xiang Zhang visit http://xlab.me.berkeley.edu/

Abstract-Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides

Manjakavahoaka. Razanoelina, Filchito Renee Bagsican, Iwao Kawayama, Xiang Zhang, Lulu Ma, Hironaru Murakami, Robert Vajtai, Pulickel M. Ajayan, Junichiro Kono, Masayoshi Tonouchi, 

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-4-3885

As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 1011 cm−2, which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.

© 2016 Optical Society of America

Full Article  |  PDF Article

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.

Related Links

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

Article by David Bradley

• Read more about David and our other columnists >>>

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

Tiny laser sensor heightens bomb detection sensitivity

The plasmon laser sensor consists of a thin slab of semiconductor separated from the metal surface by a dielectric gap layer. Surface defects on the semiconductor interact with molecules of the explosive DNT. (Image by Ren-Min Ma and Sadao Ota)

http://www.nanotech-now.com/news.cgi?story_id=49833

Abstract:
New technology under development at the University of California, Berkeley, could soon give bomb-sniffing dogs some serious competition.

Tiny laser sensor heightens bomb detection sensitivity

Berkeley, CA | Posted on July 19th, 2014

A team of researchers led by Xiang Zhang, UC Berkeley professor of mechanical engineering, has found a way to dramatically increase the sensitivity of a light-based plasmon sensor to detect incredibly minute concentrations of explosives. They noted that it could potentially be used to sniff out a hard-to-detect explosive popular among terrorists.

Their findings are to be published Sunday, July 20, in the advanced online publication of the journal Nature Nanotechnology.

They put the sensor to the test with various explosives - 2,4-dinitrotoluene (DNT), ammonium nitrate and nitrobenzene - and found that the device successfully detected the airborne chemicals at concentrations of 0.67 parts per billion, 0.4 parts per billion and 7.2 parts per million, respectively. One part per billion would be akin to a blade of grass on a football field.

The researchers noted that this is much more sensitive than the published results to date for other optical sensors.

"Optical explosive sensors are very sensitive and compact," said Zhang, who is also director of the Materials Science Division at the Lawrence Berkeley National Laboratory and director of the National Science Foundation Nanoscale Science and Engineering Center at UC Berkeley. "The ability to magnify such a small trace of an explosive to create a detectable signal is a major development in plasmon sensor technology, which is one of the most powerful tools we have today."

The new sensor could have many advantages over current bomb-screening methods.

"Bomb-sniffing dogs are expensive to train and they can become tired," said study co-lead author Ren-Min Ma, an assistant professor of physics at Peking University who did this work when he was a postdoctoral researcher in Zhang's lab. "The other thing we see at airports is the use of swabs to check for explosive residue, but those have relatively low-sensitivity and require physical contact. Our technology could lead to a bomb-detecting chip for a handheld device that can detect the tiny-trace vapor in the air of the explosive's small molecules."

The sensor could also be developed into an alarm for unexploded land mines that are otherwise difficult to detect, the researchers said. According to the United Nations, landmines kill 15,000 to 20,000 people every year. Most of the victims are children, women and the elderly.

Unstable and hungry for electrons

The nanoscale plasmon sensor used in the lab experiments is much smaller than other explosive detectors on the market. It consists of a layer of cadmium sulfide, a semiconductor, laid on top of a sheet of silver with a layer of magnesium fluoride in the middle.

In designing the device, the researchers took advantage of the chemical makeup of many explosives, particularly nitro-compounds such as DNT and its more well-known relative, TNT. Not only do the unstable nitro groups make the chemicals more explosive, they are also characteristically electron deficient, the researchers said. This quality increases the interaction of the molecules with natural surface defects on the semiconductor. The device works by detecting the increased intensity in the light signal that occurs as a result of this interaction.

Potential use to sense hard-to-detect explosive

"We think that higher electron deficiency of explosives leads to a stronger interaction with the semiconductor sensor," said study co-lead author Sadao Ota, a former Ph.D. student in Zhang's lab who is now an assistant professor of chemistry at the University of Tokyo.

Because of this, the researchers are hopeful that their plasmon laser sensor could detect pentaerythritol tetranitrate, or PETN, an explosive compound considered a favorite of terrorists. Small amounts of it pack a powerful punch, and because it is plastic, it escapes x-ray machines when not connected to detonators. It is the explosive found in Richard Reid's shoe bomb in 2001 and Umar Farouk Abdulmtallab's underwear bomb in 2009.

U.S. Attorney General Eric Holder Jr. was recently quoted in news reports as having "extreme, extreme concern" about Yemeni bomb makers joining forces with Syrian militants to develop these hard-to-detect bombs, which can be hidden in cell phones and mobile devices.

