Graphene Can Be Used for Terahertz Hyperlens

From left to right: Andrei Andryieuski, Dmitry Chigrin, Andrei Lavrinenko








Author: Andrei Andryieuski
Affiliation: DTU Fotonik, Technical University of Denmark, Denmark

Andrei Andryieuski and Andrei Lavrinenko, the researchers from the Metamaterials group at Technical University of Denmark (DTU) -- in collaboration with Dmitry Chigrin from the University of Wuppertal (BUW) -- made the first theoretical description of graphene hyperlens, able to work in the terahertz range.

Terahertz radiation, which occupies the spectral range between infrared and microwaves, is harmless to humans and animals and passes easily through many dielectric materials. Terahertz waves, which are able to detect drugs even in sealed vessels, to reveal hidden weapon and to detect cancer tumors, may revolutionize spectroscopy, defense and medical analysis.

 Fig. 1. Artistic view of the graphene hyperlens in action. Multiple structured graphene layers resolve and magnify two subwavelength sources.

The wavelength of terahertz radiation is, however, relatively large (300 µm at 1 THz) so the image-quality is seriously limited by the natural diffraction limit. To overcome this limit, artificially structured metamaterial lenses or metallic funnels can be employed. The very fine details hindered in the evanescent waves, which rapidly decay from the radiating object, can be captured by negative index superlens or indefinite medium (hyperbolic dispersion) hyperlens. While negative index metamaterials are still very lossy and far from being employed for practical purposes, the hyperlens present a realistic approach to imaging.

To make the hyperlens a very special material is required. It should behave as a metal in one direction and as an insulator in another. To obtain such properties, thin multiple metal-dielectric layers are normally used in optics. The theoretical concept -- proposed by Evgenii Narimanov’s group from Princeton University (USA) in 2006 [1] -- was checked experimentally by Xiang Zhang’s group from UC Berkeley (USA) in 2007 [2]. To realize the hyperlens the researchers deposited many ultrathin (35 nm) layers of silver and alumina. Even though the metal based hyperlens shows a subwavelength resolution, it is prohibited in tunability; basically, once being fabricated, its properties cannot be changed.

Contrary to metals, graphene, an atomically thin layer of carbon atoms, easily changes its properties under the influence of electrostatic field, magnetic field or chemical doping. This is why the researchers from DTU and BUW decided to employ it for the hyperlens. They propose to construct the hyperlens from narrow (starting from 40 nm) tapered graphene wires embedded into polymer.

Fig. 2. The building block of the hyperlens is the narrow graphene wire embedded into polymer. Arranging such tapered wires radially gives the required material properties for the hyperlens, namely, negative dielectric permittivity in radial direction and positive in azimuthal direction.

Such structured graphene arranged into multiple layers has the very properties needed for hyperbolic dispersion: the wave feels it as metal along the wires and as dielectric perpendicular to the wires. The hyperlens showed subwavelength resolution at the wavelength 50 µm of two sources separated with 10 µm, thus giving the possibility to image the points as close as 1/5 of the wavelength. The resolution of the hyperlens depends on its radii, so with proper selection of geometrical parameter it is possible to separate the images far enough to be captured by the conventional terahertz camera. The hyperlens can be tuned by applying voltage to various graphene layers. It works reciprocally, thus being able not only to image, but also to concentrate terahertz radiation in small volumes.

Fig. 3. Electromagnetic waves emitted by two line sources are captured by the hyperlens and magnified to such extent that they can be resolved with a conventional imaging device.

DTU’s researchers are looking forward to realize and test the graphene hyperlens experimentally.

The paper presenting the above work was published last week in Physical Review B Rapid Communications [3].

References:
[1] Zubin Jacob, Leonid V. Alekseyev, and Evgenii Narimanov, “Optical Hyperlens: Far-field imaging beyond the diffraction limit”, Optics Express, 14, 8247 (2006). Abstract.
[2] Zhaowei Liu, Hyesog Lee, Yi Xiong, Cheng Sun and Xiang Zhang, “Far-field optical hyperlens magnifying sub-diffraction-limited objects”, Science, 315,  1686 (2007). Abstract.
[3] Andrei Andryieuski, Andrei V. Lavrinenko, Dmitry N. Chigrin, "Graphene hyperlens for terahertz radiation", Physical Review B, 86, 121108(R) (2012). Abstract. Also available at: arXiv:1209.3951.

A Newtonian Approach to Negative Index Metamaterials

Hosang Yoon (left) and Donhee Ham (right)














Authors: Hosang Yoon and Donhee Ham 
Affiliation:
School of Engineering and Applied Sciences, Harvard University, USA

Link to Donhee Ham Research Group >>

Negative index metamaterials have been celebrated due to their unusual ability to manipulate electromagnetic waves, such as bending light in the ‘wrong’ direction [1] and focusing light below the diffraction limit [2], which may prove technologically useful. To achieve negative refraction, a variety of material systems with engineered electric and/or magnetic properties have been developed. Reporting in Nature [3], we have demonstrated a ‘kinetic’ route to negative refraction, where exploitation of acceleration of electrons in a two-dimensional (2D) conductor leads to extraordinarily strong negative refraction with refractive index as large as -700.

Electrons in a conductor subjected to an electric field collectively accelerate according to Newton’s 2nd law of motion, creating a current that lags the electric field by 90°, as in a magnetic inductor. The conductor may then be considered as an inductor, but with its inductance being of kinetic origin [4]. This kinetic inductance is usually not much appreciable in ordinary three-dimensional conductors, but in a 2D conductor, it is orders-of-magnitude larger than magnetic inductance, which is what we exploit to create the dramatically strong negative refraction.

The acceleration of electrons is continually interrupted by their collisions with vibrations, impurities, or defects of the crystal lattice. Imagine a time-varying sinusoidal electric field at a given frequency, which accelerates/decelerates electrons into an oscillation motion. If the mean scattering rate is too high to accommodate an appreciable fraction of an oscillation period, the acceleration effect is hard to observe, and the conductor acts as an Ohmic resistor. This can be avoided in two ways: one is to cool the conductor to lower the scattering rate as in our work [3]; the other is to increase the frequency at room temperature, as currently pursued in our lab.

Figure 1. Optical micrograph (left) and schematic (right) of the metamaterial. The 2DEG strip array is connected to electromagnetic waveguides to the left and right. (Image reproduced from the paper published in Nature [3])

We employ a GaAs/AlGaAs 2D electron gas (2DEG) as a demonstrational 2D conductor. Our metamaterial designed in the GHz frequency range is an array of 2DEG strips (Fig. 1). Signal (S) lines flanked by ground (G) lines on the left and right of the strip array are coplanar waveguides (CPWs) that guide electromagnetic waves to and from the metamaterial. As an electromagnetic wave arrives from the left CPW, its electric field between the S and G lines accelerates electrons in/along the leftmost few strips. This inductive movement of electrons is capacitively coupled to the strip on the right, accelerating electrons there. This dynamics repeats to propagate an ‘electro-kinetic’ wave from left to right, perpendicular to the strips acting as kinetic inductors. It is this electro-kinetic wave that is negatively refracting. The negative refractive index is large due to the large kinetic inductance of the GaAs/AlGaAs 2DEG strip.

