Metaflex: Flexible Metamaterial at Visible Wavelengths

A. Di Falco (left) and T. F. Krauss

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

Authors: Andrea Di Falco and Thomas F. Krauss

Affiliation: School of Physics and Astronomy, Univ. of St Andrews, UK

Andrea Di Falco, Martin Plöschner and Thomas Krauss of the School of Physics and Astronomy of the Scottish University of St Andrews, in an article published by the New Journal of Physics [1], have recently reported on the fabrication of a key building block for flexible metamaterials for visible light, Metaflex.

Figure 1: Artist's impression of the Metaflex concept. The green sphere is made invisible and not reflected by the mirror.

Metamaterials have engineered properties that are not available with naturally occuring materials. For example, they can exhibit negative refraction, which means that light refracts in the opposite direction to the one we are used to. They can also be used to build superlenses, which are lenses that can form images with “unlimited” resolution, well beyond the diffraction limit and invisibility cloaks that can guide light around an object as if it did not exist. For these effects to take place, the smallest building blocks of metamaterials, called “meta-atoms”, have to be much smaller than the wavelength of the incident light. Therefore, at visible wavelengths, which are typically 400-600 nanometres, the meta-atoms have to be in the range of few tens of nanometers. For this reason, researchers have to employ the sophisticated techniques developed in the semiconductor industry, i.e. the same techniques that are used to densely pack the semiconductor circuits that are required in modern computer processors. As a result, most metamaterials are realised on flat and rigid substrates, which limits the range of applications that can be accessed.

The work carried out at St Andrews overcomes this limitation by demonstrating metamaterials on flexible substrates. This achievement can almost be understood as a transition from the hard and rigid “stone-age” of nanophotonics to a modern age marked by flexibility [2,3]. While some examples of stretchable and deformable metamaterials have previously appeared [4-6], the St Andrews researchers were the first to demonstrate such flexible metamaterials at visible wavelengths.

Metaflex consists of very thin, and self-supporting polymer membranes. The metamaterial property arises from an array of gold nanostructures that are resonant in the visible range. In particular, Di Falco et al. have “written” a nanometer sized gold fishnet pattern (in an area of few mm²), which interacts with light at a wavelength of 630 nm, i.e. the wavelength of red light. Because metaflex is so thin, multiple layers can be stacked together as well as wrapped around an object. Such multilayer metaflex will be demonstrated as the next step, which will allow the demonstration of more complex behaviors such as negative refraction in flexible substrates at optical wavelengths.

Metaflex is also a useful tool for exploring the paradigm of Transformation Optics, which is the concept behind the ideas of invisibility cloaks that are so inspiring [7]. Transformation Optics requires materials with “designer” refractive properties that go far beyond those available with natural materials, so are ideally suited to the application of metamaterials; flexibility then adds a key ingredient. Metaflex, being supple and modifiable, is the natural choice for applications where, for example, a curved geometry is required.

Figure 2 : A layer of Metaflex placed on a disposable contact lens to show its potential use in visual prostheses.

In addition to enabling such exciting ideas as invisibility cloaks, metaflex offers more immediately feasible and practical applications such as enhanced visual prostheses, whereby the designer refractive properties can be used to improve the performance of everyday objects such as contact lenses.



References
[1] Andrea Di Falco, Martin Ploschner and Thomas F Krauss, "Flexible metamaterials at visible wavelengths", New Journal of Physics, vol.12, 113006 (2010). Abstract.
[2] John A. Rogers, Takao Someya and Yonggang Huang, "Materials and Mechanics for Stretchable Electronics", Science, vol.327, 1603 (2010). Abstract.
[3] I. Park, S. H. Ko, H. Pan, C. P. Grigoropoulos, A. P. Pisano, J. M. J. Fréchet, E.-S. Lee, J.-H. Jeong, "Nanoscale patterning and electronics on flexible substrate by direct nanoimprinting of metallic nanoparticles", Advanced Materials, vol. 20, 489 (2008). Abstract.
[4] Hu Tao, A. C. Strikwerda, K. Fan, W. J. Padilla, X. Zhang and R. D. Averitt, "Reconfigurable Terahertz Metamaterials", Phys. Rev. Lett., vol. 103, 147401 (2009). Abstract.
[5] H.O. Moser, L.K. Jian, H.S. Chen, M. Bahou, S.M.P. Kalaiselvi, S. Virasawmy, S.M. Maniam, X.X. Cheng, S.P. Heussler, Shahrain bin Mahmood, and B.-I. Wu, "All-metal self-supported THz metamaterial - the meta-foil", Opt Express (2009) vol. 17, 23914 (2009). Abstract.
[6] Imogen M. Pryce, Koray Aydin, Yousif A. Kelaita, Ryan M. Briggs, and Harry A. Atwater, "Highly Strained Compliant Optical Metamaterials with Large Frequency Tunability", Nano Lett., vol. 10, 4222 (2010). Abstract.
[7] Ulf Leonhardt and Thomas Philbin, "Geometry and Light: The Science of Invisibility" (Mineola, NY: Dover, 2010)

Optical Nano-antenna Controls Single Quantum Dot Emission

Niek F. van Hulst
[This is an invited article based on recent works by the author and his collaborators -- 2Physics.com]

Author: Niek F. van Hulst
Affiliation: ICFO – Institute of Photonic Sciences, 08860 Castelldefels - Barcelona, Spain.
ICREA – Institució Catalana de Recerca i Estudis Avançats, 08015 Barcelona, Spain.

Can one imagine a TV-antenna to send a beam of light? Yes, nanoscale TV-antennas have now been fabricated and brought into action to steer and brighten up the light of molecules and quantum dots by researchers at ICFO – the Institute of Photonic Sciences, in Barcelona, Spain. The achievement was reported in the Science issue of 20 August 2010 [1].

Everywhere. Antennas are all around in our modern wireless society: they are the front-ends in satellites, cell-phones, laptops, etc., that establish the communication by sending and receiving signals, typically MHz-GHz. Characteristic for any town is the chaotic forest of TV antennas covering roofs: metal bar constructions forming sub-wavelength structures, optimized to receive (or send) directional electro-magnetic fields with the wavelengths of the TV/radio signal.

Scaling. Can the proven antenna technology be scaled up towards the optical domain, i.e. from some 100 MHz towards typically a million times higher frequency of around 500 THz? Inevitably, this implies scaling down to a million times smaller structures, with dimensions of typically 100 nm, requiring nanofabrication accuracy down to a few nm. Moreover metals at optical frequencies are far from ideal, very dispersive and usually lossy. These are definite challenges in scaling antennas towards visible light, but the promise is clear: light, despite its submicron wavelength, is conventionally guided by rather bulky elements, such as lenses, mirrors and optical fibres. Optical antennas hold the promise to realize optical logics on truly sub-wavelength scale, comparable to the scale of electronic integrated circuitry [2]. Indeed this has motivated the exploration of modern nanofabrication methods, such as focussed electron and ion beams, to fabricate nanostructures and antennas with optical resonances [3, 4].

