Unusual ‘Quasiparticles’ in Tri-Layer Graphene

Liyuan Zhang and Igor Zaliznyak at the Center for Functional Nanomaterials, Brookhaven National Laboratory, USA

By studying three layers of graphene — sheets of honeycomb-arrayed carbon atoms — stacked in a particular way, scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have discovered a “little universe” populated by a new kind of “quasiparticles” — particle-like excitations of electric charge. Unlike massless photon-like quasiparticles in single-layer graphene, these new quasiparticles have mass, which depends on their energy (or velocity), and would become infinitely massive at rest.

That accumulation of mass at low energies means this trilayer graphene system, if magnetized by incorporating it into a heterostructure with magnetic material, could potentially generate a much larger density of spin-polarized charge carriers than single-layer graphene — making it very attractive for a new class of devices based on controlling not just electric charge but also spin, commonly known as spintronics.

“Our research shows that these very unusual quasiparticles, predicted by theory, actually exist in three-layer graphene, and that they govern properties such as how the material behaves in a magnetic field — a property that could be used to control graphene-based electronic devices,” said Brookhaven physicist Igor Zaliznyak, who led the research team. Their work measuring properties of tri-layer graphene as a first step toward engineering such devices was published online in Nature Physics [1].

Graphene has been the subject of intense research since its discovery in 2004, in particular because of the unusual behavior of its electrons, which flow freely across flat, single-layer sheets of the substance. Stacking layers changes the way electrons flow: Stacking two layers, for example, provides a “tunable” break in the energy levels the electrons can occupy, thus giving scientists a way to turn the current on and off. That opens the possibility of incorporating the inexpensive substance into new types of electronics.

With three layers, the situation gets more complicated, scientists have found, but also potentially more powerful.

One important variable is the way the layers are stacked: In “ABA” systems, the carbon atoms making up the honeycomb rings are directly aligned in the top and bottom layers (A) while those in the middle layer (B) are offset; in “ABC” variants, the honeycombs in each stacked layer are offset, stepping upwards layer by layer like a staircase. So far, ABC stacking appears to give rise to more interesting behaviors — such as those that are the subject of the current study.

ABC trilayer graphene, where the three layers are offset from one another like stair steps [Image courtesy: Brookhaven National Laboratory]

For this study, the scientists created the tri-layer graphene at the Center for Functional Nanomaterials (CFN) at Brookhaven Lab, peeling it from graphite, the form of carbon found in pencil lead. They used microRaman microscopy to map the samples and identify those with three layers stacked in the ABC arrangement. Then they used the CFN’s nanolithography tools, including ion-beam milling, to shape the samples in a particular way so they could be connected to electrodes for measurements.

At the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, the scientists then studied the material’s electronic properties — specifically the effect of an external magnetic field on the transport of electronic charge as a function of charge carrier density, magnetic field strength, and temperature.

“Ultimately, the success of this project relied on hard work and rare experimental prowess of talented young researchers with whom we engaged in these studies, in particular, Liyuan Zhang, who at the time was research associate at Brookhaven, and Yan Zhang, then a graduate student from Stony Brook University,” said Igor Zaliznyak.

The measurements provide the first experimental evidence for the existence of a particular type of quasiparticle, or electronic excitation that acts like a particle and serves as a charge carrier in the tri-layer graphene system. These particular quasiparticles, which were predicted by theoretical studies, have ill-defined mass — that is, they behave as if they have a range of masses — and those masses diverge as the energy level decreases with quasiparticles becoming infinitely massive.

Ordinarily such particles would be unstable and couldn’t exist due to interactions with virtual particle-hole pairs — similar to virtual pairs of oppositely charged electrons and positrons, which annihilate when they interact. But a property of the quasiparticles called chirality, which is related to a special flavor of spin in graphene sytems, keeps the quasiparticles from being destroyed by these interactions. So these exotic infinitively massive particles can exist.

“These results provide experimental validation for the large body of recent theoretical work on graphene, and uncover new exciting possibilities for future studies aimed at using the exotic properties of these quasiparticles,” Zaliznyak said.

For example, combining magnetic materials with tri-layer graphene could align the spins of the charge-carrier quasiparticles. “We believe that such graphene-magnet heterostructures with spin-polarized charge carriers could lead to real breakthroughs in the field of spintronics,” Zaliznyak said.

Reference
[1] Liyuan Zhang, Yan Zhang, Jorge Camacho, Maxim Khodas, Igor Zaliznyak, "The experimental observation of quantum Hall effect of l=3 chiral quasiparticles in trilayer graphene", Nature Physics, doi:10.1038/nphys2104 (Published online September 25, 2011). Abstract.

Digital Plasmonics

Bergin Gjonaj

Authors:
Bergin Gjonaj¹, Jochen Aulbach¹, Patrick M. Johnson¹, L. Kuipers¹, Ad Lagendijk¹ & Allard P. Mosk<sup>2

1</sup>Center for Nanophotonics, FOM Institute AMOLF, Amsterdam, The Netherlands

²Complex Photonic Systems, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands

Researchers from the FOM Institute AMOLF in Amsterdam and the University of Twente have developed a way to digitally control tiny electrical waves, so-called surface plasmons. These waves play an important role in nanophotonic research, which studies materials at a very small scale. The new knowledge could lead to an interface that makes nanophotonic devices more user-friendly (comparable to Windows on a computer). The advance online publication of their work appeared in the leading journal Nature Photonics on May 22nd.

When light reflects off gold nanostructures under the right conditions, it creates tiny electrical waves on the surface called surface plasmon polaritons (SPPs). The researchers have now achieved SPP control in a remarkably simple and universal manner. They did this by changing the shape of the incident light with a computer-controlled megapixel device called a phase plate (similar to a liquid crystal display).

Figure 1. Controlling the surface plasmons via a pixilated phase plate device. Light from a laser impinges on the phase plate. Each pixel of the device can be programmed to alter the amplitude and phase of the outgoing light. Light from each pixel will later be directed toward the sample surface using a lens system, as shown in figure for three different pixels. The sample is a nanohole grating engraved on a very thin gold film. When light from a pixel of the phase plates impinges on the sample it generates surface plasmon waves. These tiny waves propagate along the surface as indicated by the blue arrows. By proper tuning of the phase plate (pixel by pixel) it is possible to control the blue arrows.

Figure 2. Flexible control of the plasmonic waves using a pixelated phase plate. (a) Light from each pixel of the phase plate acts as a source of plasmons on the gold film. The computer tunes the relative phases of the pixels, and thus of the plasmonic sources, to achieve constructive interference at a chosen spot. This creates a sharp plasmonic focus. (b) Relocation of the focus to a newly chosen spot. The position of the plasmonic focus is fully selectable from the computer.

Unlike previous approaches, which rely on fixed prefabricated surface structures to control SPPs, phase plate control is cheap and highly flexible. The researchers demonstrated this flexibility by creating a sharply focused SPP spot and scanning it across a gold surface. Such a scanned focused spot could be used to create the first super-resolution SPP microscope. In addition, the method could result in new interfaces for nanophotonic devices. This would make them more accessible for industry.

Reference:
[1] Bergin Gjonaj, Jochen Aulbach, Patrick M. Johnson, Allard P. Mosk, L. Kuipers & Ad Lagendijk, "Active spatial control of plasmonic fields,” Nature Photonics, 6, 360 (May, 2011). Abstract.