Figure 2. (Click on the image to see high resolution version) Dispersion relation (left) and refractive index (middle) of a 13-strip metamaterial with strip length 112 μm and periodicity 1.25 μm measured at different cryogenic temperatures. (right) Refractive index measured for metamaterials with various strip length (l) and periodicity (a). (Image reproduced from the paper published in Nature[3]) 

Microwave scattering experiments over 1~50 GHz confirms negative refraction; the measured dispersion of the electro-kinetic wave (Fig. 2, left) shows opposite signs for the phase and group velocities above a cutoff frequency. The corresponding negative refractive index is hundreds in magnitude, two orders of magnitude larger than typical negative refractive indices (Fig. 2, middle). With varying geometric parameters, an index as large as -700 is obtained (Fig. 2, right).

The very large negative index brings the science of negative refraction into drastically miniaturized scale, enabling ultra-subwavelength manipulation of electromagnetic waves. We expect to see the similar result at room temperatures by increasing the frequency towards the THz range, which is the direction our lab is now taking.

References:
[1] V. G. Veselago, "The electrodynamics of substances with simultaneously negative values of ε and μ," Soviet Physics Uspekhi, 10, 509–514 (1968). Abstract.
[2] J. B. Pendry, "Negative refraction makes a perfect lens," Physical Review Letters, 85, 3966–3969 (2000). Abstract.
[3] H. Yoon, K. Y. M. Yeung, V. Umansky, and D. Ham, "A Newtonian approach to extraordinarily strong negative refraction," Nature, 488, 65–69 (2012). Abstract.
[4] R. Meservey, "Measurements of the kinetic inductance of superconducting linear structures," Journal of Applied Physics, 40, 2028–2034 (1969). Abstract.

Importance of Electron-Electron Interactions in Graphene

Michael Crommie [Photo by Roy Kaltschmidt, Courtesy: Lawrence Berkeley National Laboratory]

 Perhaps no other material is generating as much excitement in the electronics world as graphene, sheets of pure carbon just one atom thick through which electrons can race at nearly the speed of light – 100 times faster than they move through silicon. Superthin, superstrong, superflexible and superfast as an electrical conductor, graphene has been touted as a potential wonder material for a host of electronic applications, starting with ultrafast transistors. For the vast potential of graphene to be fully realized, however, scientists must first learn more about what makes graphene so super. The latest step in this direction has been taken by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley.

Michael Crommie, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s Physics Department, led a study in which the first direct observations at microscopic lengths were recorded of how electrons and holes respond to a charged impurity – a single Coulomb potential – placed on a gated graphene device. The results provide experimental support to the theory that interactions between electrons are critical to graphene’s extraordinary properties. This work has been published online on July 29th in the journal 'Nature Physics'[1].

“We’ve shown that electrons in graphene behave very differently around charged impurities than electrons in other materials,” Crommie says. “Some researchers have held that electron-electron interactions are not important to intrinsic graphene properties while others have argued they are. Our first-time-ever pictures of how ultra-relativistic electrons re-arrange themselves in response to a Coulomb potential come down on the side of electron-electron interactions being an important factor.”

Graphene sheets are composed of carbon atoms arranged in a two-dimensional hexagonally patterned lattice, like a honeycomb. Electrons moving through this honeycomb lattice perfectly mimic the behavior expected of highly relativistic charged particles with no mass: think of a ray of light that is electrically charged. Because this is the same behavior displayed by highly relativistic free electrons, charge-carriers in graphene are referred to as “Dirac quasiparticles,” after Paul Dirac, the scientist who first described the behavior of relativistic fermions in 1928.

“In graphene, electrons behave as massless Dirac fermions,” Crommie says. “As such, the response of these electrons to a Coulomb potential is predicted to differ significantly from how non-relativistic electrons behave in traditional atomic and impurity systems. However, until now, many key theoretical predictions for this ultra-relativistic system had not been tested.”

Image 1: This zoom-in STM topograph shows one of the cobalt trimers placed on graphene for the creation of Coulomb potentials – charged impurities – to which electrons and holes could respond. (Image courtesy of Crommie group)

Working with a specially equipped scanning tunneling microscope (STM)in ultra-high vacuum, Crommie and his colleagues probed gated devices consisting of a graphene layer deposited atop boron nitride flakes which were themselves placed on a silicon dioxide substrate, the most common of semiconductor substrates.

“The use of boron-nitride significantly reduced the charge inhomogeneity of graphene, thereby allowing us to probe the intrinsic graphene electronic response to individual charged impurities,” Crommie says. In this study, the charged impurities were cobalt trimers constructed on graphene by atomically manipulating cobalt monomers with the tip of an STM.”

Image 2: The response of ultrarelativistic electrons in graphene to Coulomb potentials created by cobalt trimers was observed to be signficantly different the response of non-relativistic electrons in traditional atomic and impurity systems. (Image courtesy of Crommie group)

The STM used to fabricate the cobalt trimers was also used to map (through spatial variation in the electronic structure of the graphene) the response of Dirac quasiparticles – both electron-like and hole-like – to the Coulomb potential created by the trimers. Comparing the observed electron–hole asymmetry to theoretical simulations allowed the research team to not only test theoretical predictions for how Dirac fermions behave near a Coulomb potential, but also to extract graphene’s dielectric constant.

“Theorists have predicted that compared with other materials, electrons in graphene are pulled into a positively-charged impurity either too weakly, the subcritical regime; or too strongly, the supercritical regime,” Crommie says. “In our study, we verified the predictions for the subcritical regime and found the value for the dielectric to be small enough to indicate that electron–electron interactions contribute significantly to graphene properties. This information is fundamental to our understanding of how electrons move through graphene.”

Reference:
[1] Yang Wang, Victor W. Brar, Andrey V. Shytov, Qiong Wu, William Regan, Hsin-Zon Tsai, Alex Zettl, Leonid S. Levitov, Michael F. Crommie, "Mapping Dirac quasiparticles near a single Coulomb impurity on graphene", Nature Physics, doi:10.1038/nphys2379 (Published online July 29, 2012). Abstract.