Bright quantum emitters. Yet beyond scaling, optical antennas offer a more fundamental advantage. Conventional antennas are connected to electronic circuitry by wires, impedance matched, to afford efficient communication between the local circuit and a certain far field directional signal. What about optical frequency electronic circuitry? Optical sources and detectors are atoms, molecules, quantum dots: quantum systems. Thus hooking up an atom to an antenna (resonant with the atom) does “impedance match” the atom to the surrounding vacuum. The result is an improved emitter or receiver with optimized communication between the localized near-field and the far field: a bright quantum emitter, or an efficient absorber. Indeed fluorescent molecules close to metallic nanoparticles do show enhanced signal and faster radiative decay rate [5].

Quantum emitter @ TV-antenna. With all potential advantages clearly in mind we decided to focus on the icon of optimized antenna technology, the TV-antenna, and strive for interfacing to the quantum world; thus obtaining full control on a directed bright quantum emitter. The “TV-antenna” is actually called Yagi-Uda antenna after the design of Hidetsugu Yagi and Shintaro Uda at Tohoku University in Japan in 1926 and was first widely used in the 2nd world war radar systems. The multi-element Yagi-Uda antenna is made of parallel metallic bars: a central half-wavelength dipole bar acts as the active “feed” element for emission or collection; the surrounding passive elements act as reflector and directors. As result the Yagi-Uda antenna has strongly unidirectional gain profile. This is why a TV-antenna on a roof has to be mounted with the right direction to catch signal. In recent years optical “Yagi’s” have been proposed, simulated [6] and in 2010 the first directional scattering of red light on an array of Yagi-Uda antennas was presented (fittingly) by a Japanese group [7]. In parallel, the interfacing of a quantum emitter to such optical Yagi-Uda antenna, to achieve active control of the direction of light emission, has been theoretically predicted [8, 9]. Now, can one do this in practise? In 2008 we achieved first encouraging results in observing the redirection of the dipolar photon emission pattern of a single molecule by scanning a resonant monopole antenna probe in its direct proximity [10].

Scanning electron microscopy (SEM) image of a five-element Yagi-Uda antenna consisting of a feed element, one reflector, and three directors, fabricated by e-beam lithography. Overall dimension of the antenna is 800 nm, equal to the wavelength of operation. A quantum dot is attached to one end of the feed element.

Getting it right. The final realization of our idea required a real team effort of ICFO researchers, involving both the research groups of Niek van Hulst and Romain Quidant. First Tim Taminiau designed a 5-element gold Yagi antenna, resonant enough to the red, around 800 nm, such that the elements still act as efficient radiation sources, while at the same time providing spectral overlap with the luminescence of CdSeTe quantum dots. Next, to drive the antenna by a quantum dot, it is essential to position the quantum dot at a high field point of the ~140 nm feed element. Giorgio Volpe and Mark Kreuzer developed double e-beam lithography and surface functionalization to position a quantum dot with ~20 nm accuracy at the end of each feed element in an array of Yagi antennas. Finally Alberto Curto adapted a single molecule detection microscope to scan and identify single quantum dots on individual antennas, by monitoring spectra, polarization, blinking and antibunching. Most importantly using a super high 1.46NA objective and detection in the back focal plane on an emCCD camera Alberto could record the angular luminescence for each single dot-antenna system. Indeed, after getting all the details right, we could observe unidirectional emission of a quantum emitter when resonantly coupled to an optical Yagi antenna [1]. The narrow forward angular cone of quantum dot luminescence shows a forward-to-backward ratio of about 5 times [1]. Also the luminescence becomes strongly linearly polarized, corresponding to the antenna dipolar mode. Moreover the directivity of the quantum dot emission is sensitive to the tuning of the antenna resonance, e.g. by changing antenna dimensions, even such that at certain mistuning conditions backward emission is created. Finally it should be noted that the Yagi antenna is very compact with its largest dimension only one wavelength, here 800 nm; thus directional emission is realized from a truly compact area.

Artist's impression of the directional emission of the Yagi-Uda antenna driven by a single luminescent quantum dot.

Perspective. Clearly our experiment demonstrates how photonic antennas are key nano-elements to control single photon emitters. Obviously this provides inspiration to interface such antennas to individual molecules, color centers, proteins, etc., allowing us to explore new avenues in the fields of active photonic circuits, bio-sensing and quantum information technology [2].

References
[1] A.G. Curto, G. Volpe, T.H. Taminiau, M.P. Kreuzer, R. Quidant, N.F. van Hulst, "Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna", Science 329, 930 (2010) . Abstract.
[2] P. Bharadwaj, B. Deutsch, L. Novotny, "Optical antennas", Adv. Opt. Photon. 1, 438 (2009). Abstract.
[3] H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, T. W. Ebbesen, "Beaming light from a subwavelength aperture", Science 297, 820 (2002). Abstract.
[4] P.Mühlschlegel, H.-J.Eisler, O.J.F.Martin, B.Hecht, D.W.Pohl, "Resonant Optical Antennas", Science 308, 1607 (2005). Abstract.
[5] S. Kühn, U. Hakanson, L. Rogobete, V. Sandoghdar, "Enhancement of single-molecule fluorescence using a gold nanoparticle as an optical nanoantenna", Phys. Rev. Lett. 97, 017402 (2006). Abstract.
[6] J. J. Li, A. Salandrino, N. Engheta, "Shaping light beams in the nanometer scale: A Yagi-Uda nanoantenna in the optical domain", Phys. Rev. B 76, 245403 (2007). Abstract.
[7] T.Kosako, Y.Kadoya, H.F.Hofmann, "Directional control of light by a nano-optical Yagi-Uda antenna", Nature Photonics, 4, 312 (2010). Abstract.
[8] T.H.Taminiau F.D.Stefani & N.F. van Hulst, "Enhanced directional excitation and emission of single emitters by a nano-optical Yagi-Uda antenna", Optics Express 16, 10858 (2008). Abstract.
[9] A. F. Koenderink, "Plasmon Nanoparticle Array Waveguides for Single Photon and Single Plasmon Sources", Nano Letters, 9, 4228 (2009). Abstract.
[10] T.H.Taminiau, F.D.Stefani, F.B.Segerink. & N.F. van Hulst, "Optical antennas direct single-molecule emission", Nature Photonics, 2, 234 (2008). Abstract.

New Technique Allows 3-D Mapping of the Magnetic Vector Potential

Amanda Petford-Long [Photo courtesy: Argonne National Laboratory]

Scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have developed a new technique [1] that maps the magnetic vector potential — one of the most important electromagnetic quantities and a foundation of quantum mechanics — in three dimensions. The vector potential is central to a number of areas of condensed matter physics, such as superconductivity and magnetism.

"The vector potential of magnetic structures is essential to the understanding of several areas in condensed matter physics and magnetism on a quantum level, but until now it has never been visualized in three dimensions,” Argonne Distinguished Fellow Amanda Petford-Long said. “If you want to understand the way magnetic nanostructures behave, then you have to understand the magnetic vector potential.”

According to Petford-Long, research into the creation and manipulation of magnetic nanostructures will enable the development of the next generation of data storage in the form of magnetic random access memory.

Charudatta Phatak [Photo Courtesy: Argonne National Laboratory]

Petford-Long and post-doctoral researcher Charudatta Phatak used a transmission electron microscope (TEM) to examine a series of different nanostructures. The theoretical and numerical reconstruction procedure was developed in collaboration with Prof. Marc De Graef at Carnegie Mellon University.