Scattering Lens Yields Unprecedented Sharp Images

Elbert G. van Putten

Author: E.G. van Putten

Affiliation: Complex Photonic Systems, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands

It is generally believed that disorder always degrades the sharpness of optical images. Now scientists of the MESA+ Institute at the University of Twente, University of Florence and the FOM Institute AMOLF have shown that a scattering and disordered layer in conjunction with a high refractive index material can be used as an imaging device with a sub-100 nm resolution thereby beating the most expensive microscope objectives. The robustness of this scattering lens against distortion and aberrations, together with the ease of manufacturing and its very high resolution are highly favorable features to improve the performance of a wide range of cutting-edge microscopy techniques.

Even the most expensive microscope objectives offer only a limited resolution. This restriction is due to the wave nature of light that force any focus to be larger than half the wavelength of light (the diffraction limit). This theoretical limit is usually impossible to reach due to practical problems like aberrations that cause focal distortion. Paradoxically a completely disordered layer naturally creates very small and intense light spots when illuminated by a laser. The price to pay is that these spots, which are known as speckle, are arranged in a dense and random pattern making them useless for imaging purposes.

The new scattering lens developed by the scientists, uses light scattering to couple light efficiently into a high refractive index material. By a fine control over the light that illuminates the disordered layer they can concentrate the speckle spots in the same place, effectively creating a single very small focus. Taking advantage of what is known as the "memory effect" the scientists were able to scan this nano-sized focus in the object plane of the lens. They then placed small gold nano particles in the object plane and used the scattering lens to resolve the particles with a sub-100 nm resolution.

Figure 1. Comparison of light focusing with a conventional lens and a scattering lens. (a) A plane light wave sent through a normal lens forms a focus. The focal size is determined by the range of angles in the converging beam as and by the refractive index of the medium that the light is propagating in. The microscope image shows a collection of gold spheres as imaged with a commercial high quality oil immersion microscope objective. Inset on left is a photo of an ordinary lens. (b) The scientists send a shaped wave through a scattering layer on top of a high refractive index material. The wave front is carefully shaped so that, after traveling through the layer, it forms a perfectly spherical, converging wave front. The large range of angles contributing to the converging beam, combined with the high refractive index, give rise to a nanometer-sized focal spot. The microscope image shows the same collection of gold spheres as in (a) imaged with the scattering lens. Inset on left is a photo of the lens with the scattering layer on top.

The combination of a high-index scattering material with the complete control over the illumination provides the first lens to create such a small and scannable focus, which makes it a favorable tool to improve the performance of all the imaging methods that require accurate focusing.

Reference
[1] E.G. van Putten, D. Akbulut, J. Bertolotti, W.L. Vos, A. Lagendijk, A.P. Mosk, “Scattering lens resolves sub-100 nm structures with visible light”, Phys. Rev. Lett. 106, 193905: 1-4 (2011). Abstract.

Measurements of the 'Edge States' of Graphene Nanoribbons

Michael Crommie [photo courtesy: Lawrence Berkeley National Laboratory]

As far back as the 1990s, long before anyone had actually isolated graphene – a honeycomb lattice of carbon just one atom thick – theorists were predicting extraordinary properties at the edges of graphene nanoribbons. Now physicists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and their colleagues at the University of California at Berkeley, Stanford University, and other institutions, have made the first precise measurements of the “edge states” of well-ordered nanoribbons.

A graphene nanoribbon is a strip of graphene that may be only a few nanometers wide. Theorists have envisioned that nanoribbons, depending on their width and the angle at which they are cut, would have unique electronic, magnetic, and optical features, including band gaps like those in semiconductors, which sheet graphene doesn’t have.

“Until now no one has been able to test theoretical predictions regarding nanoribbon edge-states, because no one could figure out how to see the atomic-scale structure at the edge of a well-ordered graphene nanoribbon and how, at the same time, to measure its electronic properties within nanometers of the edge,” says Michael Crommie of Berkeley Lab’s Materials Sciences Division (MSD) and UC Berkeley’s Physics Division, who led the research. “We were able to achieve this by studying specially made nanoribbons with a scanning tunneling microscope.”

Graphene nanoribbons are narrow sheets of carbon atoms only one layer thick. Their width, and the angles at which the edges are cut, produce a variety of electronic states, which have been studied with precision for the first time using scanning tunneling microscopy and scanning tunneling spectroscopy. [image courtesy: Lawrence Berkeley National Laboratory]

The team’s research not only confirms theoretical predictions but opens the prospect of building quick-acting, energy-efficient nanoscale devices from graphene-nanoribbon switches, spin-valves, and detectors, based on either electron charge or electron spin. Farther down the road, graphene nanoribbon edge states open the possibility of devices with tunable giant magnetoresistance and other magnetic and optical effects.

Crommie and his colleagues have published their research in Nature Physics, available May 8, 2011 in advanced online publication [1].

The well-tempered nanoribbon

“Making flakes and sheets of graphene has become commonplace,” Crommie says, “but until now, nanoribbons produced by different techniques have exhibited, at best, a high degree of inhomogeneity” – typically resulting in disordered ribbon structures with only short stretches of straight edges appearing at random. The essential first step in detecting nanoribbon edge states is access to uniform nanoribbons with straight edges, well-ordered on the atomic scale.

Hongjie Dai of Stanford University’s Department of Chemistry and Laboratory for Advanced Materials, a member of the research team, solved this problem with a novel method of “unzipping” carbon nanotubes chemically. Graphene rolled into a cylinder makes a nanotube, and when nanotubes are unzipped in this way the slice runs straight down the length of the tube, leaving well-ordered, straight edges.

By "unzipping" carbon nanotubes, regular edges with differing chiralities can be produced between the extremes of the zigzag configuration and, at a 30-degree angle to it, the armchair configuration. [image courtesy: Lawrence Berkeley National Laboratory]


Graphene can be wrapped at almost any angle to make a nanotube. The way the nanotube is wrapped determines the pitch, or “chiral vector,” of the nanoribbon edge when the tube is unzipped. A cut straight along the outer atoms of a row of hexagons produces a zigzag edge. A cut made at a 30-degree angle from a zigzag edge goes through the middle of the hexagons and yields scalloped edges, known as “armchair” edges. Between these two extremes are a variety of chiral vectors describing edges stepped on the nanoscale, in which, for example, after every few hexagons a zigzag segment is added at an angle.

These subtle differences in edge structure have been predicted to produce measurably different physical properties, which potentially could be exploited in new graphene applications. Steven Louie of UC Berkeley and Berkeley Lab’s MSD was the research team’s theorist; with the help of postdoc Oleg Yazyev, Louie calculated the expected outcomes, which were then tested against experiment.

Chenggang Tao of MSD and UCB led a team of graduate students in performing scanning tunneling microscopy (STM) of the nanoribbons on a gold substrate, which resolved the positions of individual atoms in the graphene nanoribbons. The team looked at more than 150 high-quality nanoribbons with different chiralities, all of which showed an unexpected feature, a regular raised border near their edges forming a hump or bevel. Once this was established as a real edge feature – not the artifact of a folded ribbon or a flattened nanotube – the chirality and electronic properties of well-ordered nanoribbon edges could be measured with confidence, and the edge regions theoretically modeled.

Electronics at the edge

“Two-dimensional graphene sheets are remarkable in how freely electrons move through them, including the fact that there’s no band gap,” Crommie says. “Nanoribbons are different: electrons can become trapped in narrow channels along the nanoribbon edges. These edge-states are one-dimensional, but the electrons on one edge can still interact with the edge electrons on the other side, which causes an energy gap to open up.”