[This article is written by Lynn Yarris of Lawrence Berkeley National Laboratory]

Capturing, Tuning and Controlling Light with a Single Sheet of Carbon Atoms

Group Leaders: (From Left to Right) Javier Garcia de Abajo, Rainer Hillenbrand, Frank Koppens  







Authors: Jianing Chen^1,2, Michela Badioli³, Pablo Alonso-González¹, Susokin Thongrattanasiri⁴, Florian Huth^1,5, Johann Osmond³, Marko Spasenović³, Alba Centeno⁶, Amaia Pesquera⁶, Philippe Godignon⁷, Amaia Zurutuza⁶, Nicolas Camara⁸, Javier García de Abajo⁴, Rainer Hillenbrand^1,9, Frank Koppens³

Affiliation:
¹CIC nanoGUNE Consolider, 20018 Donostia-San Sebastián, Spain
²Centro de Fisica de Materiales (CSIC-UPV/EHU) and Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain
³ICFO-Institut de Ciéncies Fotoniques, Barcelona, Spain
⁴IQFR-CSIC, Madrid, Spain
⁵Neaspec GmbH, Munich, Germany
⁶Graphenea SA, 20018 Donostia-San Sebastián, Spain
⁷CNM-IMB-CSIC–Campus UAB, Barcelona, Spain
⁸GREMAN, UMR 7347, Université de Tours/CNRS, France
⁹IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain

Graphene, a remarkable one-atom-thick material consisting of a lattice of carbon atoms possesses extraordinary and gate-tunable optical properties. Interestingly, graphene can also carry strongly confined optical fields that travel along the surface of the sheet. These surface waves, based on the coupling between optical fields and charge carrier oscillations, are also called plasmons. For graphene, these plasmons have unique properties, as they can be tuned by electric fields, they propagate with speeds more than 100 times below the velocity of light, and they have a wavelength that is more than 100 times below the wavelength of light in free space. This makes it possible to confine light to extremely small volumes and to guide light along nanometer scale waveguides.

Since graphene was discovered, many theoretical physicists predicted the existence of graphene plasmons, but no experimental observations of propagating plasmons in graphene were reported so far. Due to the large mismatch in momentum between photons and graphene plasmons, it is not trivial to excite graphene plasmons by just shining light on a graphene sheet. This work has overcome this problem by focusing light on a sharp tip which is placed close to the graphene sheet. Because the tip acts as a nanoantenna, it can provide the extra momentum needed for the plasmons to be created (also called scattering near-field microscopy, s-SNOM). Moreover, the same tip can be used to probe the plasmons, which are reflected at the edges, and propagate back to the tip.

Interestingly, due to the interference between the plasmon waves that propagate away from the tip and towards the tip, it was possible to make real-space images of the plasmon waves with nanometer scale resolution (see Figure). For this experiment, a tapered graphene sheet was used where the variable width allowed for the observation of plasmon resonances defined by the standing plasmon wave between the edges. Similar to what happens with the standing waves on strings, only waves with appropriate characteristics can appear for a certain width.

Tuning the plasmon properties is a novel and unique aspect of graphene. This work, along with the work by Fei et al [2] (see 2Physics article of last week), shows not only the plasmon wavelength can be tuned over a wide range, it’s also possible to completely switch on and off the existence of the plasmons. In this way, it’s possible to electrically control light in a similar fashion as is traditionally achieved with electrons in a transistor. These capabilities, which until now were impossible with other existing plasmonic materials, enable new highly efficient nano-scale optical switches, which can perform calculations using light instead of electricity. In addition, the capability of trapping light in very small volumes could give rise to a new generation of nano-sensors, with applications in diverse areas such as medicine and bio-molecules, solar cells and light detectors, as well as quantum information processing.

Reference:
[1]  Jianing Chen, Michela Badioli, Pablo Alonso-González, Susokin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza, Nicolas Camara, Javier García de Abajo, Rainer Hillenbrand, Frank Koppens, "Optical nano-imaging of gate-tunable graphene plasmons", Nature, DOI: 10.1038/nature1125 (Published online June 20, 2012). Abstract. 
[2] Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, D. N. Basov, "Gate-tuning of graphene plasmons revealed by infrared nano-imaging", Nature, DOI:10.1038/nature11253 (published online June 20, 2012). Abstract. 2Physics Article.

Contributions and institutes:
• Optical nano-imaging: CIC nanoGUNE Consolider (San Sebastian, Spain), CFM-CSIC-UPV/EHU (San Sebastian, Spain), Neaspec GmbH (Martinsried, Germany), Ikerbasque (Bilbao, Spain)
• Graphene nano-photonics and optoelectronics: ICFO (Barcelona, Spain)
• Theory: IQFR-CSIC (Madrid, Spain)
• Graphene synthesis: Graphenea (San Sebastian, Spain) University of Tours (Tours, France), and CNM-IMB-CSIC (Barcelona, Spain)

Direct Imaging of Plasmons in Graphene

(From Left to Right) Zhe Fei, Aleksandr S. Rodin, Michael M. Fogler, Dimitri N. Basov

Authors:
Zhe Fei¹, Aleksandr S. Rodin², Michael M. Fogler¹, Dimitri N. Basov¹,

Affiliation:
¹Department of Physics, University of California, San Diego, USA
²Physics Department of Boston University, Boston, USA

Plasmonics is an emerging field, stemming from electronics and nanophotonics. At its heart lies plasmons—collective electronic oscillations where electrons in the system bunch up and spread out as a group. This rapidly developing technology enables localization and manipulation of electromagnetic energy at the nanometer length scale [1,2]. Applications of plasmonics are numerous and include such diverse examples as stained glass coloring, imaging nanoscopy, photovoltaics, metamaterials, optoelectronic devices, medical sensing and so on [3,4]. In addition, plasmonic technology holds promise for future applications in ultrafast information processing and transformation optics [5,6].

Plasmonic materials with high tunability and nano-scale confinement are required to fabricate high-speed devices with functionalities mimicking current state-of-the-art electronics. However, presently used plasmonic materials—noble metals and doped semiconductors—are not easily tunable. In fact, only limited plasmon tunablilties were reported before in sophisticated metal nanostructures or metamaterials. Graphene, on the other hand, is predicted to be capable of carrying gate-tunable surface plasmons in a wide frequency range with high confinement and low losses [7]. Recently, spectroscopic studies of graphene nanostructures verified the existence of plasmon resonances and their gate tunablilties in the terahertz and infrared frequencies [8,9]. Nevertheless, all the other essential properties of plasmons, such as confinement, damping, reflection, and interference, were still unknown.

Figure 1 Schematics of tip-launched plasmon waves close to the edge of graphene.

Our recent work [10], along with a similar one by Chen et al. [11], presents infrared nano-imaging results of graphene plasmons. In our experiments, we launched plasmons using a metalized tip illuminated by infrared light (Figure 1). The light polarizes the tip and this enables strong confined field close to the tip apex— the so-called “lighting-rod effect”. This electric field drives the electrons inside graphene back and forth, forming collective electronic oscillations known as plasmons. The plasmon waves propagate away like water ripples after throwing a stone into a pond [12].

Interestingly, we are able to detect plasmons using the same tip that produces them. While the tip launches plasmons, the plasmonic energy also enhances the polarization of the tip. The polarized tip is basically an antenna, so any enhancement in polarization of the tip will increase its far-field radiation that will be collected by a detector. Therefore, the signal we obtained is a direct measure of the plasmonic energy underneath the tip.

When the tip is close to the edge of graphene, the plasmons are able to travel to the edge, reflect from it, and, eventually, return back to the tip. In this case, the plasmonic energy underneath the tip is governed by both launched and reflected plasmon waves, which add up constructively when they are in phase or destructively when they are out of phase. Since their phase difference is solely determined by the distance between the tip and the edge of graphene, one would expect plasmonic energy underneath the tip to oscillate as the tip scans towards the edge. Final outcomes are images like Figure 2, where plasmon fringes parallel to the edge (or line defect) of graphene are observed. The distance between these fringes is well defined—exactly half the plasmon wavelength.