Using the TEM, the researchers were able to take images from several different angles and then rotate the structure by 90 degrees until they had several series of images. The scientists then extracted the vector potential by reconstructing how the electrons in the material shifted phase.

“The development of next generation magnetic sensors and devices requires studying the physics underlying the magnetic interactions at the nanoscale,” Phatak said. “This 3-D map is the first step to truly understanding those interactions.”

Marc De Graef [Photo courtesy: Carnegie Mellon University]

Funding for the research, including the TEM situated in the Materials Science Division, was provided by the U.S. Department of Energy’s Office of Science. The patterned structures were prepared at the Center for Nanoscale Materials with Alexandra Imre.

The Center for Nanoscale Materials at Argonne National Laboratory is one of the five DOE Nanoscale Science Research Centers (NSRCs), premier national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. For more information about the DOE NSRCs, visit http://nano.energy.gov/.

Reference
[1] Charudatta Phatak, Amanda K. Petford-Long, Marc De Graef, "Three-Dimensional Study of the Vector Potential of Magnetic Structures", Phys. Rev. Lett. 104, 253901 (2010). Abstract.

[We thank Argonne National Laboratory for materials used in this posting]

Physics Nobel Prize 2010: Graphene

Andre Geim (photo courtesy: Sergeom, Wikimedia Commons) and Konstantin Novoselov (photo courtesy: University of Manchester, UK)

The Nobel Prize in Physics 2010 was awarded jointly to Andre Geim and Konstantin Novoselov of the University of Manchester "for groundbreaking experiments regarding the two-dimensional material graphene"

Graphene is a form of carbon. As a material it is completely new – not only the thinnest ever but also the strongest. As a conductor of electricity it performs as well as copper. As a conductor of heat it outperforms all other known materials. It is almost completely transparent, yet so dense that not even helium, the smallest gas atom, can pass through it. Carbon, the basis of all known life on earth, has surprised us once again.

Homepage of Andre Geim >> Homepage of Konstantin Novoselov >>
Link to the Mesoscopic Physics Group, University of Manchester, UK >>

Geim and Novoselov extracted the graphene from a piece of graphite such as is found in ordinary pencils. Using regular adhesive tape they managed to obtain a flake of carbon with a thickness of just one atom. This at a time when many believed it was impossible for such thin crystalline materials to be stable.

However, with graphene, physicists can now study a new class of two-dimensional materials with unique properties. Graphene makes experiments possible that give new twists to the phenomena in quantum physics. Also a vast variety of practical applications now appear possible including the creation of new materials and the manufacture of innovative electronics. Graphene transistors are predicted to be substantially faster than today’s silicon transistors and result in more efficient computers.

Since it is practically transparent and a good conductor, graphene is suitable for producing transparent touch screens, light panels, and maybe even solar cells.

When mixed into plastics, graphene can turn them into conductors of electricity while making them more heat resistant and mechanically robust. This resilience can be utilised in new super strong materials, which are also thin, elastic and lightweight. In the future, satellites, airplanes, and cars could be manufactured out of the new composite materials.

This year’s Laureates have been working together for a long time now. Konstantin Novoselov, 36, first worked with Andre Geim, 51, as a PhD-student in the Netherlands. He subsequently followed Geim to the United Kingdom. Both of them originally studied and began their careers as physicists in Russia. Now they are both professors at the University of Manchester.

Playfulness is one of their hallmarks, one always learns something in the process and, who knows, you may even hit the jackpot. Like now when they, with graphene, write themselves into the annals of science.

New Physics from Graphene Quartet's Quantum Harmonies

Joseph Stroscio [Photo courtesy: Center for Nanoscale Science & Technology (CNST), Gaithersburg, MD, USA]

An international team of researchers from the National Institute of Standards and Technology (NIST), the University of Maryland, Seoul National University, the Georgia Institute of Technology, and the University of Texas at Austin, have "unveiled" a quartet of graphene's electron states and discovered that electrons in graphene can split up into an unexpected and tantalizing set of energy levels when exposed to extremely low temperatures and extremely high magnetic fields.

The team, led by Joseph Stroscio of the Electron Physics Group in the NIST Center for Nanoscale Science and Technology (CNST), published their results in Sept. 9, 2010, issue of Nature [1]. The new research raises several intriguing questions about the fundamental physics of this exciting material and reveals new effects that may make graphene even more powerful than previously expected for practical applications.

Graphene is one of the simplest materials—a single-atom-thick sheet of carbon atoms arranged in a honeycomb-like lattice—yet it has many remarkable and surprisingly complex properties. Measuring and understanding how electrons carry current through the sheet is important to realizing its technological promise in wide-ranging applications, including high speed electronics and sensors. For example, the electrons in graphene act as if they have no mass and are almost 100 times more mobile than in silicon. Moreover, the speed with which electrons move through graphene is not related to their energy, unlike materials such as silicon where more voltage must be applied to increase their speed, which creates heat that is detrimental to most applications.

To fully understand the behavior of graphene's electrons, scientists must study the material under an extreme environment of ultra-high vacuum, ultra-low temperatures and large magnetic fields. Under these conditions, the graphene sheet remains pristine for weeks, and the energy levels and interactions between the electrons can be observed with precision [2].

NIST recently constructed the world's most powerful and stable scanning-probe microscope, with an unprecedented combination of low temperature (as low as 10 millikelvin, or 10 thousandths of a degree above absolute zero), ultra-high vacuum and high magnetic field. In the first measurements made with this instrument, the team has used its power to resolve the finest differences in the electron energies in graphene, atom-by-atom.



















[Image credit: T. Schindler and K. Talbott/NIST] This artist's rendition illustrates the electron energy levels in graphene as revealed by a unique NIST instrument. Because of graphene's properties, an electron in any given energy level (the wide, purple band) comprises four quantum states (the four rings), called a "quartet." This quartet of levels split into different energies when immersed in a magnetic field. The two smaller bands on the outermost ring represent the further splitting of a graphene electronic state.

"Going to this resolution allows you to see new physics," said Young Jae Song, a postdoctoral researcher who helped develop the instrument at NIST and make these first measurements.

And the new physics the team saw raises a few more questions about how the electrons behave in graphene than it answers.

Because of the geometry and electromagnetic properties of graphene's structure, an electron in any given energy level populates four possible sublevels, called a "quartet." Theorists have predicted that this quartet of levels would split into different energies when immersed in a magnetic field, but until recently there had not been an instrument sensitive enough to resolve these differences.

"When we increased the magnetic field at extreme low temperatures, we observed unexpectedly complex quantum behavior of the electrons," said NIST Fellow Joseph Stroscio.

What is happening, according to Stroscio, appears to be a "many-body effect" in which electrons interact strongly with one another in ways that affect their energy levels.

One possible explanation for this behavior is that the electrons have formed a "condensate" in which they cease moving independently of one another and act as a single coordinated unit.

"If our hypothesis proves to be correct, it could point the way to the creation of smaller, very-low-heat producing, highly energy efficient electronic devices based upon graphene," said Shaffique Adam, a postdoctoral researcher who assisted with theoretical analysis of the measurements.