A scanning tunneling microscope determines the topography and orientation of the graphene nanoribbons on the atomic scale. In spectroscopy mode, it determines changes in the density of electronic states, from the nanoribbon's interior to its edge. [image courtesy: Lawrence Berkeley National Laboratory]

Using an STM in spectroscopy mode (STS), the team measured electronic density changes as an STM tip was moved from a nanoribbon edge inward toward its interior. Nanoribbons of different widths were examined in this way. The researchers discovered that electrons are confined to the edge of the nanoribbons, and that these nanoribbon-edge electrons exhibit a pronounced splitting in their energy levels.

“In the quantum world, electrons can be described as waves in addition to being particles,” Crommie notes. He says one way to picture how different edge states arise is to imagine an electron wave that fills the length of the ribbon and diffracts off the atoms near its edge. The diffraction patterns resemble water waves coming through slits in a barrier.

For nanoribbons with an armchair edge, the diffraction pattern spans the full width of the nanoribbon; the resulting electron states are quantized in energy and extend spatially throughout the entire nanoribbon. For nanoribbons with a zigzag edge, however, the situation is different. Here diffraction from edge atoms leads to destructive interference, causing the electron states to localize near the nanoribbon edges. Their amplitude is greatly reduced in the interior.

The energy of the electron, the width of the nanoribbon, and the chirality of its edges all naturally affect the nature and strength of these nanoribbon electronic states, an indication of the many ways the electronic properties of nanoribbons can be tuned and modified.

Says Crommie, “The optimist says, ‘Wow, look at all the ways we can control these states – this might allow a whole new technology!’ The pessimist says, ‘Uh-oh, look at all the things that can disturb a nanoribbon’s behavior – how are we ever going to achieve reproducibility on the atomic scale?’”

Crommie himself declares that “meeting this challenge is a big reason for why we do research. Nanoribbons have the potential to form exciting new electronic, magnetic, and optical devices at the nanoscale. We might imagine photovoltaic applications, where absorbed light leads to useful charge separation at nanoribbon edges. We might also imagine spintronics applications, where using a side-gate geometry would allow control of the spin polarization of electrons at a nanoribbon’s edge.”

Although getting there won’t be simple — “The edges have to be controlled,” Crommie emphasizes — “what we’ve shown is that it’s possible to make nanoribbons with good edges and that they do, indeed, have characteristic edge states similar to what theorists had expected. This opens a whole new area of future research involving the control and characterization of graphene edges in different nanoscale geometries.”

Reference
[1] Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettl, Steven G. Louie, Hongjie Dai, and Michael F. Crommie, “Spatially resolving edge states of chiral graphene nanoribbons,” Nature Physics, Published online on May 8th, 2011. doi:10.1038/nphys1991. Abstract.

[The text is written by Paul Preuss of Lawrence Berkeley National Laboratory]

Negative Index Materials Reverse the Optical Doppler Effect

(Top L to R) Jiabi Chen, Yan Wang, Baohua Jia, Tao Geng; (Bottom L to R) Xiangping Li, Bingming Liang, Min Gu, and Songlin Zhuang

Authors: Jiabi Chen¹, Yan Wang^1,2, Baohua Jia³, Tao Geng¹, Xiangping Li³, Lie Feng¹, Wei Qian¹, Bingming Liang¹, Xuanxiong Zhang¹, Min Gu³, and Songlin Zhuang¹

Affiliation:
¹Shanghai Key Lab of Contemporary Optical System, Optical Electronic Information and Computer Engineering College, University of Shanghai for Science and Technology, China，
²College of physics and communication electronics, Jiangxi Normal University, China，
³Center for Micro-Photonics and CUDOS, Swinburne University of Technology, Australia

In the past couple of decades, we have witnessed a dramatic boost of the nanofabrication technology. As a result, man-made nanostructures and nanomaterials showing optical properties -- that have never been available naturally before -- came forth. Among these artificial materials, negative index materials have been intensively researched. The driving force for this is, on one hand, due to the potential fascinating applications of negative index materials in super-resolution perfect lens imaging, invisible cloaking and optical communications. On the other hand, it also originates from the curiosity to see the possibility to completely subvert the fundamental physics rules that we learned at school.

In the recent Nature Photonics paper published on March 7 [1], our team at University of Shanghai for Science and Technology in China together with collaborators from Swinburne University in Australia reported the first demonstration of the reversal of the well-known Doppler effect in the optical region with a negative index photonic crystal.

Fig. 1 (a) Normal Doppler effect in normal materials (n>0). (b) Inverse Doppler effect in negative index materials (n<0)

Our common knowledge of a Doppler effect comes from the increasing tone (frequency increase) of a whistling train approaching us and the falling tone (frequency decrease) when it recedes. The same thing happens to light waves. When a light source and an observer approach each other, blue-shifted (frequency increase) light will be observed, as illustrated in Fig.1a. The intriguing inverse Doppler effect is that red-shifted (frequency decrease) light is observed when the light source and an observer are approaching each other, as shown in Fig.1b, or vice versa. The Doppler effect is proportional to the refractive index of the medium that it passes. All naturally existing materials have a refractive index ≥ 1, therefore the normal Doppler effect is expected.

Fig 2: Measured transmission power as a function of the refraction angle θ for a normally incident beam respect to the first interface of the PC (the incident angle at the exit interface is 60°). Inset: Schematic diagram of the experimental setup.

We were able to reverse the Doppler effect for the first time in the optical region by constructing a two-dimensional silicon photonic crystal with a negative index property. In order to have the negative index property, the photonic crystal was tailored to have periodic pillars with nanometric sizes, in which a photonic bandgap can be generated. When shining a beam of CO₂ laser ( λ=10.6 μm corresponding to the 2nd band of the photonic crystal along the ΓM direction), the beam experiences a refraction with a negative index. The experimental result is presented in Fig. 2. At a refraction angle of approximately θ=-26º (incident angle is 60º as indicated in the inset of Fig. 2) high intensity signal could be measured clearly revealing that the photonic crystal prism is operating in the negative refraction region, with a measured n_p=-0.5062.

We employed a highly sensitive two-channel heterodyne interferometric experimental setup to measure the inverse Doppler effect, and at the same time used a positive-index ZnSe prism (n_p=2.403) to conduct the controlled experiment. The results shown in Fig. 3a clearly indicates the measured beat frequency Δf < f^'₂ - f₀k (where f₀ is the original frequency of the CO₂ laser, f^’₂ the reference Doppler frequency shift at the detector surface and k is defined as

Note that the Doppler shift can and can only be positive, i.e. , which indicates that the Doppler frequency is blue-shifted and larger than the original frequency of the CO₂ laser when the optical path becomes larger in the negative index materials.

In contrast the measured frequency differences Δf for four velocities are all less than f^'₂ - f₀k as shown in Fig. 3b (note that here f^'₂ - f₀k < 0), which clearly demonstrates that the Doppler effect measured in the ZnSe prism is normal.

Fig. 3 (a) Measured frequency shifts ∆f in the NIM PC prism compared with the value of f^'₂ - f₀k. (b) Measured frequency shifts ∆f in the positive index ZnSe prism compared with the value of f^'₂ - f₀k.

Our results indicate that reversed Doppler effect at the optical frequency has been observed for the first time by refracting the beam in a negative index photonic crystal. The fascinating negative index materials will lead to more counterintuitive phenomena such as the perfect lens imaging and invisible cloaking.