Figure 2: Imaging data that shows plasmonic fringe pattern close to the edge or line defect of graphene. The blue dashed line marks the edge of graphene. The green dashed line marks a line defect inside graphene. Scale bar, 100nm.

Figure 2 contains rich information about plasmon propagation, reflection, and interference. One can extract essential parameters, such as plasmon wavelength and damping rate, directly from it. We found that the plasmon wavelength of graphene is about 200 nm which is less than 2% of wavelength of incident light. Such strong confinement of electromagnetic energy hasn’t been achieved yet in infrared frequencies. The plasmon damping rate determined from Figure 2 is about 3 times higher than theoretical prediction. Detailed analysis of this observation sheds light to many-body effects in graphene.

Amazingly, the plasmon fringe pattern in Figure 2 evolves systematically when we tune the carrier density of graphene via gating. Instead of displaying all the images at different carrier densities, we show line profiles taken perpendicular to the plasmon fringes in these images as shown in Figure 3. One can see both the fringe width and amplitude increase with carrier density of graphene indicating that the plasmon wavelength and energy are tunable by gating.

Figure 3 A line profile taken perpendicular to the fringes in Figure 2 and its evolution with carrier density of graphene. Graphene is present at L>0 while SiO₂ is present at L<0.

Our work, together with the work by Chen et al., shows for the first time to the world vivid images of graphene plasmons, which give us comprehensive understanding of graphene plasmons in both fundamental and application aspects. Two open questions come up after this work: (1) How far can graphene plasmons propagate? Is there any fundamental limit? (2) What is the potential application of tip-launched plasmons? The authors will try to answer these questions in their future work.

References
[1] Harry A. Atwater, "The promise of plasmonics". Scientific American, 296, 56–62 (2007). Abstract.
[2] Jon. A. Schuller, Edward S. Barnard, Wenshan Cai, Young Chul Jun, Justin S. White, Mark L. Brongersma, "Plasmonics for extreme light concentration and manipulation". Nature Materials, 9, 193–204 (2010). Abstract.
[3] Mark I. Stockman, "Nanoplasmonics: the physics behind the applications". Physics Today, 64, 39–44 (2011). Abstract.
[4] S.A. Maier,  "Plasmonics: Fundamentals and Applications", Ch. 4 (Springer, 2007).
[5] Prashant Nagpal, Nathan C. Lindquist, Sang-Hyun Oh and David J. Norris, "Ultrasmooth patterned metals for plasmonics and metamaterials". Science 325, 594–597 (2009). Abstract.
[6] Surbhi Lal, Stephan Link, Naomi J. Halas, "Nano-optics from sensing to waveguiding". Nature Photonics, 1, 641–648 (2007). Abstract.
[7] Marinko Jablan, Hrvoje Buljan, Marin Soljačić, "Plasmonics in graphene at infrared frequencies". Physical Review B, 80, 245435 (2009). Abstract.
[8] Zhe Fei, Gregory O. Andreev, Wenzhong Bao, Lingfeng M. Zhang, Alexander S. McLeod, Chen Wang, Margaret K. Stewart, Zeng Zhao, Gerardo Dominguez, Mark Thiemens, Michael M. Fogler, Michael J. Tauber, Antonio H. Castro-Neto, Chun Ning Lau, Fritz Keilmann, Dimitri N. Basov, "Infrared nanoscopy of Dirac plasmons at the graphene-SiO₂ interface". Nano Letters, 11, 4701–4705 (2011). Abstract.
[9] Long Ju, Baisong Geng, Jason Horng, Caglar Girit, Michael Martin, Zhao Hao, Hans A. Bechtel, Xiaogan Liang, Alex Zettl, Y. Ron Shen, Feng Wang, "Graphene plasmonics for tunable terahertz metamaterials". Nature Nanotechnology, 6, 630–634 (2011). Abstract.
[10] Z. Fei, A. S. Rodin, G. O. Andreev, W. Bao, A. S. McLeod, M. Wagner, L. M. Zhang, Z. Zhao, M. Thiemens, G. Dominguez, M. M. Fogler, A. H. Castro Neto, C. N. Lau, F. Keilmann, D. N. Basov, "Gate-tuning of graphene plasmons revealed by infrared nano-imaging", Nature, doi:10.1038/nature11253 (published online June 20, 2012). Abstract.
[11] Jianing Chen, Michela Badioli, Pablo Alonso-González, Sukosin Thongrattanasiri, Florian Huth, Johann Osmond, Marko Spasenović, Alba Centeno, Amaia Pesquera, Philippe Godignon, Amaia Zurutuza Elorza, Nicolas Camara, F. Javier García de Abajo, Rainer Hillenbrand, Frank H. L. Koppens, "Optical nano-imaging of gate-tunable graphene plasmons". Nature, doi:10.1038/nature11254 (published online June 20, 2012). Abstract.
[12] Michael Dyakonov and Michael Shur, "Shallow water analogy for a ballistic field effect transistor: New mechanism of plasma wave generation by dc current". Physical Review Letters, 71, 2465–2468 (1993). Abstract.

Near-Infrared Metamaterials Go Beyond Metals

Gururaj V. Naik (left) and Alexandra Boltasseva (right)











Authors: Gururaj V. Naik and Alexandra Boltasseva 

Affiliation: School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, USA

Engineering the flow of light at the nanoscale is enabled by plasmonics and metamaterials. Research in metamaterials has progressed rapidly in the past decade, producing many breakthroughs that have changed our fundamental understanding of light propagation and interactions and pushed the frontiers of possible applications. The enormous potential of metamaterials is clogged by the limitations arising from the materials, particularly metals that constitute these metamaterials. These limitations of metal building blocks are particular detrimental to the operation of metamaterial devices in the optical range [1].

Past 2Physics article by Alexandra Boltasseva:
February 27, 2011: "New Materials Could Turn Near-Fantastic Devices like Invisibility Cloaks and Hyperlenses into Reality"
by Alexandra Boltasseva and Harry A. Atwater

Metals are the bottleneck of performance in many classes of optical metamaterials. The limitations arise from undesirable properties of metals such as high losses, large magnitude of permittivity, lack of tunability of optical properties, and challenges associated with nanofabrication and integration [2]. A possible alternative to metals that overcomes most of these problems is a semiconductor-based metal. It is well known that heavily doping semiconductors can exhibit metal-like optical properties. GaAs was demonstrated to work as a metal substitute in the mid-IR range when heavily doped (about 10^18-19 cm^-3) [3].

However, achieving metal-like optical properties in semiconductors in the near-infrared range is a tough challenge. The required very high doping (up to 10²¹ cm^-3) can hardly be accomplished in conventional semiconductors. However, some semiconductors such as zinc oxide allow ultra-high doping. Heavily doped zinc oxide for example aluminum-zinc-oxide (Al:ZnO ) belong to the class of materials called transparent conducting oxides (TCOs) that show metal-like optical properties in the near-infrared range [2].