The group's work was also recently featured in another paper in Nature Physics [3], in which they describe how the energy levels of graphene's electrons vary with position as they move along the material's crystal structure. The way in which the energy varies suggests that interactions between electrons in neighboring layers may play a role.

References
[1] Y.J. Song, A.F. Otte, Y. Kuk, Y.Hu, D.B. Torrance, P.N. First, W.A. de Heer, H. Min, S. Adam, M.D. Stiles, A.H. MacDonald and J.A. Stroscio, "High Resolution Tunnelling Spectroscopy of a Graphene Quartet", Nature, 467, 185–189 (09 September, 2010). Abstract.
[2] D.L. Miller, K.D. Kubista, G.M. Rutter, M. Ruan, W.A. de Heer, P.N. First and J.A. Stroscio. "Observing the quantization of zero mass carriers in graphene". Science, 324, 924 - 927 (May 15, 2009). Abstract.
[3] D.L. Miller, K.D. Kubista, G.M. Rutter, Ming Ruan, W.A. de Heer, M. Kindermann, P.N. First and J.A. Stroscio, "Real-space mapping of magnetically quantized graphene states", Nature Physics. Published online Aug. 8, 2010. doi:10.1038/nphys1736. Abstract.

Solving the Superconductor Puzzle

Thomas A. Maier [photo courtesy: Oak Ridge National Laboratory]

Superconducting materials, which transmit power resistance-free, are found to perform optimally when high- and low-charge density varies on the nanoscale level, according to research performed at the US Department of Energy's Oak Ridge National Laboratory (ORNL) and Institut für Theoretische Physik, Zürich, Switzerland.

In research toward better understanding the dynamics behind high-temperature superconductivity, the ORNL scientists rewrote computational code for the numerical Hubbard model that previously assumed copper-compound superconducting materials known as cuprates to be homogenous — the same electron density — from atom to atom. The paper is published in Physical Review Letters [1].

Lead author Thomas Maier and colleagues Gonzalo Alvarez, Michael Summers and Thomas Schulthess received the Association for Computing Machinery Gordon Bell Prize two years ago for their high-performance computing application. The application has now been used to examine the nanoscale inhomogeneities in superconductors that had long been noticed but left unexplained.













Researchers have found that atom clusters with inhomogenous stripes of lower density (shown in red) raise critical temperature needed to reach superconductor state [Courtsey: ORNL]

"Cuprates and other chemical compounds used as superconductors require very cold temperatures, nearing absolute zero, to transition from a phase of resistance to no resistance," said Jack Wells, director of the Office of Institutional Planning and a former Computational Materials Sciences group leader.

Liquid nitrogen is used to cool superconductors into phase transition. The colder the conductive material has to get to reach the resistance-free superconductor phase, the less efficient and more costly are superconductor power infrastructures. Such infrastructures include those used on magnetic levitation trains, hospital Magnetic Resonance Imaging, particle accelerators and some city power utilities.

In angle-resolved photoemission experiments and transport studies on a cuprate material that exhibits striped electronic inhomogeneity, scientists for years observed that superconductivity is heavily affected by the nanoscale features and in some respect even optimized.

"The goal following the Gordon Bell Prize was to take that supercomputing application and learn whether these inhomogenous stripes increased or decreased the temperature required to reach transition," Wells said. "By discovering that striping leads to a strong increase in critical temperature, we can now ask the question: is there an optimal inhomogeneity?"

In an ideal world, a material could become superconductive at an easily achieved and maintained low temperature, eliminating much of the accompanying cost of the cooling infrastructure.

"The next step in our progress is a hard problem," Wells said. "But from our lab's point of view, all of the major tools suited for studying this phenomenon — the computational codes we've written, the neutron scattering experiments that allow us to examine nanoscale properties — are available to us here."

Reference
[1] T. A. Maier, G. Alvarez, M. Summers, T. C. Schulthess, "Dynamic Cluster Quantum Monte Carlo Simulations of a Two-Dimensional Hubbard Model with Stripelike Charge-Density-Wave Modulations: Interplay between Inhomogeneities and the Superconducting State", Phys. Rev. Lett. 104, 247001 (2010). Abstract.

[We thank Oak Ridge National Laboratory for materials used in this posting]

Visualizing the Electron Wind Force in Nanostructures

Ellen D. Williams

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

Authors: Chenggang Tao, William G. Cullen, and Ellen D. Williams

Affiliation: Materials Research Science and Engineering Center & Department of Physics, University of Maryland, USA

Link to the Williams Lab >>


As electronic devices get smaller and smaller, they are more susceptible to effects of the charge carriers flowing through them. The charge carriers (electrons in metals) can push atoms around by collisions. For some specific types of atomic structures (for example, atomic “steps”, where the surface height changes by one layer of atoms), the scattering force is much stronger than had been thought [1]. These structures are ubiquitous for the surfaces of solid materials, and this becomes very important for nanoscale electronics where surfaces make up a much bigger fraction of the material.

W. G. Cullen (left) and Chenggang Tao (right)

A very careful measurement is needed to directly observe the forces that electrons exert on the atoms of the material which they are passing through. Yet over a long time, the effects of this force accumulate and can lead to failure of wires which connect components in integrated circuits - a process known as electromigration [2, 3]. In our experiment, we carefully created different types of nanoscale structures on top of a very thin wire of silver. One type of structure consists of single-atom high “islands” that contain between 100 and 100,000 atoms. Another type consists of single-atom high “steps” decorated by C₆₀ buckyballs. We then used a scanning tunneling microscope to watch the structures move or change shape when we ran current through the wire. Amazingly, when we changed the direction of the current, we could move the structures back and forth.

The force exerted by the electrons on island edge atoms is up to 20 times larger than previous theoretical calculations had predicted. However, when we decorate the island edge with a chain of C₆₀ molecules (which tend to mildly withdraw electrons locally from the silver atoms, and also change their local configuration) we find that the force is reduced by over a factor of 10. This indicates that the force is very dependent on the local environment of the atoms which comprise the step and island boundaries.

Fig. 1 Schematic of the experimental setup; inset shows STM image of silver wire surface.

The fundamentally interesting idea here is that all the different ways that electrons can move through the wire can be described by how easily an electron can be “transmitted”. Most atomic structures in a solid allow easy transmission, but the defect sites impede the transmission. This results in a local “resistivity dipole” which means that the defect sites have a local resistance. Our measurements detected the motion of atomic-scale surface structures which results from forces exerted by the passing electrons – as the atoms resist the electron flow, they in turn feel a larger “push” from the electrons.

Fig. 2A-B: Island pushed by moving electrons. The current direction is downward, and the island displacement is upward.

Here we have demonstrated that nanoscale surface structures can be moved (and even turned around) using the scattering force from electrons. Further, the scattering force can be significantly reduced by attaching C₆₀ molecules to the structures. On the other hand, a particularly exciting implication is the use of this effect to move atoms around intentionally in nanoelectronic devices, or to harness it to do work [4]. This effect might be used to self-assemble or to create structures that could be cycled through different structures under an alternating current.

Our work was supported by the NSF Materials Research Science and Engineering Center at the University of Maryland, including the use of shared experimental facilities. Additional support was provided by the University of Maryland NanoCenter and the Center for Nanophysics and Advanced Materials.