Reference:
[1] Jiabi Chen, Yan Wang, Baohua Jia, Tao Geng, Xiangping Li, Lie Feng, Wei Qian, Bingming Liang, Xuanxiong Zhang, Min Gu and Songlin Zhuang, “Observation of the inverse Doppler effect in negative-index materials at optical frequencies,” Nat. Photonics 2011 IN PRESS; Published online March 6th 2011; doi: 10.1038/nphoton.2011.17. Abstract.

Controlling Quantum Pathways for Raman Scattering in Graphene

Feng Wang beside a diagram showing how lowering the Fermi energy eliminates quantum pathways in graphene (lower left). The upper plot reveals that when destructively interfering quantum pathways are blocked, Raman scattering intensity is strongly enhanced (pale blue vertical, labeled G). At the same scattering, and at specific values of the Fermi energy, the plot reveals “hot electron luminescence” (labeled H.L.). (Click on image for best resolution.)


Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley have learned to control the quantum pathways determining how light scatters in graphene. Controlled scattering provides a new tool for the study of this unique material – graphene is a single sheet of carbon just one atom thick – and may point to practical applications for controlling light and electronic states in graphene nanodevices.

The research team, led by Feng Wang of Berkeley Lab’s Materials Sciences Division, made the first direct observation, in graphene, of so-called quantum interference in Raman scattering. Raman scattering is a form of “inelastic” light scattering. Unlike elastic scattering, in which the scattered light has the same color (the same energy) as the incident light, inelastically scattered light either loses energy or gains it.

Raman scattering occurs in graphene and other crystals when an incoming photon, a particle of light, excites an electron, which in turn generates a phonon together with a lower-energy photon. Phonons are vibrations of the crystal lattice, which are also treated as particles by quantum mechanics.

Quantum particles are as much waves as particles, so they can interfere with one another and even with themselves. The researchers showed that light emission can be controlled by controlling these interference pathways. They present their results in a forthcoming issue of the journal Nature [1].

Fig.1: The quantum pathways in Raman scattering are optically stimulated electronic excitations only possible if the initial electronic state is filled and the final state is empty (top). As pathways are removed by doping the graphene and lowering the Fermi energy (bottom), light from scattering may increase or decrease, depending on whether the removed pathways interfere constructively or destructively with the remaining pathways.

Manipulating quantum interference, in life and in the lab

“A familiar example of quantum interference in everyday life is antireflective coating on eyeglasses,” says Wang, who is also an assistant professor of physics at UC Berkeley. “A photon can follow two pathways, scattering from the coating or from the glass. Because of its quantum nature it actually follows both, and the coating is designed so that the two pathways interfere with each other and cancel light that would otherwise cause reflection.”

Wang adds, “The hallmark of quantum mechanics is that if different paths are nondistinguishable, they must always interfere with each other. We can manipulate the interference among the quantum pathways that are responsible for Raman scattering in graphene because of graphene’s peculiar electronic structure.”

In Raman scattering, the quantum pathways are electronic excitations, which are optically stimulated by the incoming photons. These excitations can only happen when the initial electronic state is filled (by a charged particle such as an electron), and the final electronic state is empty.

Quantum mechanics describes electrons filling a material’s available electronic states much as water fills the space in a glass: the “water surface” is called the Fermi level. All the electronic states below it are filled and all the states above it are empty. The filled states can be reduced by “doping” the material in order to shift the Fermi energy lower. As the Fermi energy is lowered, the electronic states just above it are removed, and the excitation pathways originating from these states are also removed.

“We were able to control the excitation pathways in graphene by electrostatically doping it – applying voltage to drive down the Fermi energy and eliminate selected states,” Wang says. “An amazing thing about graphene is that its Fermi energy can be shifted by orders of magnitude larger than conventional materials. This is ultimately due to graphene’s two-dimensionality and its unusual electronic bands.”

The Fermi energy of undoped graphene is located at a single point, where its electronically filled bands, graphically represented as an upward-pointing cone, meet its electronically empty bands, represented as a downward-pointing cone. To move the Fermi energy appreciably requires a strong electric field.

Fig.2: A flake of graphene was grown on copper and transferred onto an insulating substrate of silicon dioxide. The Fermi energy in the graphene was adjusted by varying the gate voltage on the overlying ion gel, which confines a strongly conducting liquid in a polymer matrix.

Team member Rachel Segalman, an associate professor of chemical engineering at UC Berkeley and a faculty scientist in Berkeley Lab’s Materials Sciences Division, provided the ion gel that was key to the experimental device. An ion gel confines a strongly conducting liquid in a polymer matrix. The gel was laid over a flake of graphene, grown on copper and transferred onto an insulating substrate. The charge in the graphene was adjusted by the gate voltage on the ion gel.

“So by cranking up the voltage we lowered the graphene’s Fermi energy, sequentially getting rid of the higher energy electrons,” says Wang. Eliminating electrons, from the highest energies on down, effectively eliminated the pathways that, when impinged upon by incoming photons, could absorb them and then emit Raman-scattered photons.

What comes of interference, constructive and destructive

“People have always known that quantum interference is important in Raman scattering, but it’s been hard to see,” says Wang. “Here it’s really easy to see the contribution of each state.”

Removing quantum pathways one by one alters the ways they can interfere. The changes are visible in the Raman-scattering intensity emitted by the experimental device when it was illuminated by a beam of near-infrared laser light. Although the glow from scattering is much fainter than the near-infrared excitation, changes in its brightness can be measured precisely.

“In classical physics, you’d expect to see the scattered light get dimmer as you remove excitation pathways,” says Wang, but the results of the experimenter came as a surprise to everyone. “Instead the signal got stronger!”

The scattered light grew brighter as the excitation pathways were reduced – what Wang calls “a canonical signature of destructive quantum interference.”

Why “destructively?” Because phonons and scattered photons can be excited by many different, nondistinguishable pathways that interfere with one another, blocking one path can either decrease or increase the light from scattering, depending on whether that pathway was interfering constructively or destructively with the others. In graphene, the lower and higher-energy pathways interfered destructively. Removing one of them thus increased the brightness of the emission.

“What we’ve demonstrated is the quantum-interference nature of Raman scattering,” Wang says. “It was always there, but it was so hard to see that it was often overlooked.”

In a second observation, the researchers found yet another unexpected example of inelastic light scattering. This one, “hot electron luminescence,” didn’t result from blocked quantum pathways, however.

When a strong voltage is applied and the graphene’s Fermi energy is lowered, higher-energy electron states are emptied from the filled band. Electrons that are highly excited by incoming photons, enough to jump to the unfilled band, thus find additional chances to fall back to the now-vacant states in what was the filled band. But these “hot” electrons can only fall back if they emit a photon of the right frequency. The hot electron luminescence observed by the researchers has an integrated intensity a hundred times stronger than the Raman scattering.

The road taken

The poet Robert Frost wrote of coming upon two roads that diverged in a wood, and was sorry he could not travel both. Not only can quantum processes take both roads at once, they can interfere with themselves in doing so.

The research team, working at UC Berkeley and at Berkeley Lab’s Advanced Light Source, has shown that inelastic light scattering can be controlled by controlling interference between the intermediate states between photon absorption and emission. Manipulating that interference has enabled new kinds of quantum control of chemical reactions, as well as of “spintronic” states, in which not charge but the quantum spins of electrons are affected. Strongly enhanced Raman scattering can be a boon to nanoscale materials research. Hot luminescence is potentially attractive for optoelectronics and biological research, in which near-infrared tags – even weak ones – could be very useful.