Figure 1. Field map obtained from simulations showing negative refraction occurring in a metamaterial built by stacking sixteen alternating layers of Al:ZnO and ZnO. The incident beam is TM-polarized and impinges the sample at an angle 40 degrees away from normal incidence.

Recently, we showed that Al:ZnO can be utilized as a metal substitute in a near-infrared metamaterial device and demonstrated negative refraction in this device [4]. The device consisted a stack of sixteen alternating layers of ZnO and Al:ZnO. The thickness of each layer was much smaller than the incident wavelength. Such a metamaterial produces extreme anisotropy in its dispersion, which can lead to negative refraction of the incident light. Simulations showed that the light should bend on the ‘wrong’ side of the sample normal for TM-polarized incident light. An experimental set-up was built to verify this phenomenon. The transmittance of light through the sample was measured with a blade blocking half of the transmitted beam. When negative refraction occurred, the beam shifted such that more of the beam was blocked by the blade, which led to a dip in the transmitted light intensity. This observation not only confirmed negative refraction, but it also allowed us to assess the performance of this metamaterial. We found that the performance of this metamaterial device is three orders of magnitude higher than metal-based designs.

Figure 2. a) The experiment schematic used to observe negative refraction. A blade blocks the transmitted beam partially such that the lateral shift of the beam due to refraction modulates the intensity of unblocked portion of the beam. b) The relative transmittance measured for different angles of incidence from the Al:ZnO/ZnO metamaterial. In the wavelength range 1.8-2.4 μm, the metamaterial shows negative refraction, which results in the dips in the curves.

The demonstration of a metal-free plasmonic metamaterial in the near-infrared range with super-high performance is a technologically important step. The transition from metals to doped semiconductor materials enables the efficient and practical implementation of metamaterial devices for applications such as light concentrators for solar cells, optical invisibility cloaks and super-resolution lenses. This demonstration heralds the field of metal-free optical metamaterials.

References:
[1] A. Boltasseva and H. A. Atwater, "Low-loss plasmonic metamaterials," Science 331, 290-291 (2011). Abstract. 2Physics Article.
[2] Gururaj V. Naik, Jongbum Kim and Alexandra Boltasseva, “Oxides and nitrides as alternative plasmonic materials in the optical range,” Optical Material Express, 1, 1090-1099 (2011). Abstract.
[3] Anthony J. Hoffman, Leonid Alekseyev, Scott S. Howard, Kale J. Franz, Dan Wasserman, Viktor A. Podolskiy, Evgenii E. Narimanov, Deborah L. Sivco & Claire Gmachl, “Negative refraction in semiconductor metamaterials,” Nature Materials, 6, 946-950 (2007). Abstract.
[4] Gururaj V. Naik, Jingjing Liua, Alexander V. Kildishev, Vladimir M. Shalaev and Alexandra Boltasseva, “Demonstration of Al:ZnO as a plasmonic component of near-infrared metamaterials,” Proceedings of the National Academy of Sciences of the United States of America,(published online May 16, 2012) DOI: 10.1073/pnas.112151710. Abstract.

Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices

Authors: Vincent Mourik¹, Kun Zuo¹, Sergey Frolov¹, Sébastien Plissard², Erik Bakkers^1,2, Leo Kouwenhoven¹

Affiliation:
¹Kavli Institute of Nanoscience, Delft University of Technology, Netherlands.
²Dept of Applied Physics, Eindhoven University of Technology, Netherlands.

Particle Predictors:
Paul Dirac was the very first particle predictor. In 1927, Dirac developed a formula that linked two new theories: Einstein’s special theory of relativity and quantum mechanics. Dirac’s equation, however, had several solutions. The first solution described the familiar electron: a particle with a negative charge holding a certain amount of positive energy. Another solution actually constituted its very opposite: a positively charged particle holding a certain amount of negative energy. Rather than ignoring the contradiction raised by his additional solution, Dirac surmised that there must be a particle in nature with a positive electrical charge and negative energy [1]. This particle therefore has properties that would exactly mirror the properties of an electron. Several years later, this particle was indeed found, and was named positron. Together, the electron and the positron form a particle and antiparticle pair.

A pure genius, Paul Dirac was utterly convinced of the veracity of his formula. If his equation offered a certain solution, then a corresponding particle simply had to exist in nature. Since that time, numerous other particles have been predicted and identified this way. For example, the ongoing search for the Higgs boson is set up just like that, based on a prediction from the Standard Model.

Ettore Majorana was a physicist and a contemporary of Dirac. Majorana had an enigmatic biography that formed the topic of many books and films in Italy. At some point in the 1930s, Majorana was playing around with Dirac’s equation and after slightly modifying it he found a new solution: a particle that is identical to its antiparticle. And something can only be identical to its counterpart if it has properties that are all zero. Ettore Majorana, too, had a firm belief in formulas and in 1937 he published a paper [2] predicting his new particle, which has since become known as the Majorana fermion.

For decades the Majorana particle received little attention, but in the 1970s the search began afresh. Using large accelerators and detectors, scientists started hunting for neutrino particles with Majorana properties. Indeed, these elementary Majorana particles might even solve the mystery of dark matter that fills our Universe. So far, the elementary Majorana particles have remained elusive, but this important quest is still being pursued by CERN in Geneva.

Particle Creators:

In addition to elementary particles, composite or collective particles (see box) also exist in the world of condensed matter physics. We know of heat particles (phonons), electron density waves (plasmons), magnetic waves (magnons) and a long list of other collective particles. These collective particles are particularly convenient for making the physics of materials a lot simpler. Materials hold a distinct place in physics because by combining materials we can create objects that did not exist before. Technology, for instance, abounds with remarkable material combinations, such as silicon and silicon oxide forming the backbone of electronics. But material combinations can also be used in fundamental physics to create something new. This prompted a number of theoretical physicists to reflect on whether we could combine materials in such a way that the collective particles inside them will acquire the properties of Majorana fermions.

The one-dimensional lattice proposed by Alexei Kitaev in 2001 was still highly mathematical and abstract [3]. A number of propositions then followed based on (p-wave superconducting) materials that did not yet exist. In 2008, Liang Fu and Charles Kane’s theory [4] was the first to be based on existing materials, but was still difficult to put into practice. The year 2010 saw the publication, in Physical Review Letters of two similar theories by two groups of theorists, independently of each other, which for the first time looked feasible in practice. One of the publications [5] came from theorists at the University of Maryland (Roman Lutchyn, Jay Sau and Sankar Das Sarma); the other [6] was a collaborative effort between theorists at the Weizmann Institute in Israel, California Institute of Technology of USA and the Free University of Berlin in Germany (Yuval Oreg, Gil Rafael and Felix von Oppen). The importance of the aforementioned theoretical developments was that it shifted the focus from what is found in nature to the artificial creation of Majorana particles.