Reference:
[1] Chenggang Tao, W. G. Cullen, E. D. Williams, “Visualizing the electron scattering force in nanostructures”, Science 328, 736–740 (2010). Abstract.
[2] P. S. Ho and T. Kwok, “Electromigration in metals”, Reports on Progress in Physics, 52, 301 (1989). Abstract.
[3] H. Yasunaga and A. Natori, “Electromigration on semiconductor surfaces”, Surface Science Reports 15, 205 (1992). Abstract.
[4] D. Dundas, E. McEniry and T. N. Todorov, “Current-driven atomic waterwheels”, Nature Nanotechnology, 4, 99 (2009). Abstract.

Magnetic Morphology of Nanoparticles

Kathryn Krycka [photo courtesy: University of Florida]

While attempting to solve one mystery about iron oxide-based nanoparticles, a research team working at the National Institute of Standards and Technology (NIST) stumbled upon another one. But once its implications are understood, their discovery may give nanotechnologists a new and useful tool.

The nanoparticles in question are spheres of magnetite so tiny that a few thousand of them lined up would stretch a hair’s width, and they have potential uses both as the basis of better data storage systems and in biological applications such as hyperthermia treatment for cancer. A key to all these applications is a full understanding of how large numbers of the particles interact magnetically with one another across relatively large distances so that scientists can manipulate them with magnetism.

“It’s been known for a long time that a big chunk of magnetite has greater magnetic ‘moment’—think of it as magnetic strength—than an equivalent mass of nanoparticles,” says Kathryn Krycka, a researcher at the NIST Center for Neutron Research. “No one really knows why, though. We decided to probe the particles with beams of low-energy neutrons, which can tell you a great deal about a material’s internal structure.”

The team applied a magnetic field to nanocrystals composed of 9 nm-wide particles, made by collaborators at Carnegie Mellon University. The field caused the particles to line up like iron filings on a piece of paper held above a bar magnet. But when the team looked closer using the neutron beam, what they saw revealed a level of complexity never seen before.

“When the field is applied, the inner 7 nm-wide ‘core’ orients itself along the field’s north and south poles, just like large iron filings would,” Krycka says. “But the outer 1 nm ‘shell’ of each nanoparticle behaves differently. It also develops a moment, but pointed at right angles to that of the core.”

In a word, bizarre. But potentially useful.

[Image courtesy: NIST] Schematic of a spherical magnetite nanoparticle shows the unexpected variation in magnetic moment between the particle's interior and exterior when subjected to a strong magnetic field. The core's moment (black lines in magenta region) lines up with the field's (light blue arrow), while the exterior's moment (black arrows in green region) forms at right angles to it.

The shells are not physically different than the interiors; without the magnetic field, the distinction vanishes. But once formed, the shells of nearby particles seem to heed one another: A local group of them will have their shells’ moments all lined up one way, but then another group’s shells will point elsewhere. This finding leads Krycka and her team to believe that there is more to be learned about the role that particle interaction has on determining internal, magnetic nanoparticle structure—perhaps something nanotechnologists can harness.

“The effect fundamentally changes how the particles would talk to each other in a data storage setting,” Krycka says. “If we can control it—by varying their temperature, for example, as our findings suggest we can—we might be able to turn the effect on and off, which could be useful in real-world applications.”

The research team included scientists from NIST, Carnegie Mellon University, University of Maryland, Oberlin College and Los Alamos National Laboratory of USA and Paul Scherrer Institute of Switzerland.

Reference
K.L. Krycka, R.A. Booth, C. Hogg, Y. Ijiri, J.A. Borchers, W.C. Chen, S.M. Watson, M. Laver, T.R. Gentile, L.R. Dedon, S. Harris, J.J. Rhyne and S.A. Majetich. "Core-shell magnetic morphology of structurally uniform magnetite nanoparticles", Physical Review Letters, 104, 207203 (2010). Abstract.

[We thank NIST for materials used in this report]

Interface Between Two Worlds Ultracold atoms coupled to a micromechanical oscillator

Stephan Camerer, David Hunger, and Philipp Treutlein (left to right)



[This is an invited article based on a recently published work by the authors and their collaborators from Germany and France. -- 2Physics.com]





Authors: Philipp Treutlein, David Hunger, Stephan Camerer

Affiliation: Ludwig-Maximilians-Universität München, Max-Planck-Institut für Quantenoptik, Germany

Link to the Munich atom chip experiments >>

Bose-Einstein condensates of ultracold atoms and micromechanical oscillators are usually thought to belong to different areas of science. The condensates are elusive gaseous objects that display the intriguing phenomena of quantum physics in a very clean way. Mechanical oscillators are tangible tools with widespread technological applications. In a recent experiment [1], we have coupled the vibrations of a mechanical oscillator to the motion of a Bose-Einstein condensate in a trap. Such a coupling could lead to quantum-level control and readout of mechanical oscillators [2,3], with applications in quantum information processing or precision force sensing.

Bose-Einstein condensates (BECs) of ultracold neutral atoms are quantum systems par excellence. Due to their electric neutrality, the atoms can be very well isolated from the environment. In recent years, a sophisticated experimental toolbox has been developed for cooling, readout and control of the atoms at the quantum level [4]. This has enabled many beautiful experiments in which interesting quantum states of BECs are prepared and studied.

It is an intriguing question to investigate whether this high level of control can be transferred to other systems such as micromechanical oscillators. At low temperatures, the vibrations of mechanical oscillators show quantum behaviour [5]. A sufficiently strong coupling between ultracold atoms and a mechanical oscillator would allow one to create a hybrid quantum system, in which coherent transfer of quantum information or atom-oscillator entanglement could be studied. In applications of micromechanical oscillators as force sensors, it could lead to higher sensitivity.

In our experiment, which is part of the Nanosystems Initiative Munich and involves researchers from the Ludwig-Maximilians-University in Munich, the Max-Planck-Institute of Quantum Optics in Garching, and the Ecole Normale Superieure in Paris, we have made a first step in this direction and coupled a BEC to the vibrations of a micromechanical oscillator. We use an “atom chip” – a chip with microfabricated current-carrying wires – to create magnetic trapping potentials for the atoms. In addition, the chip carries a micromechanical cantilever oscillator, similar to those used in atomic force microscopes. Using the magnetic traps on the atom chip, we prepare a Bose-Einstein condensate and position it close to the cantilever tip.

At small distances of about one micrometer, atom-surface forces such as the Casimir-Polder force result in an attraction between the atoms and the oscillator. This attractive force couples the vibrations of the oscillator and the motion of the atoms in the trap. The system can be thought of as two oscillating pendula with extremely different masses that are coupled with a spring. Through the coupling, the mechanical oscillator excites collective vibrations of the atoms in the trap – in this way we use the atoms to detect the oscillator vibrations.

Figure 1: a) Schematic setup: Micro-cantilever mounted on an atom chip with gold wires. A ⁸⁷Rb BEC can be trapped and positioned near the cantilever with magnetic fields from wire currents. Cantilever vibrations can be excited with a piezo and independently probed with a readout laser. b) Photograph of the atom chip (scale bar: 1 mm). c) Combined magnetic and surface potential. The surface potential reduces the trap depth to U₀. Cantilever oscillations modulate the potential, thereby coupling to atomic motion.