“Likewise the phenomenon of hot electron luminescence, because it immediately follows excitation by a probe laser, could become a valuable research tool,” says Wang, “particularly for studying ultrafast electron dynamics, one of the chief unusual characteristics of graphene.”

Reference
[1] Chi-Fan Chen, Cheol-Hwan Park, Bryan W. Boudouris, Jason Horng, Baisong Geng, Caglar Girit, Alex Zettl, Michael F. Crommie, Rachel A. Segalman, Steven G. Louie, and Feng Wang, “Controlling Inelastic Light Scattering Quantum Pathways in Graphene,” Nature, (Published online March 16, 2011). Abstract.

[The report is written by Paul Preuss of Lawrence Berkeley National Laboratory]

Extremely High Refractive Index Terahertz Metamaterial

(From L to R) Bumki Min, Muhan Choi and Seung Hoon Lee

Author: Bumki Min

Affiliation: Department of Mechanical Engineering and KAIST Institute for Optical Science and Technology, Korea Advanced Institute of Science and Technology, South Korea

For the past ten years, researchers in the field of metamaterials have been focusing on the demonstration of negative refractive index, as the negative side of the index could not be reached with naturally existing materials. Partly due to this overwhelming enthusiasm over the negative refractive index, the positive side of the index spectra has not been seriously explored, though the range of positive index in natural materials was still very limited.

The key idea behind the realization of high refractive index is quite simple [1-3]. From a perspective on artificial atoms (or molecules), we need to increase the dipole moment of an artificial atom, that can be induced by incident light. Though simple in its structure, I-shaped metallic patches proposed in this work possess all the requirements for the high refractive index. By periodically arranging I-shaped metallic patches with narrow gaps in-between, we can increase the capacitance of the constituting subwavelength-scale capacitors (I-shaped metallic patches). As the gap closes, the capacitance diverges rapidly and this leads to the huge accumulation of charges at the end of the I-shaped metallic patches. This huge accumulation of charges, in turn, results in extreme polarization density, and therefore the huge effective permittivity.

However, there is another problem to solve. We have to minimize the diamagnetic effect that gives rise to the decrease in effective permeability. This can be achieved simply by thinning the metallic structure and by decreasing the metallic volume fraction.

Figure 1: (left) Unit cell structure of the high-index metamaterial made of a thin I-shaped metallic patch symmetrically embedded in a dielectric material. (middle) Optical micrographs of the fabricated single, double, and triple layer metamaterials. (right) Photograph of a flexibility test for the fabricated metamaterials.

To confirm the theoretical prediction, the measurement of complex refractive index of the proposed high index metamaterials was performed with terahertz time-domain spectroscopy (THz-TDS). The experimentally-obtained refractive indices (real parts) of metamaterials having different gap width (from 80 nm to 30 μm) are plotted in Fig.1. For the sample with the smallest gap width, we obtained the peak refractive index of 38.64 and the quasi-static limiting value greater than 20.

So far, we couldn’t test metamaterials with smaller gap-width than this, but it will be interesting to see what will happen to the refractive index -- once the gap width becomes smaller than the thickness of metallic patches. If the gap width becomes smaller than the thickness of metallic patch, the increase of refractive index with respect to the reduction in gap will be more pronounced, since the subwavelength capacitor enters into the regime of parallel plate capacitor.

In addition, it is worthwhile to note that the overall refractive index is proportional to the refractive index of the substrate. For the present work, we have used a relatively low refractive index dielectric (polyimide whose real index is around 1.8) as a substrate. We expect that higher refractive index will be achieved with the use of higher index natural materials as substrates.

Figure 2: Frequency dependent effective refractive indices of single layer metamaterials with varying gap widths. Inset shows the scanning electron micrographs of a nanogap (~80 nm) high-index metamaterial.

While the proposed I-shaped metallic patch structure has shown the proof of concept, it exhibits polarization dependency owing to the structural anisotropy of the unit cell. In order to access the feasibility of isotropic high index metamaterials, we have fabricated two different types of 2D isotropic high index metamaterials and conducted additional experiments and analyses to verify the polarization independency (See Fig.3). Although the structures are different, the underlying physics is the same: Maintain small gap width for large capacitance and thin metallic patch for negligible diamagnetism.

Figure 3: (left) Polarization-angle-resolved effective refractive index for a single layer hexagonal high index metamaterial. Here, the gap width is 1.5 μm and the thickness is 1.82 μm. (right) Polarization-angle-resolved effective refractive index for a single layer window-type high index metamaterial. Here, the gap width is 1.5 μm and the thickness is 1.82 μm.

High refractive index metamaterials might provide a new way of achieving subwavelength resolution in an imaging system. Subwavelength imaging is being investigated through the utilization of negative index metamaterials (or singly negative materials). In contrast to this “perfect (or super) lens” concept, it might be possible to build a huge NA (numerical aperture) lens that provides the subwavelength-scale resolving power. In the design of high refractive index lens, spatially-varying gradient index can be obtained simply by controlling the gap between unit cells, thereby making it possible to fabricate a very thin flat metamaterial lens. However, among its limitations are the short focal length of high index lens and the working distance, which should be investigated more carefully in near future.

References:
[1] J. T. Shen, P. B. Catrysse, and S. Fan, “Mechanism for designing metallic metamaterials with a high index of refraction”, Phys. Rev. Lett. 94, 197401 (2005). Abstract.
[2] J. Shin, J. T. Shen, and S. Fan, “Three-dimensional metamaterials with an ultrahigh effective refractive index over a broad bandwidth”, Phys. Rev. Lett. 102, 093903 (2009). Abstract.
[3] M. Choi, S. H. Lee, Y. Kim, S. B. Kang, J. Shin, M. H. Kwak, K.-Y. Kang, Y.-H. Lee, N. Park, and B. Min, “A terahertz metamaterial with unnaturally high refractive index”, Nature 470, 369 (2011). Abstract.

New Materials Could Turn Near-Fantastic Devices like Invisibility Cloaks and Hyperlenses into Reality

Alexandra Boltasseva

Authors:
Alexandra Boltasseva¹ and Harry A. Atwater²

Affiliations:

1: School of Electrical & Computer Engineering and Birck Nanotechnology Center, Purdue University, IN, USA.

2: Applied Physics and Kavli Nanoscience Institute, California Institute of Technology, USA.

We have started a new research direction of developing new classes of materials that could serve as building blocks for advanced nanophotonic devices based on a novel concept of metamaterials, ranging from powerful nanoscale-resolution microscopes and improved solar cells to invisibility cloaks and new quantum optics devices.

Harry A. Atwater

We are now entering a new age of Metamaterials (MMs). These are artificial, engineered materials can be tailored for almost any application due to their extraordinary response to electromagnetic, acoustic and thermal waves that transcend the properties of “natural” materials. The astonishing MM-based designs and near-fantastic predictions by a new field of transformation optics range from a negative index of refraction, focusing and imaging with nanoscale resolution, invisibility cloaks and optical black holes to nanoscale optics and advanced quantum information applications.

Past 2Physics articles based on works of Harry A. Atwater:
May 02, 2010: "A Versatile Negative Index Metamaterial Design for Visible Light" by Stanley P. Burgos and Harry A. Atwater
March 26, 2007: "Negative Refraction of Visible Light"

We recently realized that metals like silver and gold that have traditionally been the material of choice for making MMs but suffer from high losses at operational frequencies (the visible or the near-infrared (NIR) ranges) could be successfully replaced by other materials [1]. Such development and optimization of materials has traditionally played a very important role in the development of new technologies. Similar to the infancy years of nanoelectronics, where the properties of silicon were rather poor, nanophotonics required another look at its fundamental building blocks - a step that is now marked by the recent Science article [1].