Prior knowledge: there are particles and then there are particles…

If you blow into your hand, what you will primarily feel are oxygen and nitrogen molecules. Those molecules are minute, subnanometer-scale particles that are composed, in this case, of two atoms each. In turn, each atom is made up of an atomic nucleus encircled by electrons. The electrons cannot be divided into smaller particles - they are ‘elementary’ particles. However, the protons and neutrons inside the nucleus can be shattered to create even smaller particles. This shattering process is generated in accelerators such as the one at CERN in Geneva, where the search for the Higgs particle continues unabated. Other popular particles are the neutrinos (which for a short period were believed to travel even faster than light) and the Majorana fermions. These Majorana fermions have not yet been found at CERN.The Majorana fermions may well be the key to explaining the dark matter mystery. In the universe, there is five times as much dark matter as ordinary matter, and so Majorana fermions could be the most widespread particles in the universe.

CERN are engaged in the study of fundamental particles. Each of these particles are smaller than the smallest atom, hydrogen. Our world of matter is based on atoms and clusters of atoms that form molecules. The glue that binds these atoms to molecules is described by quantum mechanics. Our bodies, for instance, are chemical factories in which atoms are stuck together with quantum glue. Apart from complex biological materials, there are also crystals that frequently hold the same atoms stacked inside a grid. Even the smallest materials may contain large numbers of atoms. For example, a nanowire with a diameter of 100 nm and a length of 1000 nm (1000 nm = 1 micrometer) alone contains some 10 billion atoms.

Next to fundamental particles and atoms there are also collective particles. The ‘wave’ in a stadium is a good example. The ‘wave’ is simply a group of spectators jumping up and down to create a wave. If we wanted to describe this wave in mathematical terms, we might do that by including everyone in a large formula. Then again, we could also approach it more simply by forgetting about all those individuals and only describe their collective behavior, that is, the wave. And for simplicity’s sake we could call the ‘wave’ a particle, in this case, a collective particle. This reduction to collective particles simplifies matters enormously and is often highly successful. An example: heat in a material is not described in terms of a bunch of vibrating atoms but, far more easily, as heat particles that are known as phonons.

You may think that ‘collective particles’ is a rather imprecise way of describing what actually happens. This may be true of the wave but phonons, for example, can in fact provide us with a very exact, realistic description. What is perhaps the most surprising fact is that collective particles can actually behave in accordance with the laws of quantum mechanics. A phonon can find itself in the superposition of both hot and cold. Such a quantum superposition may sound absurd enough for elementary particles, but is really stretching our imagination where collective particles are concerned.

The Majorana fermions in crystals are not only interesting from a fundamental viewpoint, but also have unique properties that can be used to build a quantum computer. Field medalist Michael Freedman works at Microsoft and has been carrying out active research into topological quantum computers with a team of scientists since 2005. This computer works by moving Majorana particles around each other and forming space-time braids.

The proposals put forward by Lutchyn et al [5] and Oreg et al [6] are both based on bringing semiconducting nanowires into contact with a superconducting material. We had already successfully accomplished this combination in Delft, which resulted in publications in Science (2005) and Nature (2006). Combining these specific materials suddenly made us the experimental specialists in the search for Majorana fermions.

Note that the Majorana quest had already been described at an early stage in the journal 'Science' [7].

Majorana in Delft:

How do you create a Majorana fermion? Based on the condition that the particle is identical to its antiparticle, you can do some reverse engineering. It cannot, for instance, have an electrical charge, nor have energy or spin. The theoretical proposals argue that those properties are created by combining materials consisting of a superconductor with a special semiconductor that has strong spin-orbit coupling. This semiconductor should take the form of a one-dimensional nanowire. If a magnetic field is also applied, the Majorana fermion should appear at low temperatures, just above absolute zero temperature. We combined these materials on a microchip. We developed InSb (Indium Antimonide) nanowires for the semiconductor. InSb has strong spin-orbit coupling. We used a Nb alloy as a superconductor. This material will retain its superconductive properties also in the presence of an external magnetic field. For this material we were granted permission to use the technology available in Teun Klapwijk’s group in Delft. Using nanotechnology, we produced an electronic chip that, admittedly, looks rather messy (top right). Zooming down to sub-micrometre scale, we can see the nanowire thread and the electrical contacts (below right).

In this device, the superconductor is larger than the semiconductor. The diameter of the nanowire is so small that it actually becomes a one-dimensional conductor. A portion of the superconductor is covering the nanowire, which causes the superconductivity to leak into the semiconductor, effectively creating a one-dimensional superconductor. These do not exist in nature but can be induced this way. The strong spin-orbit coupling in the InSb nanowire makes this one-dimensional superconductor particularly unique. It has a so-called p-symmetry, which again has also not been found in nature. This p-superconductor extends across the entire section where the nanowire is in contact with the superconductor. At the end points, where the p-superconductor ends, two Majorana fermions appear, one on each end point.

Image: The microchip used with three different Majorana devices. This chip is cooled down to almost absolute zero point (-273 degrees Celsius). The electrical wires are connected to measuring equipment at room temperature.
 
We can measure the Majorana fermions in the electrical conductivity. From the gold contact we send electrons into the nanowire, towards the lower Majorana fermion. Only when we send electrons inside with precisely zero energy can we measure a current. If we add voltage to the electrons to energise them further, they are reflected at the p-superconductor and we measure zero conductivity. The presence of the Majorana fermion in our system is therefore visible as a conductance peak at a voltage that is precisely zero.

In the 'Science' publication [8] we also included various control experiments, which demonstrate that each single ingredient from the original theory is essential for this observation. The results can only be interpreted if we assume the presence of Majorana fermions. The article is published online on 12 April in 'Science Express' [8].

Image: The nanowire, shown vertically in this photo, is lying flat on a substrate. Hidden in the substrate are different gate electrodes (the horizontal ‘stripes’ below the nanowire and the contacts), which can change the conductivity of the nanowire. The lower electrical contact to the nanowire is madefrom gold, a normal conductor. The contact on the top is covering half of the nanowire. This is the superconductor. The total length of the nanowire is three micrometres. The anticipated positions of two Majorana fermions are indicated with red stars. 

We have since been carrying out new experiments. As the title of our article ‘Signatures of …’ suggests, we also want to demonstrate other unique properties of Majorana fermions. And our Majorana fermions are literally one of a kind. Nature has two types of particles: fermions (such as electrons, positrons, neutrons, etc.) and bosons (photons, Higgs particles, phonons, etc.). Our Majorana particles are likely to have other properties than fermions and bosons. In terms of physics, their behaviour is described by non-Abelian statistics. If we can demonstrate these statistics in our new experiments, we add a completely new chapter to the book of physics. This new round of experiments is based on a highly theoretical approach using new concepts that are not yet quite understood. To translate abstract concepts into experiments we are working with Carlo Beenakker’s theory group from Leiden. The non-Abelian statistics also make Majorana particles useful for a topological quantum computer.