The BEC has a discrete spectrum of vibrational modes, whose frequencies can be tuned by adjusting the magnetic trapping potential. We use this feature to control the coupling and to resonantly couple the oscillator vibrations to selected mechanical modes of the BEC.

The cantilever oscillator that is used in our current experiment has a length of 200 micrometers and is excited to vibrations with several nanometers amplitude, which are then detected with the atoms. By replacing the cantilever with a nanoscale oscillator, such as a carbon nanotube, it could be possible to detect vibrations close to the quantum mechanical ground state motion of the oscillator. To prepare the oscillator close to its ground state, such an experiment has to be performed at very low temperatures in a cryogenic setup. Under these conditions, the atoms could influence the nanotube strongly, opening a path to quantum manipulations.

References:
[1] David Hunger, Stephan Camerer, Theodor W. Hänsch, Daniel König, Jörg P. Kotthaus, Jakob Reichel, and Philipp Treutlein, “Resonant Coupling of a Bose-Einstein Condensate to a Micromechanical Oscillator”, Phys. Rev. Lett. 104, 143002 (2010). Abstract.
[2] K. Hammerer, M. Wallquist, C. Genes, M. Ludwig, F. Marquardt, P. Treutlein, P. Zoller, J. Ye, and H. J. Kimble, “Strong coupling of a mechanical oscillator and a single atom”, Phys. Rev. Lett. 103, 063005 (2009). Abstract.
[3] L. Tian and P. Zoller, “Coupled Ion-Nanomechanical Systems”, Phys. Rev. Lett. 93, 266403 (2004). Abstract.
[4] S. Chu, “Cold atoms and quantum control”, Nature 416, 206 (2002). Abstract.
[5] A. D. O’Connell et al., “Quantum ground state and single-phonon control of a mechanical resonator”, Nature 464, 697 (2010). Abstract.

A Versatile Negative Index Metamaterial Design for Visible Light

Stanley P. Burgos (left) and Harry A. Atwater (right)


[This is an invited article based on a recently published work by the authors and their collaborators from the Netherlands. -- 2Physics.com]


Authors: Stanley P. Burgos and Harry A. Atwater

Affiliation: Kavli Nanoscience Institute, California Institute of Technology, USA
Link to ATWATER Research Group >>

Negative index metamaterials (NIMs) are artificial optical materials that cause light to bend in the “wrong" direction and phase fronts to move backwards in time – exactly the opposite of what is observed in naturally-occurring positive index materials [1]. What we have accomplished at the Caltech Light-Matter Interactions -- Energy
Frontier Research Center (LMI-EFRC) is to have developed the first wide-angle negative index material (NIM) operational at visible frequencies.

Our work, reported in Nature Materials on April 18th [2], presents an innovative design for an artificial material, or metamaterial, with an effective refractive index that is negative and insensitive to the direction and polarization of light over a broad range of angles – a level of isotropy which has not been possible with previous negative index metamaterial designs. By designing a nearly isotropic negative index metamaterial that operates at visible frequencies we are opening the door to such unusual – but potentially useful – phenomena as superlensing [3] (high-resolution imaging past the diffraction limit), invisibility cloaking [4], and the synthesis of materials index-matched to air, for potential enhancement of light collection in solar cells [5].

The innovation of our metamaterial design is that the source of the negative index response is fundamentally different from that of previous NIM designs. Whereas other NIM designs use multiple layers of “resonant elements” as the source of the negative index, our design is composed of a single layer of coupled “plasmonic waveguide” elements [6]. The fact that these waveguides are plasmonic allows for easy tuning of the waveguide’s negative index response into the visible simply by tuning the waveguide materials and geometry, and since the characteristic material symmetry is cylindrical, the negative index response is independent of polarization and angle of incidence over a broad range of angles [7]. By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with nearly isotropic refractive index tuned to operate in the visible.

Arrays of coupled plasmonic coaxial waveguides offer a new approach by which to realize negative-index metamaterials that are remarkably insensitive to angle of incidence and polarization in the visible range.

For practical applications, it is very important for a material’s response to be insensitive to both incident angle and polarization. Take eyeglasses for example – in order for them to properly focus light reflected off an object to the back of your eye, they must be able to accept and focus light coming from a broad range of angles, independent of polarization. Said another way, their response must be nearly isotropic. Our metamaterial has the same capabilities in terms of its response to incident light.

This means that our metamaterial design is particularly well suited for use in solar cells. The fact that the design is tunable means that the material’s index response could be tuned to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells. And the fact that the metamaterial has a wide-angle response is important because it means that it can “accept” light from a broad range of angles. For the case of solar cells, this means more light collection and less reflected or “wasted” light.

References
[1] Veselago, V. G., "The electrodynamics of substances with simultaneously negative values of ε and μ", Soviet Physics Uspekhi 10, 509–514 (1968). Abstract.
[2] Stanley P. Burgos, Rene de Waele, Albert Polman & Harry A. Atwater, "A single-layer wide-angle negative-index metamaterial at visible frequencies", Nature Materials, 9, 407-412 (2010). Abstract.
[3] Pendry, J. B., "Negative Refraction Makes a Perfect Lens", Phys. Rev. Lett. 85, 3966-3969 (2000). Abstract.
[4] Pendry, J. B., Schurig, D. & Smith, D. R. "Controlling Electromagnetic Fields". Science 312, 1780-1782 (2006). Abstract.
[5] Atwater, H. A. & Polman, A. "Plasmonics for improved photovoltaic devices". Nature Materials, 9, 205-213 (2010). Abstract.
[6] Dionne, J. A., Verhagen, E., Polman, A. & Atwater, H. A. "Are negative index materials achievable with surface plasmon waveguides? A case study of three plasmonic geometries". Optics Express, 16, 19001-19017 (2008). Abstract.
[7] de Waele, R., Burgos, S. P., Polman, A. & Atwater, H. A. Plasmon Dispersion in Coaxial Waveguides from Single-Cavity Optical Transmission Measurements. Nano Letters 9, 2832-2837 (2009). Abstract.

Playing Tug-of-War at Atomic-Scale

Douglas Smith [Photo courtesy: Nanomechanical Properties Group, NIST]

How hard do you have to pull on a single atom of—let’s say—gold to detach it from the end of a chain of like atoms?
[For answer, see the end of this article*]

It’s a measure of the astonishing progress in nanotechnology that questions that once would have interested only physicists or chemists are now being asked by engineers. To help with the answers, a research team at the National Institute of Standards and Technology (NIST) has built an ultra-stable instrument for tugging on chains of atoms, an instrument that can maneuver and hold the position of an atomic probe to within 5 picometers, or 0.000 000 000 5 centimeters.

The basic experiment uses a NIST-designed instrument inspired by the scanning tunneling microscope (STM). The NIST instrument uses as a probe a fine, pure gold wire drawn out to a sharp tip. The probe is touched to a flat gold surface, causing the tip and surface atoms to bond, and gradually pulled away until a single-atom chain (see figure) is formed and then breaks.

The trick is to do this with such exquisite positional control that you can tell when the last two atoms are about to separate, and hold everything steady; you can at that point measure the stiffness and electrical conductance of the single-atom chain, before breaking it to measure its strength.