Material space for plasmonics and metamaterial applications: The important material parameters such as carrier concentration (maximum doping concentration for semiconductors), carrier mobility and interband losses form the optimization phase space for various applications. While spherical bubbles represent materials with low interband losses, elliptical bubbles represent those with larger interband losses in the corresponding part of the electromagnetic spectrum [1].

Now, we are working on replacing silver and gold by new materials that can be created using two options: making semiconductors more metallic by doping (like transparent conducting oxides) or making metal ‘less metallic’ by adding non-metallic elements (like titanium nitride, which looks like gold but has better properties). When these new materials are used for making MM and transformation optics devices (for example, "hyperlens" that provides nanoscale resolution not achievable with conventional optics), they outperform devices made with silver and gold [2].

Researchers are developing a new class of "plasmonic metamaterials" as potential building blocks for advanced optical technologies and a range of potential breakthroughs in the field of transformation optics. This image shows the transformation optics "quality factor" for several plasmonic materials: Gallium and Aluminum-doped zinc oxide (GZO, AZO), indium tin oxide and silver. For transformation optical devices, the quality factor rises as the amount of light "lost" or absorbed by plasmonic materials falls, resulting in materials that are promising for a range of advanced technologies. (Birck Nanotechnology Center, Purdue University)

New materials could turn many other MM designs and ideas into real-life devices: novel nano-patterning techniques capable of creating nanoscale features using light, advanced sensors and new types of light-harvesting systems for more efficient solar cells, a cloak of invisibility and new generation of quantum optical devices.

This work was supported by ONR-MURI grant N00014-10-1-0942 (AB) and U.S. Department of Energy grant DOE DE-FG02-07ER46405 and AFOSR grant FA9550-09 1 0673 (HAA).

References
[1] A. Boltasseva and H. A. Atwater, "Low-loss plasmonic metamaterials," Science 331, 290-291 (2011). Abstract.
[2] G. Naik and A. Boltasseva, "Semiconductors for plasmonics and metamaterials," Physica Status Solidi RRL 4, 295-297 (2010). Abstract.

Quantum Dot Bumps in the Graphene Electronic Highway

Suyong Jung (left) and Gregory Rutter (right), the leading authors of the Nature Physics paper[1] describing quantum dot formation in graphene when placed on insulating substrates.

Electronics researchers love graphene. A two-dimensional sheet of carbon one atom thick, graphene is like a superhighway for electrons, which rocket through the material 100 times faster than they normally do in silicon. But creating graphene-based devices with a full realization of this ultrafast electron transport will be challenging, say researchers at the National Institute of Standards and Technology (NIST), because new measurements show that placing graphene on a substrate, which is essential for graphene transistor operations, transforms its bustling speedway into hills and valleys that make it harder for electrons to get around. These hills and valleys can further localize the electrons into quantum dots when then graphene is exposed to an applied magnetic field, as reported in a new article in Nature Physics [1].

According to NIST Fellow Joseph Stroscio, graphene’s ideal properties are only realized when graphene is isolated from the environment. “To get the most benefit from graphene, we have to understand fully how graphene’s properties change when put in real-world conditions, such as part of device where it is in contact with other kinds of materials like semiconductors and insulators,” Stroscio says.

To see how graphene’s ideal properties are altered when placed on a substrate, NIST postdoctoral researcher Suyong Jung made a graphene device by exfoliating a single layer of graphene onto an insulating SiO2 substrate, using the so-called “scotch tape” method. The SiO2 substrate has a highly doped Si region on the back, which serves as a back gate conductor. When the bottom conductor is charged, an equal and opposite charge is induced in the graphene. This allows the researchers to study the electronic properties of graphene with different types of carriers, electrons versus holes, and with different densities by changing the potential on the back gate conductor. This extra experimental knob allowed the NIST researchers to develop a novel “gate-mapping” spectroscopy, when combined with scanning tunneling spectroscopy.

The researchers used a home-built scanning tunneling microscope operating at 4 K to measure the electron density of states in the graphene as a function of applied magnetic field and carrier density. The researchers first identified the disorder potential hills and valleys in the graphene sheet due to the presence of the substrate by tracking the location of the so-called “Dirac point,” which is the energy location where the conduction and valence bands in graphene come to a point. At this point “ideal” graphene has no carriers, but when placed on a substrate graphene’s potential “hills” and “valleys” fill up with electrons and holes, which leads to puddles, like potholes filling up with water on a damaged highway.

The electron and hole puddles reduce the mobility of electrons in graphene and even cause them to weakly localize in space. The effect of puddles, however, is more pronounced when electrons in graphene are exposed to high magnetic fields. In a magnetic field the electrons undergo cyclotron motion, where the carriers move in circular orbits. These orbits are not random, but take on only certain radii, which are quantized in terms of Landau levels, due to the laws of quantum mechanics. The electrons -- already made sluggish by the substrate interaction -- lack the energy to scale the mountains of resistance, and settle into isolated pockets of “quantum dots,” nanometer-scale regions that confine electrical charges in all directions.

The NIST researchers were able to see the effects of the graphene quantum dots in their measurements in a number of ways. The electrons require a certain energy (charging energy) to tunnel into and out of the quantum dot, which gives rise to a pattern of Coulomb diamonds in the spectroscopic sample bias-gate voltage maps. A series of Coulomb diamonds indicate the sequential addition of single electrons to the graphene quantum dots. Interestingly, the diamonds occur in groups of four reflecting the four-fold degeneracies of electron and valley degrees of freedom in graphene. The spatial location of quantum dots was directly obtained by mapping the compressible (metallic) regions of the Landau levels at the Fermi-energy.

Motivated by the current measurements, a somewhat unique application of graphene can be considered where information on insulating substrates can be obtained by first covering them with graphene, says NIST researcher Nikolai Zhitenev. Usually insulators cannot be studied at the atomic scale with the STM, since the closed loop servo requires a tunneling current to a conducting surface to maintain a constant tip-sample distance. On an insulator, no current is available. Placing the conducting graphene on an insulator lets researchers get close enough to these substrate materials to study their electrical properties, but not so close that the substrate and probe tip are damaged.

Reference
[1] S. Jung, G. Rutter, N. Klimov, D. Newell, I. Calizo, A. Hight-Walker, N. Zhitenev and J. Stroscio, "Evolution of microscopic localization in graphene in a magnetic field from scattering resonances to quantum dots", Nature Physics. Published online Jan. 9, 2010, DOI:10.1038/nphys1866. Abstract.

GRIN Plasmonics

[Left to Right] Yongmin Liu, Xiang Zhang and Thomas Zentgraf, three of the major authors of a Nature Nanotechnology paper describing GRIN plasmonics, a practical method for achieving exotic optics.(Photo by Roy Kaltschmidt, Berkeley Lab)

They said it could be done and now they’ve done it. What’s more, they did it with a GRIN. A team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California, Berkeley, have carried out the first experimental demonstration of GRIN – for gradient index – plasmonics, a hybrid technology that opens the door to a wide range of exotic optics, including superfast computers based on light rather than electronic signals, ultra-powerful optical microscopes able to resolve DNA molecules with visible light, and “invisibility” carpet-cloaking devices.

