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

Alexandra Boltasseva

Authors:
Alexandra Boltasseva¹ and Harry A. Atwater²

Affiliations:

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

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

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

Harry A. Atwater

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

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

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

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

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

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

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

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

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

Quantum Dot Bumps in the Graphene Electronic Highway

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

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

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

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

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

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

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

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

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

Macroscopic Invisibility Cloak made from Natural Birefringent Crystals

Shuang Zhang

Author: Shuang Zhang
Affiliation: School of Physics and Astronomy, University of Birmingham, UK

Making things invisible is certainly appealing to most people - including scientists and magicians. Invisibility cloaks have existed only in movies and science fictions until 2006 when Pendry and Leonhardt independently pointed out ways to scientifically realize them [1, 2]. Transformation optics, the enabling theoretical tool to make this happen, works by optically compressing a spatial region to leave out a niche that cannot be accessed by light, while keeping its outer boundary intact so nothing seems to have happened when light exits the cloak.

This new research field has witnessed rapid progresses. Soon after the theoretical proposal, the first experimental demonstration of invisibility was shown at microwave frequencies in 2007 [3], and within 2 years, invisibility has reached Near-Infrared, the frequencies normally used for optical communications [4, 5]. These milestone works show that invisibility has become more a reality than fiction. However, all those invisibility cloaks were made from artificially engineered structures with subwavelength feature size, the so called metamaterials. At optical frequencies, the nano- and micro- fabrication of metamaterials limited the achievable invisibility cloak to a few wavelengths.

Recently two independent teams (including us at University of Birmingham, Imperial College London and Technical University of Denmark) have succeeded in scaling up the invisibility cloak to hide things of macroscopic scale [6, 7]. While previous works on optical invisibility have focused on non-uniform isotropic artificial media, we utilize uniform, anisotropic medium to construct the invisibility cloak. Specifically, our invisibility cloak was made from natural birefringent crystals, and therefore they can be easily scaled up to hide things at least thousands of times bigger than optical wavelengths. In addition, the cloaks work in the visible range, therefore the cloaking effect can directly be seen with our naked eyes.

Our macroscopic invisibility cloak consists of two triangular calcite prisms, with carefully designed geometries and crystal orientations, glued together, as shown by Fig. 1. At bottom of the cloak, there is a small triangular indentation, where objects can be hidden from view. The cloak is capable of optically transforming the triangular bump into a flat surface for a specific light polarization; light is guided around the bump without being scattered by it. As a consequence, for someone looking from outside, the bump appears to be flat and it would not be discernible when it sits on top of an opaque surface. Thus, anything that can be fitted in this triangular region of two and half centimeters width and 1.2 millimeters height can be rendered invisible.

Fig. 1: (Left) A photograph of the triangular cloak, which consists of two calcite prisms glued together. The optical axis of each calcite prism is indicated by the red arrow. The dimension of the cloak along z direction is 2 cm. (Right) Ray tracing shows that light is reflected by the bump without being scattered.

The cloak has been tested with laser beams and natural white light, both in air and liquid with carefully designed refractive index. Fig. 2 shows the reflection of an arrow-shaped green laser beam by the triangular bump at the bottom of cloak. Due to the birefringent nature of the cloak, light with different polarizations follow different paths in the cloak. Light of transverse electric (TE) polarization does not experience the cloaking effect; for TE, the cloak is no more than two pieces of glass prisms. The reflection from the bottom bulging surface split the laser beam into two, as shown by the image projected on the screen for the green laser beam (Fig. 2b). While with TM polarization, the reflected beam shows no splitting at all (Fig. 2c), serving as direct evidence that the calcite cloak transforms the protruding bottom surface into a flat mirror. An invisibility cloak should work at all incident angles. This is confirmed by the measurements at three different incident angles, as shown in Fig. 2 (d, e, f), where the central image corresponds to the TM polarization, and the two projected arrow segments images away from the centre correspond to the TE polarization. The projected images of the TM polarized beam at all incident angles show no distortion of the laser pattern.

Fig. 2: Optical characterization of the cloak using green laser beam. (a) The pattern of the laser beam as reflected by a flat surface. The projected arrow image is about 1.2 cm long in the horizontal direction. (b, c) The projected image of the laser beam reflected by the calcite cloak for TE and TM polarizations, respectively. The TM measurement shows that the laser beam is not distorted by reflection by the triangular protruding surface. (d, e, f) the projected images for mixed TE and TM polarizations at incidence angles of 39.5°, 64.5° and 88°, respectively. For all incident angles, the central TM images are not distorted, the cloaked reflective bump appears to be a flat mirror to outside observers.

The macroscopic calcite cloak is further tested by imaging of white-color alphabetic letters printed on a sheet of black paper reflected by the cloak system. For TE polarization (Fig. 3a), the mirror deformation results in distortion of the image collected by the camera, whereas switching the polarization to TM leads to imaging of consecutive letters from the same location as if the bottom surface of the cloak is flat (Fig. 3b). Due to the dispersion of calcite crystal, rainbow can be observed at the edge of the letters. Nonetheless, the overall cloaking effect looks considerably well, confirming the broadband operation of our calcite invisibility cloak.

Fig. 3: Imaging of white-color alphabetic letters (from ‘A’ to ‘Z’) printed on a sheet of black paper reflected by the cloak system. The reflected image captured by the camera for TE (a) and TM (b) polarizations, respectively.

In air, the cloak itself, though transparent, is still visible due to refraction and reflection caused by the mismatch of refractive index at its interface. When the cloak is immersed in a liquid with appropriate refractive index, both refraction and reflection can be eliminated, and the cloak itself can be made invisible as well. Fig. 4 shows the characterization of the invisibility cloak in an index matching oil of index close to 1.53. At the right polarization of light, the calcite cloak can hardly be seen if not for the scattering of light at the edges.