"PETN has more nitro functional groups and is more electron deficient than the DNT we detected in our experiments, so the sensitivity of our device should be even higher than with DNT," said Ma.

Latest generation of plasmon sensors

The sensor represents the latest milestone in surface plasmon sensor technology, which is now used in the medical field to detect biomarkers in the early stages of disease.

The ability to increase the sensitivity of optical sensors had traditionally been restricted by the diffraction limit, a limitation in fundamental physics that forces a tradeoff between how long and how small light can be trapped. By coupling electromagnetic waves with surface plasmons, the oscillating electrons found at the surface of metals, researchers were able to squeeze light into nanosized spaces, but sustaining the confined energy was challenging because light tends to dissipate at a metal's surface.

The new device builds upon earlier work in plasmon lasers by Zhang's lab that compensated for this light leakage by using reflectors to bounce the surface plasmons back and forth inside the sensor - similar to the way sound waves are reflected across the room in a whispering gallery - and using the optical gain from the semiconductor to amplify the light energy.

Zhang said the amplified sensor creates a much stronger signal than the passive plasmon sensors currently available, which work by detecting shifts in the wavelength of light. "The difference in intensity is similar to going from a light bulb for a table lamp to a laser pointer," he said. "We create a sharper signal which makes it easier to detect even smaller changes for tiny traces of explosives in the air."

The researchers noted that the sensor could have applications beyond chemical and explosive detection, such as use in biomolecular research.

The U.S. Air Force Office of Scientific Research Multi-University Research Initiative program helped support this work.

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For more information, please click here

Contacts:
Sarah Yang
Media Relations
scyang@berkeley.edu
(510) 643-7741

Xiang Zhang
xiang@berkeley.edu
(510) 225-8559

Ren-Min Ma
renminma@pku.edu.cn
(510) 984-3586
(Email first to schedule interview)

Sadao Ota
sota@chem.s.u-tokyo.ac.jp
(510) 984-4551
(Email first to schedule interview)

Copyright © University of California Berkeley

Switching terahertz waves with gate-controlled active graphene metamaterials

http://graphenetimes.com/2012/09/switching-terahertz-waves-with-gate-controlled-active-graphene-metamaterials/

Authors: Seung Hoon Lee, Muhan Choi, Teun-Teun Kim, Seungwoo Lee, Ming Liu, Xiaobo Yin, Hong Kyw Choi, Seung S. Lee, Choon-Gi Choi, Sung-Yool Choi, Xiang Zhang & Bumki Min

The extraordinary electronic properties of graphene provided the main thrusts for the rapid advance of graphene electronics. In photonics, the gate-controllable electronic properties of graphene provide a route to efficiently manipulate the interaction of photons with graphene, which has recently sparked keen interest in graphene plasmonics. However, the electro-optic tuning capability of unpatterned graphene alone is still not strong enough for practical optoelectronic applications owing to its non-resonant Drude-like behaviour. Here, we demonstrate that substantial gate-induced persistent switching and linear modulation of terahertz waves can be achieved in a two-dimensional metamaterial, into which an atomically thin, gated two-dimensional graphene layer is integrated. The gate-controllable light–matter interaction in the graphene layer can be greatly enhanced by the strong resonances of the metamaterial. Although the thickness of the embedded single-layer graphene is more than six orders of magnitude smaller than the wavelength (<λ/1,000,000), the one-atom-thick layer, in conjunction with the metamaterial, can modulate both the amplitude of the transmitted wave by up to 47% and its phase by 32.2° at room temperature. More interestingly, the gate-controlled active graphene metamaterials show hysteretic behaviour in the transmission of terahertz waves, which is indicative of persistent photonic memory effects.

Nature Materials. doi:10.1038/nmat3433

Researchers develop optically switchable chiral terahertz metamolecules

The schematic shows the chirality switching metamolecule consists of four chiral resonators with fourfold rotational symmetry. An external beam of light instantly reverses the metamolecule’s chirality from right-handed to left-handed. Credit: Courtesy of Xiang Zhang, et. al

http://phys.org/news/2012-07-optically-switchable-chiral-terahertz-metamolecules.html
A multi-institutional team of researchers that included scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab) has created the first artificial molecules whose chirality can be rapidly switched from a right-handed to a left-handed orientation with a beam of light. This holds potentially huge possibilities for the application of terahertz technologies across a wide range of fields, including biomedical research, homeland security and ultrahigh-speed communications.

Chirality is the distinct left/right orientation or "handedness" of some types of molecules, meaning the molecule can take one of two mirror image forms. The right-handed and left-handed forms of such molecules, called "enantiomers," can exhibit strikingly different properties. For example, one enantiomer of the chiral molecule limonene smells of lemon, the other smells of orange. The ability to observe or even switch the chirality of molecules using terahertz (trillion-cycles-per-second)electromagnetic radiation is a much coveted asset in the world of high technology.

