3D Polarization-Independent Invisibility Cloak at Visible Wavelengths

Authors (from left to right):
Tolga Ergin, Joachim Fischer, and Martin Wegener

Affiliation:
Institute of Applied Physics, Karlsruhe Institute of Technology, Germany
Link to Martin Wegener's Group >>

Invisibility cloaks are a fascinating subject for laymen, poets, movie directors, and scientists alike. In recent years, remarkable progress was made in realizing such devices. One design, the “carpet cloak” [1], turned out to be especially promising. Here, the cloak sits on top of a bump in a mirror. An object can be hidden under the mirror (the “carpet”), yet without cloak, the bump would still be seen as a distortion in the mirror reflection. The cloak redirects and bends the light such that this distortion is countered – the mirror appears to be flat and the object is invisible.

Past 2Physics article by this Group:
April 11, 2010: "3D Invisibility Cloaking Device at Optical Wavelengths"
by Tolga Ergin, Nicolas Stenger, Martin Wegener

Multiple exciting realizations of the carpet cloak in effectively two-dimensional geometries and at different wavelengths were demonstrated, both for microscopic [2,3,4,5,6] and macroscopic devices [7,8]. However, due to the two-dimensional nature and the consequential polarization dependence of these structures, the cloaking effect disappears once the structure is inspected from the third dimension or with a different polarization. Following our 2010 work [9], which demonstrated 3D polarization-independent cloaking at optical frequencies for the first time, we were able to shrink our device such that it now operates at wavelengths that are visible to the human eye [10]. To accomplish this, we used a lithographic technique called stimulated-emission-depletion-inspired direct laser writing [11].

The demanding part in the fabrication of these structures is to create a locally and gradually varying index of refraction. Our design accomplishes that by using a woodpile photonic crystal in the long-wavelength limit, meaning that the feature size of the woodpile structure has to be smaller than the operating wavelength. Here, the woodpile acts as an effective material – the light does not “see” the substructure of polymer rods and air, it rather “sees” a homogeneous material with a certain index of refraction at every point in space. The value of the refractive index is controlled via the local polymer-air ratio (see Fig. 1b).



Figure 1: (a) Electron micrograph of the reference (top) and the cloak (bottom) structure. (b) To show the interior, the structures are cut by focused-ion-beam milling. The constant filling fraction of the reference is clearly visible, whereas the cloak shows a locally changing polymer-air ratio. The photos in (c) and (d) are taken with a usual digital camera through a standard optical microscope at 700 nm wavelength. (c) is taken from the air side, where we expect to see the bump’s distortion (two black stripes) both in the reference and in the cloak. (d) When the sample is flipped around and inspected from the glass side, the distortions in the cloaking structure disappear. [Image reproduced from the paper published in Optics Letters. Link]

To document the distortion due to the bump that we are trying to hide, we fabricate reference samples right next to the cloak. These references have the same bump in the carpet, but a constant woodpile filling fraction (and hence, constant refractive index) – no cloaking is expected here. As a control experiment, we image the structures from the air side with a usual microscope (Fig. 1c). The depicted photographs give a very good impression of the image that one sees with the human eye and a color filter. As expected, both structures show an identical distortion (dark stripes). In sharp contrast, the distortion in the cloaking structure is almost completely gone when the sample is flipped around and inspected from the glass side (Fig. 1d) – the carpet mirror looks flat.



Figure 2: Dark-field mode. The sample is tilted by 30°. Here, the bump in the reference structure (top) lights up due to reflections off of the bump’s side. For the cloak (bottom), the bright stripe disappears – just like a flat mirror would look like. [Image reproduced from the paper published in Optics Letters. Link]

The exciting fact about this realization of the carpet invisibility cloak is that it works in three dimensions and with unpolarized light – both facts that are natural to the human sight and experience. To show the 3D performance of our cloak, we tilted the sample by 30° with respect to the optical axis (“dark-field mode”, Fig. 2). Here, in contrast to normal incidence imaging (“bright-field mode”, Fig. 1) , an ideal flat mirror looks dark, since most of the illuminating light is reflected to the side and is not collected by the microscope objective. The bump in the carpet, on the other hand, lights up as a bright stripe, since light is scattered off of it. With the cloak in place, the bright stripe disappears. Again, this corresponds to the reflection at a flat mirror.