The NIST team used a combination of clever design and obsessive attention to sources of error to achieve results that otherwise would require heroic efforts at vibration isolation, according to engineer Jon Pratt. A fiber-optic system mounted just next to the probe uses the same gold surface touched by the probe as one mirror in a classic optical interferometer capable of detecting changes in movement far smaller than the wavelength of light. The signal from the interferometer is used to control the gap between surface and probe. Simultaneously, a tiny electric current flowing between the surface and probe is measured to determine when the junction has narrowed to the last two atoms in contact. Because there are so few atoms involved, electronics can register, with single-atom sensitivity, the distinct jumps in conductivity as the junction between probe and surface narrows.

[Image credit: F. Tavazza, NIST] A quantum-mechanics-based simulation demonstrates how a new NIST instrument can delicately pull a chain of atoms apart. The chart records quantum jumps in conductivity as a gold contact is stretched 0.6 nanometer. The junction transitions from a 2-dimensional structure to a one-dimensional single-atom chain, with a corresponding drop in conductivity. Following the last point, at a wire length of 3.97 nm, the chain broke.

The new instrument can be paired with a parallel research effort at NIST to create an accurate atomic-scale force sensor—for example, a microscopic diving-board-like cantilever whose stiffness has been calibrated on NIST’s Electrostatic Force Balance.

Physicist Douglas Smith says the combination should make possible the direct measurement of force between two gold atoms in a way traceable to national measurement standards. And because any two gold atoms are essentially identical, that would give other researchers a direct method of calibrating their equipment.

“We’re after something that people that do this kind of measurement could use as a benchmark to calibrate their instruments without having to go to all the trouble we do, " Smith says. "What if the experiment you’re performing calibrates itself because the measurement you’re making has intrinsic values? You can make an electrical measurement that’s fairly easy and by observing conductance you can tell when you’ve gotten to this single-atom chain. Then you can make your mechanical measurements knowing what those forces should be and recalibrate your instrument accordingly.”

In addition to its application to nanoscale mechanics, say the NIST team, their system’s long-term stability at the picometer scale has promise for studying the movement of electrons in one-dimensional systems and single-molecule spectroscopy.

[* The answer, calculated from atomic models, should be something under 2 nanonewtons, or less than 0.000 000 007 ounces of force.]

Reference
D.T. Smith, J.R. Pratt, F. Tavazza, L.E. Levine and A.M. Chaka. An ultra-stable platform for the study of single-atom chains. J. Appl. Phys., Accepted for publication. [Link will be added, once published -- 2Physics]

[We thank NIST for materials used in this report]

Theory of Quantum Mechanics Applies to the Motion of Large Objects

(L to R) Andrew Cleland, Aaron O'Connell and John Martinis [photo credit: George Foulsham / Univ of California, Santa Barbara]

A team of physicists from University of California, Santa Barbara has provided the first clear demonstration that the theory of quantum mechanics applies to the mechanical motion of an object large enough to be seen by the naked eye. Their work satisfies a longstanding goal among physicists.

In a paper published in the March 17 issue of the advance online journal Nature [1], Aaron O'Connell, a doctoral student in physics, and John Martinis and Andrew Cleland, professors of physics, describe the first demonstration of a mechanical resonator that has been cooled to the quantum ground state, the lowest level of vibration allowed by quantum mechanics. With the mechanical resonator as close as possible to being perfectly still, they added a single quantum of energy to the resonator using a quantum bit (qubit) to produce the excitation. The resonator responded precisely as predicted by the theory of quantum mechanics.

"This is an important validation of quantum theory, as well as a significant step forward for nanomechanics research," said Cleland.

The researchers reached the ground state by designing and constructing a microwave-frequency mechanical resonator that operates similarly to –– but at a higher frequency than –– the mechanical resonators found in many cellular telephones. They wired the resonator to an electronic device developed for quantum computation, a superconducting qubit, and cooled the integrated device to temperatures near absolute zero. Using the qubit as a quantum thermometer, the researchers demonstrated that the mechanical resonator contained no extra vibrations. In other words, it had been cooled to its quantum ground state.

Micrograph of the resonator

The researchers demonstrated that, once cooled, the mechanical resonator followed the laws of quantum mechanics. They were able to create a single phonon, the quantum of mechanical vibration, which is the smallest unit of vibrational energy, and watch as this quantum of energy exchanged between the mechanical resonator and the qubit. While exchanging this energy, the qubit and resonator become "quantum entangled," such that measuring the qubit forces the mechanical resonator to "choose" the vibrational state in which it should remain.

In a related experiment, they placed the mechanical resonator in a quantum superposition, a state in which it simultaneously had zero and one quantum of excitation. This is the energetic equivalent of an object being in two places at the same time. The researchers showed that the resonator again behaved as expected by quantum theory.

Reference
[1] A. D. O’Connell, M. Hofheinz, M. Ansmann, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, John M. Martinis & A. N. Cleland, "Quantum ground state and single-phonon control of a mechanical resonator", Nature advance online publication 17 March 2010 [doi:10.1038/nature08967]. Abstract.

Nanobubbles on Super Non-Stick Surfaces

Antonio Checco [Photo courtesy: Brookhaven National Laboratory]

A team of physicists at the U.S. Department of Energy’s Brookhaven National Laboratory has obtained the first glimpse of miniscule air bubbles that keep water from wetting a super non-stick surface. Detailed information about the size and shape of these bubbles — and the non-stick material the scientists created by “pock-marking” a smooth material with cavities measuring mere billionths of a meter — was published online on February 24th in the journal Nano Letters.

“Our results explain how these nanocavities trap tiny bubbles which render the surface extremely water repellent,” said Brookhaven physicist and lead author Antonio Checco. The research could lead to a new class of non-stick materials for a range of applications, including improved-efficiency power plants, speedier boats, and surfaces that are resistant to contamination by germs.

Non-stick surfaces are important to many areas of technology, from drag reduction to anti-icing agents. These surfaces are usually created by applying coatings, such as Teflon, to smooth surfaces. But recently — taking the lead from observations in nature, notably the lotus leaf and some varieties of insects — scientists have realized that a bit of texture can help. By incorporating topographical features on surfaces, they’ve created extremely water repellant materials.

“We call this effect ‘superhydrophobicity,’” said Brookhaven physicist Benjamin Ocko. “It occurs when air bubbles remain trapped in the textured surfaces, thereby drastically reducing the area of liquid in contact with the solid.” This forces the water to ball up into pearl shaped drops, which are weakly connected to the surface and can readily roll off, even with the slightest incline.

“To get the first glimpse of nanobubbles on a superhydrophobic surface we created a regular array of more than a trillion nano-cavities on an otherwise flat surface, and then coated it with a wax-like surfactant,” said Charles Black, a physicist at Brookhaven’s Center for Functional Nanometerials.

This coated, nanoscale textured surface was much more water repellant than the flat surface alone, suggesting the existence of nanobubbles. However, because the nanoscale is not accessible using ordinary microscopes, little is known about these nanobubbles.

Image 1: The central image is the optical profile of a water drop placed on "nanopitted" silicon; the right image is a scanning electron micrograph of the nanocavities; and the left image is a cartoon illustrating the nanobubbles' shape as inferred from x-ray measurements.