Working with composites featuring a dielectric (non-conducting) material on a metal substrate, and “grey-scale” electron beam lithography, a standard method in the computer chip industry for patterning 3-D surface topographies, the researchers have fabricated highly efficient plasmonic versions of Luneburg and Eaton lenses. A Luneburg lens focuses light from all directions equally well, and an Eaton lens bends light 90 degrees from all incoming directions.

“This past year, we used computer simulations to demonstrate that with only moderate modifications of an isotropic dielectric material in a dielectric-metal composite, it would be possible to achieve practical transformation optics results,” says Xiang Zhang, who led this research. “Our GRIN plasmonics technique provides a practical way for routing light at very small scales and producing efficient functional plasmonic devices.”

Zhang, a principal investigator with Berkeley Lab’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center (SINAM), is the corresponding author of a paper in the journal Nature Nanotechnology, describing this work titled, “Plasmonic Luneburg and Eaton Lenses.” Co-authoring the paper were Thomas Zentgraf, Yongmin Liu, Maiken Mikkelsen and Jason Valentine.

GRIN plasmonics combines methodologies from transformation optics and plasmonics, two rising new fields of science that could revolutionize what we are able to do with light. In transformation optics, the physical space through which light travels is warped to control the light’s trajectory, similar to the way in which outer space is warped by a massive object under Einstein’s relativity theory. In plasmonics, light is confined in dimensions smaller than the wavelength of photons in free space, making it possible to match the different length-scales associated with photonics and electronics in a single nanoscale device.

“Applying transformation optics to plasmonics allows for precise control of strongly confined light waves in the context of two-dimensional optics,” Zhang says. “Our technique is analogous to the well-known GRIN optics technique, whereas previous plasmonic techniques were realized by discrete structuring of the metal surface in a metal-dielectric composite.”

Like all plasmonic technologies, GRIN plasmonics starts with an electronic surface wave that rolls through the conduction electrons on a metal. Just as the energy in a wave of light is carried in a quantized particle-like unit called a photon, so, too, is plasmonic energy carried in a quasi-particle called a plasmon. Plasmons will interact with photons at the interface of a metal and dielectric to form yet another quasi-particle, a surface plasmon polariton (SPP).

















On left is a scanning electron micrograph of a plasmonic Luneburg lens on a gold film. On the right, fluorescence imaging shows intensity of the SPPs propagated by the Luneburg lens (dotted circle). X marks the launching position of the electron beam and Z is the direction in which the SPPs propogate. (Image courtesy of Zhang group)

The Luneburg and Eaton lenses fabricated by Zhang and his co-authors interacted with SPPs rather than photons. To make these lenses, the researchers worked with a thin dielectric film (a thermplastic called PMMA) on top of a gold surface. When applying grey-scale electron beam lithography, the researchers exposed the dielectric film to an electron beam that was varied in dosage (charge per unit area) as it moved across the film’s surface. This resulted in highly controlled differences in film thickness across the length of the dielectric that altered the local propagation of SPPs. In turn, the “mode index,” which determines how fast the SPPs will propagate, is altered so that the direction of the SPPs can be influenced.
















On left, a scanning electron micrograph of Eaton lenses on a gold film. On right, fluorescence imaging shows the intensity of SPPs propagating in z-direction (arrow) and bending to the right when passing through an Eaton lens. The solid line marks the outer diameter of the lens and the dashed line marks the high index region. (Image by Zhang group)

“By adiabatically tailoring the topology of the dielectric layer adjacent to the metal surface, we’re able to continuously modify the mode index of SPPs,” says Zentgraf. “As a result, we can manipulate the flow of SPPs with a greater degree of freedom in the context of two-dimensional optics.”

Says Liu, “The practicality of working only with the purely dielectric material to transform SPPs is a big selling point for GRIN plasmonics. Controlling the physical properties of metals on the nanometer length-scale, which is the penetration depth of electromagnetic waves associated with SPPs extending below the metal surfaces, is beyond the reach of existing nanofabrication techniques.”

Adds Zentgraf, “Our approach has the potential to achieve low-loss functional plasmonic elements with a standard fabrication technology that is fully compatible with active plasmonics.”

In the Nature Nanotechnology paper, the researchers say that inefficiencies in plasmonic devices due to SPPs lost through scattering could be reduced even further by incorporating various SPP gain materials, such as fluorescent dye molecules, directly into the dielectric. This, they say, would lead to an increased propagation distance that is highly desired for optical and plasmonic devices. It should also enable the realization of two-dimensional plasmonic elements beyond the Luneburg and Eaton lenses.

Says Mikkelsen, “GRIN plasmonics can be immediately applied to the design and production of various plasmonic elements, such as waveguides and beam splitters, to improve the performance of integrated plasmonics. Currently we are working on more complex, transformational plasmonic devices, such as plasmonic collimators, single plasmonic elements with multiple functions, and plasmonic lenses with enhanced performance.”

Reference
[1] Thomas Zentgraf,Yongmin Liu, Maiken H. Mikkelsen, Jason Valentine, Xiang Zhang,"Plasmonic Luneburg and Eaton Lenses", Nature Nanotechnology, Published online January 23 (2011). doi:10.1038/nnano.2010.282. Abstract.

[The text is written by Lynn Yarris of Lawrence Berkeley National Laboratory]

Enhanced Coupling of Mesoscopic Quantum Dots to Plasmons

[Left to Right] Mads Lykke Andersen, Peter Lodahl and Søren Stobbe

[This is an invited article based on a recent work by the authors.
-- 2Physics.com Team]

Authors: Mads Lykke Andersen, Søren Stobbe, and Peter Lodahl

Affiliation : Quantum Photonics Group, DTU Fotonik, Technical University of Denmark.

Today it is possible to fabricate and tailor highly efficient solid-state light-sources that emit a single photon at a time. Such solid-state emitters are referred to as quantum dots and consist of thousands of atoms. Despite the expectations reflected in this terminology, quantum dots cannot be described as point sources of light, which leads to the surprising conclusion: quantum dots are not dots!

In collaboration with Anders Søndberg Sørensen from the Niels Bohr Institute at University of Copenhagen, we have in an article in Nature Physics [1] recently reported on the discovery that light emission from quantum dots is fundamentally different than hitherto believed. The new insight may find important applications as a way to improve the coupling between light and matter, which is a prerequisite for efficient quantum information devices [2].

In our experiments we recorded the photon emission rate from quantum dots positioned close to a metallic mirror. Using this simple nanostructure it was possible to directly compare the experimental findings to the expectation for a point-dipole source, and a pronounced discrepancy was observed. Point sources of light have the same properties whether or not they are flipped upside down, and this was expected to be the case for quantum dots as well. However, this fundamental symmetry was violated in the experiments at DTU where a very pronounced dependence of the photon emission rate on the orientation of the quantum dots was observed.

Figure 1: Measured decay rates of quantum dots as a function of distance to the silver mirror for the direct (a) and inverted (b) structure at a wavelength of λ=1,030 nm. The dashed curves are the predictions for a point-dipole emitter. The solid curves show the results of a new theory valid for mesoscopic emitters, and are found to match the experimental data very well. The insets show the orientation of the quantum dots relative to the silver mirrors for the direct and inverted structures. [From Ref. [1] -- Thanks to 'Nature Physics']

The observation that the photon emission rate is dependent on the orientation of the quantum dots relative to the silver mirror is the experimental tell-tale that the point-dipole description breaks down and that the mesoscopic character of the quantum dot leads to modified light-matter interaction. The experimental data are found to be in excellent agreement with a new theory for light-matter interaction that takes the spatial extent of the quantum dots into account, see Figure 1.