References:
[1] P. A. M. Dirac, "The Quantum Theory of the Electron". Proceedings of the Royal Society of London: Series A 117, 610–624 (1928). Full Article.
[2] Ettore Majorana, "Teoria simmetrica dell’elettrone e del positrone", Il Nuovo Cimento, 171 (1937). Abstract. 
[3] A. Yu. Kitaev, "Unpaired Majorana fermions in quantum wires". Physics Uspekhi, 44, 131 (2001). Full Article. 
[4] Liang Fu and Charles Kane, "Superconducting Proximity Effect and Majorana Fermions at the Surface of a Topological Insulator", Phys. Rev. Lett. 100, 096407 (2008). Abstract.   
[5] Roman M. Lutchyn, Jay D. Sau and Sankar Das Sarma, "Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures", Physical Review Letters, 105, 077001 (2010). Abstract.
[6] Yuval Oreg, Gil Refael, and Felix von Oppen, "Helical Liquids and Majorana Bound States in Quantum Wires", Physical Review Letters, 105, 177002 (2010). Abstract.
[7] Robert F. Service, "Search for Majorana Fermions Nearing Success at Last?", Science, 332, 193 (2011). Abstract.
[8] V. Mourik, K. Zuo, S.M. Frolov, S.R. Plissard, E.P.A.M. Bakkers, L.P. Kouwenhoven, "Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices", Science Express, DOI: 10.1126/science.1222360 (Published Online April 12 2012). Abstract.

Strong Coupling and Its Dynamic Control of Distant Nanocavities

Susumu Noda (left) and Yoshiya Sato (right)



Authors: Susumu Noda and Yoshiya Sato

Affiliation: Department of Electronic Science and Engineering, Kyoto University, Japan

Noda's Quantum Optoelectronics Laboratory >>

The dynamic manipulation of photons in nanostructures is essential for various applications including advanced photonic circuits, stopping light, and quantum information processing. In particular, the formation and dynamic control of a coupled state among on-chip, photonic nanoelements at arbitrary positions should have great impact on these directions. However, when we couple individual nanocavities, they must be placed in close proximity since the light is confined so tightly in each cavity that their evanescent fields extends only a few microns. With this limitation, a nanocavity can only couple to adjacent nanocavities, which restricts the architecture of the system and makes it difficult to achieve on-demand dynamic control of the coupled states.

In our recent work, we have obtained strong coupling between nanocavities even though they are separated by a large distance (> 80 μm) and achieve dynamic control over their coupling to freeze the photon state on demand. Photonic states can now be separated without being isolated, opening the door to the development of advanced functional photonic circuits in scalable classical and quantum information processing. These results have recently been published in the journal "Nature Photonics"[1]





















Fig. 1.a, Schematic model of two indirectly coupled photonic nanocavities through a waveguide with reflecting boundary walls. b, The resonant spectrum of the isolated individual nanocavities A and B (red line) and their resonant spectrum when coupled to an open waveguide (dashed green line). The green line spectrum has a line width of δ_in, which corresponds to the coupling bandwidth between the nanocavities and the waveguide. c, The resonant spectrum of the bounded waveguide discretized by Fabry-Perot (FP) resonant effect.

At first, we discuss how to realize strong coupling between distant nanocavities. We employ a system as shown in Fig. 1. Two nanocavities (A and B) are connected by a waveguide, which is terminated on both sides by reflecting walls (C and D). The individual nanocavities each have a single resonant mode with the same frequency (see Fig. 1b, red line). The bounded waveguide has many standing wave modes due to Fabry-Perot (FP) resonance (see Fig. 1c). To realize strong cavity-cavity coupling, we theoretically investigated the system in detail and found that all FP waveguide modes should be detuned far from the nanocavity modes (by more than the coupling bandwidth δ_in between the nanocavities and the waveguide) as shown in Fig. 1b and c. Even under such a condition, the nanocavities can still couple to each other indirectly through a forced oscillation of the FP waveguide modes, while concentrating photons in either nanocavity, not the waveguide.

Fig. 2. a, Overall view of the fabricated silicon-based photonic crystal observed by SEM. Two photonic crystal nanocavities (A & B) are placed 202a apart with a line defect waveguide nearby. b, Magnified image of a nanocavity. This is based on a multi-step hetero structure with a1 of 415 nm and a2 of 420 nm. c, Magnified image of the waveguide and a partial reflector. d, e Spectra of the vertically emitted light observed at cavities A (d) and B (e) respectively, obtained by introducing a tunable continuous-wave laser through partial reflector C. f, Time resolved amplitude of vertically emitted light from cavities A & B. A pulse laser with duration of 4 ps and a centre wavelength of ~ 1539.45 nm (width = 1 nm) was introduced through the partial reflector C, and vertically emitted light from each cavity was observed in the time domain by a cross-correlation method.

According to this scheme, we fabricated a silicon-based photonic crystal sample as shown in Fig. 2a-c. Two multistep-hetero nanocavities[2], A and B, with original Q factors of ~ 1 million, were placed 83µm apart, with a line defect waveguide nearby. Both ends of the waveguide were bound by partial reflectors (C and D), formed by narrowing the waveguide’s width. Figure 2d, e show resonant spectra of the fabricated sample observed from nanocavity A and B, respectively. Two resonant peaks with similar intensities were observed from both cavities with exactly the same wavelengths (1539.39 and 1539.54 nm). These peaks correspond to the coupled nanocavity modes. The splitting of the peaks (150 pm) is 50 times larger than the resonant peak’s width (~3pm), indicating that the system is within the strong coupling regime.

Next, we carried out time domain measurements. We excited the coupled cavity modes by introducing a short optical pulse through partial reflector C. The results are shown in Fig. 2f. Clear exchange of photons between the distant nanocavities was observed with a period of ~54 ps. This exchange is seen to continue more than 400 ps, demonstrating the long coherence time of photons in this system. Note that the photon lifetime is much larger than that of the FP waveguide modes (~40ps). This indicates that the photons are predominantly concentrated in the nanocavities rather than in the waveguide when the coupled nanocavity modes are excited.























Fig. 3. Dynamic control of the coupling state between the nanocavities. a, Schematic of the experimental set-up. A control pulse is irradiated into cavity B, causing a dynamic wavelength shift by the carrier plasma effect. b, Time-resolved amplitude of emitted light from cavities A and B, where the control pulse was irradiated into cavity B when the photons populated only cavity A.

The next important step was to demonstrate dynamic control over these coupling states. Because the nanocavities are sufficiently far apart, it is possible to induce a dynamic change in either cavity, completely independently, to control the coupling state. Here, we attempted to induce a wavelength shift of cavity B using a control pulse with duration of 4 ps, and a wavelength of 770 nm at a specified time during the photon exchange. (See Fig. 3a) The control pulse is absorbed by the cavity, generating free carriers, which lowers the refractive index and induces a blueshift of the resonant wavelength [3]. This blueshift cuts off the coupling between cavities A and B, and this decoupled state continues for ~1 ns due to the long carrier lifetime in silicon. Figure 3b shows the results obtained for the control pulse irradiation. The control pulse is irradiated onto the cavity B with the photons populating only cavity A. This figure clearly shows that the photon exchange was stopped successfully, and froze the photon population in the state at the moment of control pulse irradiation. This finding suggests that the behaviour of photons can be controlled even in regions where they are not present which therefore enables remote control of photons. We have also observed similar phenomena even at single photon power levels.