To unambiguously prove that these ultra-small bubbles were present, the Brookhaven team carried out x-ray measurements at the National Synchrotron Light Source. “By watching how the x-rays diffracted, or bounced off the surface, we are able to image extremely small features and show that the cavities were mostly filled with air,” said Brookhaven physicist Elaine DiMasi.

Checco added, “We were surprised that water penetrates only about 5 to 10 nanometers into the cavities — an amount corresponding to only 15 to 30 layers of water molecules — independent of the depth of the cavities. This provides the first direct evidence of the morphology of such small bubbles.”

According to the scientists’ observations, the bubbles are only about 10 nanometers in size — about ten thousand times smaller than the width of a single human hair. And the team’s results conclusively show that these tiny bubbles have nearly flat tops. This is in contrast to larger, micrometer-sized bubbles, which have a more rounded top.

“This flattened configuration is appealing for a range of applications because it is expected to increase hydrodynamic slippage past the nanotextured surface,” Checco said. “Moreover, the fact that water hardly penetrates into the nano-textures, even if an external pressure is applied to the liquid, implies that these nanobubbles are very stable.”

Therefore, in contrast to materials with larger, micrometer-sized textures, the surfaces fabricated by the Brookhaven team may exhibit more stable superhydrophobic properties.

“These findings provide a better understanding of the nanoscale aspects of superhydropobicity, which should help to improve the design of future superhydrophobic non-stick surfaces,” Checco said.

Reference
"Morphology of Air Nanobubbles Trapped at Hydrophobic Nanopatterned Surfaces",
Antonio Checco, Tommy Hofmann, Elaine DiMasi, Charles T. Black and Benjamin M. Ocko,
Nano Letters, published online on Feb 24, 2010. DOI: 10.1021/nl9042246. Abstract.

[We thank Brookhaven National Laboratory for materials used in this posting]

Filming Photons of Nanoscale Structures with Electrons

Ahmed Zewail [Image courtesy: Caltech]

In two recent papers published in the December 17 issue of Nature [1] and the October 30 issue of Science [2], a team of researchers from the California Institute of Technology (Caltech) reported the invention of techniques that allow the real-time, real-space visualization of fleeting changes in the structure of nanoscale matter. These novel techniques have been used to image the evanescent electrical fields produced by the interaction of electrons and photons, and to track changes in atomic-scale structures.

Four-dimensional (4D) microscopy-the methodology upon which the new techniques were based-was developed at Caltech's Physical Biology Center for Ultrafast Science and Technology. The center is directed by Ahmed Zewail, the Linus Pauling Professor of Chemistry and professor of physics at Caltech, and winner of the 1999 Nobel Prize in Chemistry.

Zewail was awarded the Nobel Prize for pioneering the science of femtochemistry, the use of ultrashort laser flashes to observe fundamental chemical reactions occurring at the timescale of the femtosecond (one-millionth of a billionth of a second). The work "captured atoms and molecules in motion," Zewail says, but while snapshots of such molecules provide the "time dimension" of chemical reactions, they don't give the dimensions of space of those reactions-that is, their structure or architecture.

Zewail and his colleagues were able to visualize the missing architecture through 4D microscopy, which employs single electrons to introduce the dimension of time into traditional high-resolution electron microscopy, thus providing a way to see the changing structure of complex systems at the atomic scale.

Image 1: The diffraction obtained for silicon with 4D electron microscopy. From the patterns the structure can be determined on the nanoscale. [Credit: AAAS / Science / Zewail / Caltech]

In the research detailed in the Science paper [2], Zewail and postdoctoral scholar Aycan Yurtsever were able to focus an electron beam onto a specific nanoscale-sized site in a specimen, making it possible to observe structures within that localized area at the atomic level.

In electron diffraction, an object is illuminated with a beam of electrons. The electrons bounce off the atoms in the object, then scatter and strike a detector. The patterns produced on the detector provide information about the arrangement of the atoms in the material. However, if the atoms are in motion, the patterns will be blurred, obscuring details about small-scale variations in the material.

The new technique devised by Zewail and Yurtsever addresses the blurring problem by using electron pulses instead of a steady electron beam. The sample under study - in the case of the Science paper [2], a wafer of crystalline silicon-is first heated by being struck with a short pulse of laser light. The sample is then hit with a femtosecond pulse of electrons, which bounce off the atoms, producing a diffraction pattern on a detector.

Since the electron pulses are so incredibly brief, the heated atoms don't have time to move much; this shorter "exposure time" produces a sharp image. By adjusting the delay between when the sample is heated and when the image is taken, the scientists can build up a library of still images that can be strung together into a movie.

"Essentially all of the specimens we deal with are heterogeneous," Zewail explains, with varying compositions over very small areas. "This technique provides the means for examining local sites in materials and biological structures, with a spatial resolution of a nanometer or less, and time resolution of femtoseconds."

The new diffraction method allows the structures of materials to be mapped out at an atomic scale. With the second technique-introduced in the Nature paper [1], which was coauthored by postdoctoral scholars Brett Barwick and David Flannigan - the light produced by such nanostructures can be imaged and mapped.

The concept behind this technique involves the interaction between electrons and photons. Photons generate an evanescent field in nanostructures, and electrons can gain energy from such fields, which makes them visible in the 4D microscope.

Image 2: Photons imaged in nanoscale structures (carbon nanotubes) using pulsed electrons at very high speed. Shown are the evanescent fields for two time frames and for two polarizations. [Credit: Zewail/Caltech]

In what is known as the photon-induced near-field electron microscopy (PINEM) effect, certain materials-after being hit with laser pulses-continue to "glow" for a short but measurable amount of time (on the order of tens to hundreds of femtoseconds).

In their experiment, the researchers illuminated carbon nanotubes and silver nanowires with short pulses of laser light as electrons were being shot past. The evanescent field persisted for femtoseconds, and the electrons picked up energy during this time in discrete amounts (or quanta) corresponding to the wavelength of the laser light. The energy of an electron at 200 kilo-electron volts (keV) increased by 2.4 electron volts (eV), or by 4.8 eV, or by 7.2 eV, etc.; alternatively, an electron might not change in energy at all. The number of electrons showing a change is more striking if the timing is just right, i.e., if the electrons are passing the material when the field is at its strongest.

The power of this technique is that it provides a way to visualize the evanescent field when the electrons that have gained energy are selectively identified, and to image the nanostructures themselves when electrons that have not gained energy are selected.

"As noted by the reviewers of this paper, this technique of visualization opens new vistas of imaging with the potential to impact fields such as plasmonics, photonics, and related disciplines," Zewail says. "What is interesting from a fundamental physics point of view is that we are able to image photons using electrons. Traditionally, because of the mismatch between the energy and momentum of electrons and photons, we did not expect the strength of the PINEM effect, or the ability to visualize it in space and time."

References
[1] "Photon-induced near-field electron microscopy",
Brett Barwick, David J. Flannigan & Ahmed H. Zewail,
Nature, Vol. 462, pp 902-906 (17 December 2009). Abstract.
[2] "4D Nanoscale Diffraction Observed by Convergent-Beam Ultrafast Electron Microscopy",
Aycan Yurtsever and Ahmed H. Zewail,
Science, Vol. 326. no. 5953, pp 708 - 712 (October 30, 2009). Abstract.

[We thank Media relations, Caltech for materials used in this posting]