Fig. 4: Characterization of the invisibility cloak in the index matching fluid. (a, b) Reflected images for TE and TM polarizations, respectively, viewed right above the cloak. (c) Reflected image viewed at an oblique angle.

Our work represents the first macroscopic cloak operating at visible frequencies, which transforms a deformed mirror into a flat one from all viewing angles. The cloak is capable of hiding three-dimensional objects three to four orders of magnitudes larger than optical wavelengths. Indeed, the cloak can be further scaled up as it does not require time consuming nanofabrication techniques. Because our work solves several major issues typically associated with cloaking: size, bandwidth, loss, and image distortion, it paves the way for future practical cloaking devices.

References:
[1] J.B. Pendry, D. Schurig, D.R. Smith, “Controlling electromagnetic fields.” Science 312, 1780–1782 (2006). Abstract.
[2] U. Leonhardt, “Optical conformal mapping.” Science 312, 1777–1780 (2006). Abstract.
[3] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr and D.R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies.” Science 314, 977–980 (2006). Abstract.
[4] J. Valentine, J. Li, T. Zentgraf, G. Bartal, X. Zhang, “An optical cloak made of dielectrics.” Nature Materials, 8, 568–571 (2009). Abstract.
[5] L.H. Gabrielli, J. Cardenas, C.B. Poitras, M. Lipson, “Silicon nanostructure cloak operating at optical frequencies.” Nature Photonics, 3, 461–463 (2009). Abstract.
[6] Xianzhong Chen, Yu Luo, Jingjing Zhang, Kyle Jiang, John B. Pendry & Shuang Zhang, “Macroscopic invisibility cloaking of visible light.” Nature Communications, 2:176 doi: 10.1038/ncomms1176 (published online February 01, 2011). Article.
[7] Baile Zhang, Yuan Luo, Xiaogang Liu, George Barbastathis,“Macroscopic invisibility cloak for visible light”, Phys. Rev. Lett. 106, 033901 (2011). Abstract.

Quantum Quirk: Packing Atoms Together to Prevent Collisions in Atomic Clock

Jun Ye [photo courtesy: JILA/University of Colorado]

In a paradox typical of the quantum world, JILA scientists have eliminated collisions between atoms in an atomic clock by packing the atoms closer together. The surprising discovery, described in the Feb. 3 issue of Science Express [1], can boost the performance of experimental atomic clocks made of thousands or tens of thousands of neutral atoms trapped by intersecting laser beams.

[JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder. Once upon a time, JILA was the Joint Institute for Laboratory Astrophysics. These days, that name doesn't encompass the breadth of science conducted at JILA. So, after extended discussion in 1994, JILA's fellows decided to keep the word JILA but drop the meaning.]

JILA scientists demonstrated the new approach using their experimental clock made of about 4,000 strontium atoms. Instead of loading the atoms into a stack of pancake-shaped optical traps as in their previous work, scientists packed the atoms into thousands of horizontal optical tubes. The result was a more than tenfold improvement in clock performance because the atoms interacted so strongly that, against all odds, they stopped hitting each other. The atoms, which normally like to hang out separately and relaxed, get so perturbed from being forced close together that the ensemble is effectively frozen in place.

The idea was proposed by JILA theorist Ana Maria Rey and demonstrated in the lab by Ye's group.

Ana Maria Rey [photo courtesy: JILA/University of Colorado]

"The atoms used to have the whole dance floor to move around on and now they are confined in alleys, so the interaction energy goes way up," says NIST/JILA Fellow Jun Ye, leader of the experimental team.

How exactly does high interaction energy—the degree to which an atom's behavior is modified by the presence of others—prevent collisions? The results make full sense in the quantum world. Strontium atoms are a class of particles known as fermions. If they are in identical energy states, they cannot occupy the same place at the same time—that is, they cannot collide. Normally the laser beam used to operate the clock interacts with the atoms unevenly, leaving the atoms dissimilar enough to collide [Read past 2Physics report dated May 2, 2009] But the interaction energy of atoms packed in optical tubes is now higher than any energy shifts that might be caused by the laser, preventing the atoms from differentiating enough to collide.

Intersecting laser beams create "optical tubes" to pack atoms close together, enhancing their interaction and the performance of JILA's strontium atomic clock.[Image Credit: Baxley/JILA]

Given the new knowledge, Ye believes his clock and others based on neutral atoms will become competitive in terms of accuracy with world-leading experimental clocks based on single ions (electrically charged atoms). The JILA strontium clock is currently the best performing experimental clock based on neutral atoms and, along with several NIST ion and neutral atom clocks, a possible candidate for a future international time standard. The devices provide highly accurate time by measuring oscillations (which serve as "ticks") between the energy levels in the atoms.

In addition to preventing collisions, the finding also means that the more atoms in the clock, the better. "As atom numbers increase, both measurement precision and accuracy increase accordingly," Ye says.

To trap the atoms in optical tubes, scientists first use blue and red lasers to cool strontium atoms to about 2 microKelvin in a trap that uses light and magnetic fields. A vertical lattice of light waves is created using an infrared laser beam that spans and traps the atom cloud. Then a horizontal infrared laser beam is turned on, creating optical tube traps at the intersection with the vertical laser.

Reference:
[1] Matthew D. Swallows, Michael Bishof, Yige Lin, Sebastian Blatt, Michael J. Martin, Ana Maria Rey, Jun Ye, "Suppression of collisional shifts in a strongly interacting lattice clock", Science Express (Posted online Feb.3, 2011). DOI: 10.1126/science.1196442. Abstract.

[We thank National Institute of Standards and Technology for materials used in this report]