Figure 3: Measurements of the cloak similar to Fig. 1c, but for different illumination wavelengths. Cloaking persists down to 650 nm. [Image reproduced from the paper published in Optics Letters. Link]

We also measured the wavelength dependence of the cloaking effect (Fig. 3). We found that for all wavelengths, for which the woodpile photonic crystal can be regarded as a good effective medium, the cloak works very well. This is true for wavelengths larger than 650 nm. In fact, we expect the cloak to be extremely broadband and to keep its functionality up to about 3 µm, where absorption of the polymer sets in. For wavelengths shorter than 650 nm, the light starts to “see” the polymer rods and air holes, which leads to scattering and refraction.

The carpet cloak is a fascinating example of newly developed devices based on the theory of transformation optics and demonstrates the fabrication finesse that is possible with the 3D laser lithography of present days.

References:
[1] Jensen Li and J. B. Pendry, "Hiding under the Carpet: A New Strategy for Cloaking", Phys. Rev. Lett. 101, 203901 (2008). Abstract.
[2] R. Liu, C. Ji, J. J. Mock, J. Y. Chin, T. J. Cui, D. R. Smith, "Broadband Ground-Plane Cloak", Science 323, 366 (2009). Abstract.
[3] Jason Valentine, Jensen Li, Thomas Zentgraf, Guy Bartal & Xiang Zhang, "An optical cloak made of dielectrics", Nature Materials, 8, 568 (2009). Abstract. 2Physics Article.
[4] Lucas H. Gabrielli, Jaime Cardenas, Carl B. Poitras & Michal Lipson, Nature Photonics, 3, 461 (2009). Abstract.
[5] J. H. Lee, J. Blair, V. A. Tamma, Q. Wu, S. J. Rhee, C. J. Summers, and W. Park, "Direct visualization of optical frequency invisibility cloak based on silicon nanorod array", Optics Express, 17, 12922 (2009). Abstract.
[6] Majid Gharghi, Christopher Gladden, Thomas Zentgraf, Yongmin Liu, Xiaobo Yin, Jason Valentine, Xiang Zhang, "A Carpet Cloak for Visible Light", Nano Letters. To be published. DOI:10.1021/nl201189z
[7] Baile Zhang, Yuan Luo, Xiaogang Liu, George Barbastathis, “Macroscopic invisibility cloak for visible light”, Phys. Rev. Lett. 106, 033901 (2011). Abstract.
[8] Xianzhong Chen, Yu Luo, Jingjing Zhang, Kyle Jiang, John B. Pendry & Shuang Zhang, "Macroscopic invisibility cloaking of visible light", Nature Communications, 2, 176 (2011). Abstract. 2Physics Article.
[9] Tolga Ergin, Nicolas Stenger, Patrice Brenner, John B. Pendry and Martin Wegener, "Three-Dimensional Invisibility Cloak at Optical Wavelengths", Science 328, 337 (2010). Abstract. 2Physics Article.
[10] Joachim Fischer, Tolga Ergin, Martin Wegener, "Three-dimensional polarization-independent visible-frequency carpet invisibility cloak", Optics Letters, 36, 2059 (2011). Abstract.
[11] J. Fischer et al., Optics Materials Express, submitted (2011).

Towards Very Compact Invisibility Cloaks

Jingjing Zhang (left) and Niels Asger Mortensen (right) of Technical University of Denmark

Authors: Jingjing Zhang¹, Shuang Zhang², and Niels Asger Mortensen¹

Affiliations:
¹DTU Fotonik - Dept of Photonics Engineering, Technical University of Denmark, Denmark
²School of Physics and Astronomy, University of Birmingham, UK

Much effort has been made to realize invisibility cloaks ever since the theoretical proposal based on transformation optics were first put forward in 2006 [1, 2]. There have been two main trends in the realization of invisibility cloaks: One is to extend the bandwidth of the cloak, and the other is to push the working frequencies to optical spectrum. These concerns have been addressed very recently where objects of millimeter size scale were successfully concealed over the whole visible range [3,4].