The significant breakdown of the point-dipole description observed in our experiments is strongly promoted by the vicinity of the quantum dot to the silver mirror. At the mirror surface highly confined optical surface modes exist; the so-called surface plasmons, see Figure 2. Plasmonics is a very active and promising research field and the strong confinement of light available in plasmonics may have applications for quantum information science and solar energy harvesting [3]. In our experiments the plasmons at the mirror surface give rise to a strongly varying electric field over the spatial extent of the quantum dot, see Figure 2. Quantum dots coupled to surface plasmons have been suggested as a way to achieve very efficient light-matter interaction enabling, e.g., highly efficient single-photon sources [4]. Our work demonstrates that the excitation of plasmons can be even more efficient than previously thought. Thus the fact that quantum dots are extended over areas much larger than atomic dimensions implies that they can interact more efficiently with plasmons.

Figure 2 : Sketch of the studied system. A quantum dot (green trapezoid) is placed a distance z below a metal mirror. The lateral extension of a quantum dot is typically a=20 nm. The plasmon wavelength is λ_pl=262 nm (figure is not to scale). The field amplitude of the plasmon decays exponentially away from the interface leading to a variation of the electric field over the extension of the quantum dot. In describing light-matter interaction for such a system both the point-dipole moment μ as well as a mesoscopic moment Λ must be taken into account. [From Ref. [1] -- Thanks to 'Nature Physics']

The discovery that light emission can be strongly modified for quantum dots in optical nanostructures may pave the way for new nanophotonic devices that exploit the spatial extent of quantum dots as a novel resource. This may have important implications also in other research areas where quantum dots are applied, including photonic crystals, cavity quantum electrodynamics, and light harvesting.

Figure 3: Artist's impression of the discovery. Quantum dots are made up of thousands of atoms (yellow spheres) embedded in a semiconductor (blue spheres). Due to their mesoscopic dimensions, the point-emitter description is revealed to break down by comparing photon emission from quantum dots with opposite orientations relative to a metallic mirror.

References
[1] M. L. Andersen, S. Stobbe, A. S. Sørensen, P. Lodahl, "Strongly-modified Plasmon-matter interaction with mesoscopic quantum emitters", Nature Physics, (published online December 19, 2010). doi:10.1038/nphys1870.
[2] T.D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe & J.L. O’Brien, "Quantum computers", Nature 464, 45–53 (2010). Abstract.
[3] H.A. Atwater & A. Polman, "Plasmonics for improved photovoltaic devices", Nature Materials, 9, 205–213 (2010). Abstract.
[4] A.V. Akimov, A. Mukherjee, C.L. Yu, D.E. Chang, A.S. Zibrov, P.R. Hemmer, H. Park & M.D. Lukin, "Generation of single optical plasmons in metallic nanowires coupled to quantum dots", Nature 450, 402–406 (2007). Abstract.

Plasmon Laser @ Room Temperature

Xiang Zhang [photo courtesy: UC Berkeley]

Researchers at the University of California, Berkeley, have developed a new technique that allows plasmon lasers to operate at room temperature, overcoming a major barrier to practical utilization of the technology.

The achievement, described in an advanced online publication of the journal Nature Materials [1], is a "major step towards applications" for plasmon lasers, said the research team's principal investigator, Xiang Zhang, UC Berkeley professor of mechanical engineering and faculty scientist at Lawrence Berkeley National Laboratory.

"Plasmon lasers can make possible single-molecule biodetectors, photonic circuits and high-speed optical communication systems, but for that to become reality, we needed to find a way to operate them at room temperature," said Zhang, who also directs at UC Berkeley the Center for Scalable and Integrated Nanomanufacturing, established through the National Science Foundation’s (NSF) Nano-scale Science and Engineering Centers program.

In recent years, scientists have turned to plasmon lasers, which work by coupling electromagnetic waves with the electrons that oscillate at the surface of metals to squeeze light into nanoscale spaces far past its natural diffraction limit of half a wavelength. In 2009, Zhang's team reported a plasmon laser that generated visible light in a space only 5 nanometers wide, or about the size of a single protein molecule [2] (Read past 2Physics report).

[Image credit: Renmin Ma and Rupert Oulto] Schematic of a plasmon laser showing a cadmium sulfide (CdS) square atop a silver (Ag) substrate separated by a 5 nanometer gap of magnesium fluoride (MgF2). The cadmium sulfide square measures 45 nanometers thick and 1 micrometer long. The most intense electric fields of the device reside in the magnesium fluoride gap.

But efforts to exploit such advancements for commercial devices had hit a wall of ice.

"To operate properly, plasmon lasers need to be sealed in a vacuum chamber cooled to cryogenic temperatures as low as 10 kelvins, or minus 441 degrees Fahrenheit, so they have not been usable for practical applications," said Renmin Ma, a post-doctoral researcher in Zhang's lab and co-lead author of the Nature Materials paper.

In previous designs, most of the light produced by the laser leaked out, which required researchers to increase amplification of the remaining light energy to sustain the laser operation. To accomplish this amplification, or gain increase, researchers put the materials into a deep freeze.

[Image credit: Renmin Ma and Rupert Oulto] Electron microscope image of the plasmon laser

To plug the light leak, the scientists took inspiration from a whispering gallery, typically an enclosed oval-shaped room located beneath a dome in which sound waves from one side are reflected back to the other. This reflection allows people on opposite sides of the gallery to talk to each other as if they were standing side by side. (Some notable examples of whispering galleries include the U.S. Capitol's Statuary Hall, New York's Grand Central Terminal, and the rotunda at San Francisco's city hall.)

Instead of bouncing back sound waves, the researchers used a total internal reflection technique to bounce surface plasmons back inside a nano-square device. The configuration was made out of a cadmium sulfide square measuring 45 nanometers thick and 1 micrometer long placed on top of a silver surface and separated by a 5 nanometer gap of magnesium fluoride.

The scientists were able to enhance by 18-fold the emission rate of light, and confine the light to a space of about 20 nanometers, or one-twentieth the size of its wavelength. By controlling the loss of radiation, it was no longer necessary to encase the device in a vacuum cooled with liquid helium. The laser functioned at room temperature.

"The greatly enhanced light matter interaction rates means that very weak signals might be observable," said Ma. "Lasers with a mode size of a single protein are a key milestone toward applications in ultra-compact light source in communications and biomedical diagnostics. The present square plasmon cavities not only can serve as compact light sources, but also can be the key components of other functional building-blocks in integrated circuits, such as add-drop filters, direction couplers and modulators."

Rupert Oulton, a former post-doctoral researcher in Zhang's lab and now a lecturer at Imperial College London, is the other co-lead author of the paper. Other co-authors are Volker Sorger, a UC Berkeley Ph.D. student in mechanical engineering, and Guy Bartal, a former research scientist in Zhang's lab. The U.S. Air Force Office of Scientific Research and the NSF helped support this work.

Reference
[1] Ren-Min Ma, Rupert F. Oulton, Volker J. Sorger, Guy Bartal, Xiang Zhang, "Room-temperature sub-diffraction-limited plasmon laser by total internal reflection", Nature Materials, (published online December 19th, 2010), doi:10.1038/nmat2919 .
[2] Rupert F. Oulton, Volker J. Sorger, Thomas Zentgraf, Ren-Min Ma, Christopher Gladden, Lun Dai, Guy Bartal & Xiang Zhang, "Plasmon lasers at deep subwavelength scale", Nature 461, 629-632 (2009). Abstract.

[The text of this report is written by Sarah Yang of University of California, Berkeley]