The results obtained in this work are expected to be applicable to various nanophotonic circuits that require distant coupling of on-chip cavities and will become a fundamental building block for areas including the stopping (or slowing) of light and even photonic quantum information processing.

References
[1] Y. Sato, Y. Tanaka, J. Upham, Y. Takahashi, T. Asano, and S. Noda “Strong coupling between distant photonic nanocavities and its dynamic control”. Nature Photonics. 6, 56-61 (2012). Abstract.
[2] Yasushi Takahashi, Yoshinori Tanaka, Hiroyuki Hagino, Tomoyuki Sugiya, Yoshiya Sato, Takashi Asano, and Susumu Noda, "Design and demonstration of high-Q photonic heterostructure nanocavities suitable for integration". Optics Express. 17, 18093-18102 (2009). Abstract.
[3] Yoshinori Tanaka, Jeremy Upham, Takushi Nagashima, Tomoaki Sugiya, Takashi Asano & Susumu Noda, "Dynamic control of the Q factor in a photonic crystal nanocavity". Nature Materials, 6, 862-865 (2007). Abstract.

Nanorods that can Self-assemble into 1-, 2- and even 3-dimensional Microstructures

(From left to right) Ting Xu, Kari Thorkelsson and Peter Ercius (Photo by Roy Kaltschmidt, Lawrence Berkeley National Laboratory)

A relatively fast, easy and inexpensive technique for inducing nanorods – rod-shaped semiconductor nanocrystals – to self-assemble into one-, two- and even three-dimensional macroscopic structures has been developed by a team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab). This technique should enable more effective use of nanorods in solar cells, magnetic storage devices and sensors. It should also help boost the electrical and mechanical properties of nanorod-polymer composites.

Leading this project was Ting Xu, a polymer scientist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and the University of California (UC) Berkeley’s Departments of Materials Sciences and Engineering, and Chemistry. Xu and her research group used block copolymers – long sequences or “blocks” of one type of monomer bound to blocks of another type of monomer – as a platform to guide the self-assembly of nanorods into complex structures and hierarchical patterns. Block copolymers have an innate ability to self-assemble into well-defined arrays of nano-sized structures over macroscopic distances.

“Ours is a simple and versatile technique for controlling the orientation of nanorods within block copolymers,” Xu says. “By varying the morphology of the block copolymers and the chemical nature of the nanorods, we can provide the controlled self-assembly in nanorods and nanorod-based nanocomposites that is critical for their use in the fabrication of optical and electronic devices.”

Transmission electron micrograph (a) shows cadmium sulfide nanorods forming arrays that are aligned and oriented parallel to the cylindrical microdomains of block copolymers. Schematic drawing (b) illustrates copolymers with nanorods.

Xu is the corresponding author of a paper [1] describing this research that has been published in the journal Nano Letters under the title “Direct Nanorod Assembly Using Block Copolymer-Based Supramolecules.” Co-authoring the paper were Kari Thorkelsson, Alexander Mastroianni and Peter Ercius.

Nanorods – particles of matter a thousand times smaller than the stuff of today’s microtechnologies – display highly coveted optical, electronic and other properties not found in macroscopic materials. To fully realize their vast technological promise, however, nanorods must be able to assemble themselves into complex structures and hierarchical patterns, similar to what nature routinely accomplishes with proteins.

Xu and her research group first enlisted block copolymers as allies in this self-assembly effort in 2009, working with the spherical nanoparticles commonly known as quantum dots. In that study, they lashed quantum dots to block copolymers via a “mediator” of small adhesive molecules. In this latest development, Xu and her group again made use of adhesive molecules, but this time to mediate between the nanorods and supramolecules of block copolymers. A supramolecule is a group of molecules that act as a single molecule able to perform a specific set of functions.

“Block copolymer supramolecules self-assemble and form a wide range of morphologies that feature microdomains typically a few to tens of nanometers in size,” Xu says. “As their size is comparable to that of nanoparticles, the microdomains of block copolymer supramolecules provide an ideal structural framework for the co-assembly of nanorods.”

Xu and her group incorporate nanorods into solutions of block copolymer supramolecules that form spherical, cylindrical and lamellar microdomains. During the drying process energy is contributed to the system from the interactions between nanorod ligands and polymers, the entropy associated with polymer chain deformation upon nanorod incorporation, and the interactions between individual nanorods. Xu and her group observed that these energetic contributions determine the placement and distribution of the nanorods, as well as the overall morphology of the nanorod-block copolymer composites. These energetic contributions can be easily tuned by varying the supramolecular morphology, which is accomplished simply by attaching different types of small molecules to the side chains of the block copolymers.

“We can readily access a wide library of nanorod assemblies including arrays of nanorods aligned parallel to block copolymer cylindrical microdomains, continuous nanorod networks, and nanorod clusters,” Xu says. “Since the macroscopic alignment of block copolymer microdomains can be obtained in bulk and in thin films by the application of external fields, our technique should open up a viable route to manipulate the macroscopic alignments of nanorods.”









This TEM tomography reconstruction of cadmium sulfide nanorods that self-assembled within block copolymers shows their ordered macroscopic alignment.

This new technique can produce ordered arrays of nanorods that are macroscopically aligned with tunable distances between individual rods – a morphology that lends itself to the production of plasmonics, which are materials that hold great promise for superfast computers, ultra-powerful optical microscopes, and even the creation of invisibility carpets. It is also a straightforward nanoparticle self-assembly technique that can produce a continuous network of nanorods with nanoscopic separation distances. Such networks can enhance the macroscopic properties of nanocomposites, including electrical conductivity and material strength.

Xu credits much of the success of this research to the exceptional capabilities and staff at the National Center for Electron Microscopy (NCEM), a DOE national user facility at Berkeley Lab, which is home to the world’s most powerful electron microscopes.

“For the study of three-dimensional nanorod assemblies, we needed to implement high-resolution tomography and this posed a challenge not only for collecting the imaging data but also for processing it,” Xu says. “The expertise and skill of NCEM’s Peter Ercius was invaluable.”

Most of this work was done on NCEM’s 200kV monochromated UT Tecnai microscope, an instrument designed to produce optimum high resolution performance in both transmission and scanning transmission electron microscopy modes.

Xu and her group are now investigating the self-assembly of semiconductor nanocrystals that take the shapes of cubes or tetrapods, both of which have important potential applications for photovoltaic and other technologies.

“We’d also like to investigate the self-assembly of nanoparticles into combinations of different shapes,” Xu says.

Reference:
[1] Kari Thorkelsson, Alexander J. Mastroianni, Peter Ercius, and Ting Xu, "Direct Nanorod Assembly Using Block Copolymer-Based Supramolecules", Nano Latters, vol. 12, pp 498–504 (2012). DOI: 10.1021/nl2040089. Abstract.

[This article is written by Lynn Yarris of Lawrence Berkeley National Laboratory]