Shuang Zhang of University of Birmingham, UK

However, there is still one major problem that needs to be solved for the cloak design and realization, which is an important issue in practical applications. The size of the cloak device is usually much (one to two orders of magnitude) larger than that of the cloaked object, meaning that even hiding a tiny object requires a fairly large device. Our team, Jingjing Zhang, Liu Liu, and Niels Asger Mortensen from Technical University of Denmark, Yu Luo from Imperial College London, and Shuang Zhang from University of Birmingham makes the first attempt to address this concern and demonstrates at optical frequencies a cloak whose size is only four times that of the hidden object [5].

Past 2Physics article by Shuang Zhang:
February 13, 2011: "Macroscopic Invisibility Cloak made from Natural Birefringent Crystals"

Liu Liu of Technical University of Denmark (left) and Yu Luo from Imperial College London (right)

In the previous works on visible invisibility cloak, the ratio between the size of the object to be hidden and the size of cloak is limited by the small birefringence of the constituent calcite crystal (~10% difference between n_e and n_o), resulting in cloaks 20 times larger than the object. Here we construct an invisibility cloak from nano-structured homogeneous anisotropic composite materials, which are obtained by adopting semiconductor manufacturing techniques that involve patterning the top silicon layer of an SOI wafer with subwavelength gratings of appropriate filling factor (see Fig. 1). The effective media consisting of silicon gratings bypass the limitation of natural material at hand and give us extra freedom to design the devices as desired, especially those with miniaturized thickness, and the shape of the cloak can be custom designed by properly arranging the composing materials.

Fig. 1 Scanning electron microscopic image of a fabricated carpet cloak from (a) top view (b) oblique view. The inset shows the detail of the silicon gratings.


In the measurement, the light with TM polarization from a tunable laser is used to characterize the cloak. The cloak works by essentially disguising an object from light, making it appear like a flat ground plane. By precisely restoring the path of the reflecting wave from the surface, the cloak creates an illusion of a flat plane for a triangular bump on the surface and any objects underneath, hiding their presence over wavelengths ranging from 1480nm to 1580nm (see Fig. 2).

Fig. 2 The measured output image from a flat surface (left) and a cloaked protruded surface (right) at 1480nm (a), 1550nm (b), and 1580nm (c).

The cloak is made exclusively of dielectric materials, enabling the broadband and low-loss invisibility. In contrast to previous works based on nanostructures [6, 7], our cloak also shows advantages of easier design and fabrication. More importantly, the uniform grating profiles may be fabricated using large area interferometric lithography technique, and therefore, there is no size limitation on the invisibility cloak.

This design approach can be well extended to other frequency ranges. In microwave frequencies where we have access to dielectric materials with very high permittivity, the relative size of the cloak compared with the hidden object can be even more significantly minimized. With more precise fabrication, our scheme also holds promise for a true invisibility cloak that works in the visible parts of the spectrum and at a larger size, and has potential applications in integrated photonics and plasmonics.

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] Xianzhong Chen, Yu Luo, Jingjing Zhang, Kyle Jiang, John B. Pendry & Shuang Zhang, “Macroscopic invisibility cloaking of visible light.” Nature Communications, 2:176, (2011). Abstract.
[4] Baile Zhang, Yuan Luo, Xiaogang Liu, George Barbastathis,“Macroscopic invisibility cloak for visible light”, Phys. Rev. Lett. 106, 033901 (2011). Abstract.
[5] Jingjing Zhang, Liu Liu, Yu Luo, Shuang Zhang, and Niels Asger Mortensen, "Homogenous optical cloak constructed with uniform layered structures," Optics Express 19, 8625-8631, (2011). Abstract.
[6] J. Valentine, J. Li, T. Zentgraf, G. Bartal, X. Zhang, “An optical cloak made of dielectrics.” Nature Materials, 8, 568–571 (2009). Abstract.
[7] L.H. Gabrielli, J. Cardenas, C.B. Poitras, M. Lipson, “Silicon nanostructure cloak operating at optical frequencies.” Nature Photonics, 3, 461–463 (2009). Abstract.