"Natural materials can be induced to change their chirality but the process, which involves structural changes to the material, is weak and slow. With our artificial molecules, we've demonstrated strong dynamic chirality switching at light-speed," says Xiang Zhang, one of the leaders of this research and a principal investigator with Berkeley Lab's Materials Sciences Division.

Working with terahertz (THz) metamaterials engineered from nanometer-sized gold strips with air as the dielectric - Zhang and his colleagues fashioned a delicate artificial chiral molecule which they then incorporated with a photoactive silicon medium. Through photoexcitation of their metamolecules with an external beam of light, the researchers observed handedness flipping in the form of circularly polarized emitted THz light. Furthermore, the photoexcitation enabled this chirality flipping and thecircular polarization of THz light to be dynamically controlled.

"In contrast to previous demonstrations where chirality was merely switched on or off in metamaterials using photoelectric stimulation, we used an optical switch to actually reverse the chirality of our THz metamolecules," Zhang says.

Related articles

• Metamolecules that switch handedness at light-speed (eurekalert.org)

Revolution in Communication: Berkeley Scientists Create World’s Smallest, Fastest Optical Modulator Using Graphene

http://techie-buzz.com/science/fast-modulators-using-graphene.html

by Debjyoti Bardhan

The wonder material Graphene continues to amaze. A research team, headed by Xiang Zhang, a UC Berkeley engineering professor, has built an ultra-small optical device that can control the switching on and off of light pulses, using Graphene. This extraordinary device is guaranteed to revolutionise communication, both in terms of speed and how we do it, in the very near future.

Graphene is a one-atom thick sheet of carbon atoms arranged in a hexagonal pattern – the so called sp²hybridized structure. (Read more about Graphene here). Graphene can be switched on and off extremely fast and this is the property exploited here. According to an externally applied voltage, Graphene can modulate pulses of light by letting some go through and restricting some others.

An artist's impression of Graphene

But why Graphene?

The modulator will not only be fast, it will be very compact. It only takes a 25 micron (that’s roughly 400 times thinner than a human hair) a side square of Graphene to make this modulator. Graphene also has the added advantage of supporting a huge bandwidth of optical frequencies. Any frequency of light ranging across the optical range up to ultra-violet and down to even infra-red can be effectively modulated. Graphene is also quite cheap to produce, especially with improved techniques like Chemical Vapor Deposition (CVD). It can be easily integrated with other materials, like Silicon, without bothering much about contamination. Graphene has the tremendous advantage of being highly conducting in pure form. It needs no doping, unlike Silicon. Further, its conductivity remains constant down to very low temperatures, like a few Kelvin.

A disk of Graphene (coated with an oxide)

How the modulation happens: Interplay between electrons and light

UC Berkeley researchers were attracted by the behavior of electrons and light within Graphene, especially their interaction. Here’s the watered down version of the technicalities: The electrons that matter are called valence electrons. They lie near the top of a so-called valence band, having energy equal to that of the Fermi level (the level till which the electrons are filled – here the top of the valence band). Electrons can jump from just below the Fermi level to above the level by absorbing light. The Fermi level can, however, be varied by an external voltage. Given enough negative voltage, electrons can be drawn out of the bands completely or packed tight (Technically, external voltage alters the Fermi level). In both these cases, light cannot be absorbed by the electrons (either because they are absent or because they have no place to jump to). The material is then transparent to light. Researchers hit upon the ‘Goldilocks voltage range’ in which Graphene is opaque and used it to turn the modulator off. Due to the high mobility of electrons in Graphene, a square voltage pulse with Gigahertz frequency easily dumps Graphene in and out of the opaque state, effectively modulating the transmission of light pulses. This is the first time light has been modified and guided at such small scales. Generally, light requires bulky mirrors or photonic crystals.

Schematic for Graphene Modulators. The honeycomb layer is the Graphene which sits on top of the Silicon waveguide. Pulses of light are sent through the waveguide and according to the voltage applied (the square pulses on the left), light is either transmitted or absorbed. (Courtesy: UC Berkeley)

The team put a Graphene layer on top of a Silicon waveguide (images above and below). They were able to achieve 1GHz modulation speed. Theoretically, 500 GHz is possible, so the 1 GHz figure will definitely be revised.

An image showing the actual fabrication (Courtesy: UC Berkeley)

The Future … is Here

In the near future, Zhang says, “Instead of broadband, we will have ‘extremeband’”, because the bandwidth offered will be huge, up to 10 nanometers (above 1000 Terahertz).

So there is another glimpse of the future, courtesy Graphene: It’s super-fast, super-small, cheap and offers huge bandwidth. Hope you download a lot of HD movies in 3D on your phone in future – it’ll, after all, take just a few seconds.