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

Alexandra Boltasseva

Authors:
Alexandra Boltasseva¹ and Harry A. Atwater²

Affiliations:

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

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

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

Harry A. Atwater

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

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

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

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

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

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

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

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

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

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.

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)

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.

3D Invisibility Cloaking Device at Optical Wavelengths

Martin Wegener, Tolga Ergin, Nicolas Stenger (from left to right)

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




Authors: Tolga Ergin, Nicolas Stenger, Martin Wegener

Affiliation: Institut für Angewandte Physik, Karlsruher Institut für Technologie (KIT), Germany

In recent years, the emerging field of transformation optics has been shown to be a powerful design tool for a variety of innovative and new optical elements. In this design process, the inverse problem of electromagnetism can be addressed: If a certain functionality of a device is needed, what materials should it be composed of? Once the desired “path” that light should take inside the device is established, the mathematical rules of transformation optics dictate the optical parameters (electric and magnetic response) of the material.

Fig.1: (click on the image to see higher resolution version) Numerical raytracing calculations showing (a) a room with mirror on the floor, (b) with an additional bump (outlined in green) and (c) with the cloaking structure on top (outlined in red).

A thrilling possibility that arose alongside these new design options is the fact that light can now be guided around objects. The light wave will “flow” around an object, just like water around a rock, and reform afterwards. Since there is no interaction between the light wave and the central region (the “rock”), this region will become invisible to an external observer. Such invisibility cloaks have high demands on the optical parameters (inhomogeneity, anisotropy), which makes them challenging to fabricate. Nevertheless, the proof of concept has been shown in the microwave regime [1].

In 2008, Jensen Li and Sir John Pendry proposed a different approach to invisibility [2]. Suppose an object would be hidden under a reflecting mirror, the resulting bump could be seen in the distortions of the reflected image. By building a so-called “carpet cloak” on top of that bump, these distortion disappear since the light waves are guided such that an observer perceives a flat mirror – the bump and the object are effectively invisible. This design has relaxed requirements on the optical parameters of the material, whereby “only” a spatial distribution of an isotropic refractive index is needed. This makes it very interesting for experiments [3,4].

Although originally designed to work in a purely 2D scenario, we could show with numerical raytracing calculations [5] that one still expects a cloaking effect at angles out of the design plane. Following this theoretical work, we built the carpet cloak as a full 3D structure by means of Direct Laser Writing (DLW). Here, a photo resist is illuminated by a focused laser beam. Just like a pencil tip, the laser focus can “write” a programmed three-dimensional structure into the resist. After development, the non-illuminated parts of the resist are washed out and the written 3D polymer structure remains.

Fig.2 : Gold film with bump and invisibility cloaking structure (woodpile photonic crystal) on glass substrate.

We realized the needed refractive index distribution for the carpet cloak by using a woodpile photonic crystal [6]. It is composed of small polymer rods with a distance of 800 nm, whereas the size of the rods changes locally. This leads to a spatially varying filling fraction of polymer and air, which controls the refractive index. The structure is coated with a thin gold film (the “carpet”).

We measured the cloaking device with an experimental setup similar to an optical microscope: The sample is illuminated with unpolarized light from an incandescent lamp and a microscope objective images and magnifies the structure onto an image plane. This plane is scanned with the tip of a glass fiber, which ends in a Fourier Transform Infrared Spectrometer. By scanning the glass fiber across the magnified image, we can measure a cross-section of the structure and simultaneously acquire the full spectrum at every position. The cloaking effect was observed from 1.4 µm to 2.7 µm and spans almost one octave. Furthermore, it is expected to still work on even longer wavelengths, since the effective medium condition (wavelength has to be larger than local nanostructure) still holds and the polymer does not show strong absorption. In the measured interval, the visibility of the bump was drastically reduced.

With this experiment, we have taken a first step to bring the concepts of transformation optics into the third dimension. Furthermore, wavelengths at which cloaking is observed (near infrared) are close to the visible wavelength regime.

Reference
[1] D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, "Metamaterial Electromagnetic Cloak at Microwave Frequencies", Science 314, 977(2006). Abstract.
[2] J. Li, J. B. Pendry, "Hiding under the Carpet: A New Strategy for Cloaking", Phys. Rev. Lett. 101, 203901 (2008). Abstract.
[3] J. Valentine, J. Li, T. Zentgraf, G. Bartal, X. Zhang, "An optical cloak made of dielectrics", Nature Materials, 8, 568 (2009). Abstract.
[4] L. H. Gabrielli, J. Cardenas, C. B. Poitras, M. Lipson, "Silicon nanostructure cloak operating at optical frequencies", Nature Photonics, 3, 461 (2009). Abstract.
[5] J. C. Halimeh, T. Ergin, J. Mueller, N. Stenger, M. Wegener, "Photorealistic images of carpet cloaks", Optics Express, 17, 19328 (2009). Abstract.
[6] Tolga Ergin, Nicolas Stenger, Patrice Brenner, John B. Pendry, Martin Wegener, "Three-Dimensional Invisibility Cloak at Optical Wavelengths", Science, in press, (Published Online March 18, 2010 Science DOI: 10.1126/science.1186351). Abstract.

Large Broadband Invisibility Cloak for Visible Light

Vera Smolyaninova (Towson University)


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

Authors: Vera Smolyaninova¹ and Vlad Shalaev<sup>2

1</sup>Dept. of Physics Astronomy and Geosciences, Towson University, Towson, MD, USA

²Birck Nanotechnology Center, School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA


Most researchers believe that sophisticated artificially engineered materials are required to build an invisibility cloak. Such “metamaterials” exhibit high losses and work for only one color. The resulting invisibility cloaks are tiny, and cannot hide anything if another color of light is used. Our team, Igor Smolyaninov from BAE Systems, Vera Smolyaninova from Towson University, Alexander Kildishev and Vlad Shalaev form Purdue University demonstrated a different approach to cloaking.

Vlad Shalaev (Purdue University)

Instead of sophisticated metamaterials, we used a waveguide, which is curved to mimic the metamaterial properties. This approach leads to all-color invisibility cloak with much lower losses. As a result, we built a large optical cloak, which is about hundred times larger than the cloaks built previously. This “see-through” cloak bends light around itself and thus differs from the “invisibility carpet,” which camouflages bumps on a metal surface. We believe that further size increase is possible, and that the same technique may be applied to other tasks, which require the use of metamaterials, such as building new “hyperlenses” which considerably surpass the resolution limit of conventional lenses.

This work is reported in the May 29 issue of Physical Review Letters [1]. In the experiments, conducted at Towson University, electromagnetic cloaking is achieved using a specially tapered waveguide. An area with a radius ~100 times larger than the wavelengths of light shined by a laser into the device has been cloaked, an unprecedented achievement. This is the first experiment on optical cloaking performed with normal visible light.

Previous experiments with metamaterials, which require complex nanofabrication, have been limited to cloaking regions only a few times larger than the wavelengths of visible light [2,3]. The new design is a far simpler device: waveguides represent established technology - including fiber optics - used in communications and other commercial applications. Because the new method enabled us to dramatically increase the cloaked area, the technology offers hope of cloaking larger objects. All previous attempts at optical cloaking have involved very complicated nanofabrication of metamaterials containing many elements, which makes it very difficult to cloak large objects. Here, we showed that if a waveguide is tapered properly it acts like a sophisticated nanostructured material. The waveguide is inherently broadband, meaning it could be used to cloak the full range of the visible light spectrum. Unlike metamaterials, which contain many light-absorbing metal components, only a small portion of the new design contains metal.

Igor Smolyaninov (BAE Systems)

Theoretical work for the design was led by Purdue, with BAE Systems and Towson University leading work to fabricate the device and demonstrate its cloaking properties. The cloaking device is formed by two gold-coated surfaces, one a curved lens and the other a flat sheet. We cloaked an object about 50 microns in diameter, or roughly the width of a human hair, in the center of the waveguide. Instead of being reflected as normally would happen, the light flows around the object and shows up on the other side, like water flowing around a stone.

This research falls within a new field called transformation optics, which may usher in a host of radical advances, including cloaking; powerful "hyperlenses" resulting in microscopes 10 times more powerful than today's and able to see objects as small as DNA; computers and consumer electronics that use light instead of electronic signals to process information; advanced sensors; and more efficient solar collectors.

Alexander Kildishev (Purdue University)

Unlike natural materials, metamaterials are able to reduce the "index of refraction" to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material. Natural materials typically have refractive indices greater than one. Metamaterials, however, can be designed to make the index of refraction vary from zero to one, which is needed for cloaking. The precisely tapered shape of the new waveguide alters the refractive index in the same way as metamaterials, gradually increasing the index from zero to 1 along the curved surface of the lens. Previous cloaking devices have been able to cloak only a single frequency of light, meaning many nested devices would be needed to render an object invisible.

We reasoned that the same nesting effect might be mimicked with the waveguide design. Subsequent experiments and theoretical modeling proved the concept correct. We do not know of any fundamental limit to the size of objects that could be cloaked, but additional work will be needed to further develop the technique.

Recent cloaking findings reported by researchers at other institutions have concentrated on a technique that camouflages features against a background. Those works, which use metamaterials, are akin to rendering bumps on a carpet invisible by allowing them to blend in with the carpet, whereas our work concentrates on enabling light to flow around an object.

The work was funded by the ARO-MURI and the National Science Foundation.

References
[1] “Anisotropic metamaterials emulated by tapered waveguides: application to electromagnetic cloaking”, I.I. Smolyaninov, V.N. Smolyaninova, A.V. Kildishev, and V.M. Shalaev,
Phys. Rev. Letters, 102, 213901 (2009). Abstract.
[2] “Metamaterial electromagnetic cloak at microwave frequencies”, D. Schurig, J.J. Mock, B.J. Justice, S.A. Cummer, J.B. Pendry, A.F. Starr, D.R. Smith, , Science 314, 977-980 (2006). Abstract.
[3] “Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm”, I.I. Smolyaninov, Y.J. Hung, and C.C. Davis, Optics Letters 33, 1342-1344 (2008). Abstract.

Invisibility Cloak for Near-Infrared Light

Xiang Zhang [Photo courtesy: Roy Kaltschmidt/ Lawrence Berkeley National Laboratory]

In a paper published in 'Nature Materials' [1], a team led by Xiang Zhang, a principal investigator with Lawrence Berkeley National Laboratory’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, reported the creation of a “carpet cloak” from nanostructured silicon that conceals the presence of objects placed under it from optical detection in near-infrared region at a wavelength range of 1,400–1,800 nm.

Previous demonstrations of cloaking, where objects are rendered invisible at certain frequencies, have been limited to the microwave regime. This new development takes a significant step closer to achieving invisibility in a region that can be seen (no ... not 'seen' ...must be changed to 'experienced'!) by human eye.

In recent years, several theories for 'invisibility' schemes have been proposed for cloaking devices using transformation optics and conformal mapping. The necessary medium for enabling precise control over the flow of electromagnetic waves was provided by Metamaterial which gains its properties from its spatially tailored structure rather than directly from its composition. The first microwave cloaking with the use of metamaterials was demonstrated in 2006 by Duke University's Pratt School of Engineering [2] but the realization of cloaking at optical frequencies, a key step towards achieving actual invisibility, has eluded scientists mainly because the metal elements absorb too much light.

Previous work by Zhang and his group with invisibility devices involved complex metamaterials - composites of metals and dielectrics whose extraordinary optical properties arise from their unique structure rather than their composition. They constructed one material out of an elaborate fishnet of alternating layers of silver and magnesium fluoride, and another out of silver nanowires grown inside porous aluminum oxide. With these metallic metamaterials, Zhang and his group demonstrated that light can be bent backwards, a property unprecedented in nature.

“We have come up with a new solution to the problem of invisibility based on the use of dielectric (nonconducting) materials,” says Zhang. “Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step towards transformation optics, opening the door to manipulating light at will for the creation of powerful new microscopes and faster computers.”

[Image courtesy: Thomas Zentgraf] These three images depict how light striking an object covered with the carpet cloak acts as if there were no object being concealed on the flat surface. In essence, the object has become invisible.

The optical 'carpet' cloak designed by the Berkeley team used quasi-conformal mapping to conceal an object that is placed under a curved reflecting surface by imitating the reflection of a flat surface. The cloak consists only of isotropic dielectric materials, which enables broadband and low-loss invisibility at a wavelength range of 1,400–1,800 nm. While the carpet itself can still be seen, the bulge of the object underneath it disappears from view. Shining a beam of light on the bulge shows a reflection identical to that of a beam reflected from a flat surface, meaning the object itself has essentially been rendered invisible.

[Video by Jensen Li] This video shows how a beam of light is obstructed by an object in a flat surface and casts a shadow until the object is cloaked, at which point the light is reflected as if the surface were still perfectly flat.

The new cloak created by Zhang and his team is made exclusively from dielectric materials, which are often transparent at optical frequencies. The cloak was demonstrated in a rectangular slab of silicon (250 nanometers thick) that serves as an optical waveguide in which light is confined in the vertical dimension but free to propagate in the other two dimensions. A carefully designed pattern of holes - each 110 nanometers in diameter - perforates the silicon, transforming the slab into a metamaterial that forces light to bend like water flowing around a rock. In the experiments reported in Nature Materials, the cloak was used to cover an area that measured about 3.8 microns by 400 nanometers. It demonstrated invisibility at variable angles of light incident.

[Image courtesy: Xiang Zhang] Image (a) is a schematic diagram showing the cloaked region (marked with green) which resides below the reflecting bump (carpet) and can conceal any arbitrary object by transforming the shape of the bump back into a virtually flat object. Image (b) was taken with a scanning electron microscope image of the carpet coated bump.

Right now the cloak operates for light between 1,400 and 1,800 nanometers in wavelength, which is the near-infrared portion of the electromagnetic spectrum, just slightly longer than light that can be seen with the human eye. However, because of its all dielectric composition and design, Zhang says the cloak is relatively easy to fabricate and should be upwardly scalable. He is also optimistic that with more precise fabrication this all dielectric approach to cloaking should yield a material that operates for visible light - in other words, true invisibility to the naked eye.

“Even with the advances that have been made in optical metamaterials, scaling sub-wavelength metallic elements and placing them in an arbitrarily designed spatial manner remains a challenge at optical frequencies", says Zhang,"In this experiment, we have demonstrated a proof of concept for optical cloaking that works well in two dimensions. Our next goal is to realize a cloak for all three dimensions, extending the transformation optics into potential applications.”

Reference
[1] "An optical cloak made of dielectrics"
Jason Valentine, Jensen Li, Thomas Zentgraf, Guy Bartal & Xiang Zhang,
Nature Materials, Published online: 29 April 2009 doi:10.1038/nmat2461. Abstract
[2] "Metamaterial Electromagnetic Cloak at Microwave Frequencies"
D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, D. R. Smith,
Science, Vol. 314, pp. 977 - 980 (2006). Abstract

[We thank Lawrence Berkeley National Laboratory media relations for materials used in this posting]

'Invisibility Cloaks' Could Break Sound Barriers

Steven Cummer

Contrary to earlier predictions, Duke University engineers report in Physical Review Letters that a three-dimensional sound cloak is possible, at least in theory.

Such an acoustic veil would do for sound what the "invisibility cloak" previously demonstrated by the research team does for microwaves -- allowing sound waves to travel seamlessly around it and emerge on the other side without distortion. Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering said that such a cloak might hide submarines in the ocean from detection by sonar, he said, or improve the acoustics of a concert hall by effectively flattening a structural beam.

As in the case of the microwave cloak, the properties required for a sound cloak are not found among materials in nature and would require the development of artificial, composite metamaterials. (For more about metamaterials, click this link) The engineering of acoustic metamaterials lags behind those that interact with electromagnetic waves (i.e. microwaves or light), but "the same ideas should apply," Cummer said.

In 2006, researchers at Duke and the Imperial College London used a new design theory to create a blueprint for an electromagnetic invisibility cloak. Only a few months later,the team demonstrated the first such cloak, designed to operate at microwave frequencies.

Cummer and David Schurig, a former research associate at Duke who is now at North Carolina State University, later reported in "The New Journal of Physics" a theory showing that an acoustic cloak could be built. But that theory relied on a "special equivalence" between electromagnetic and sound waves that is only true in two dimensions. A report by another team had also suggested that a 3-D acoustic cloak couldn't exist. It appeared they had reached a dead end.

This time, he started instead from a shell like the microwave cloak his team had already devised and attempted to derive the mathematical specifications required to prevent such a shell from reflecting sound waves, a key characteristic for achieving invisibility. On paper, at least, it worked.

Image Description: Analytical computation of the interaction of an acoustic wave with the ideal cloaking shell. The peaks and valleys in the color scale represent the instantaneous pressure field of a uniform plane wave traveling from left to right as it impinges on a rigid sphere surrounded by a cloaking shell whose thickness is equal to the radius of the interior sphere. The shell smoothly bends the acoustic wavefronts around the interior rigid sphere so that they smoothly pass around the object. No acoustic wave is scattered back toward the source, nor does the object cast any acoustic shadow. From the pressure field outside the shell, there's no way to know whether there is an object there or not.

Although the theory used to design such acoustic devices so far isn't as general as the one used to devise the microwave cloak, the finding nonetheless paves the way for other acoustic devices, for instance,those meant to bend or concentrate sound. The existence of an acoustic cloaking solution also indicates that cloaks might possibly be built for other wave systems, including seismic waves that travel through the earth and the waves at the surface of the ocean.

Reference
"Scattering Theory Derivation of a 3D Acoutic Cloaking Shell"
S. Cummer, B.-I. Popa, D. Schurig, D.R. Smith, J. Pendry, M. Rahm and A.Starr,
Physical Review Letters, 100, 024301 (2008), Abstract Link.

Negative Refraction of Visible Light

Harry A. Atwater (photo courtesy: Caltech)

In the online publication 'Science Express', applied physics researchers from California Institute of Technology, Henri Lezec, Jennifer Dionne, and Professor Harry Atwater, reported their success in constructing a nanofabricated photonic material that creates a negative index of refraction in the blue-green region of the visible spectrum. Their device makes visible light travel in the opposite direction and not refract or bend when passing from one material to another, like from air through water or glass.

Researchers in recent years have created materials with negative diffraction for microwave and infrared frequencies. These achievements have exploited the relatively long wavelengths at those frequencies--the wavelength of microwaves being a few centimeters, and that of infrared frequencies about the width of a human hair. Visible light, because its wavelength is at microscopic dimensions--about one-hundredth the width of a hair--has defeated this conventional approach.

The physicists of the Atwater group at Caltech came up with a clever new idea that if new optical materials could be constructed at the nanoscale level in a certain way, it might be possible to make the light bend at the same angle. The datails are complicated, but have to do with the speed of light through the material itself.

They employed a few ideas from the emerging field of work on 'plasmonics', in which light is "squeezed" with specially designed materials to create a wave known as a 'plasmon'. In this case, the plasmons act in a manner somewhat similar to a wave carrying ripples across the surface of a lake, carrying light along the silver-coated surface of a silicon-nitride material, and then across a nanoscale gold prism so that the light reenters the silicon-nitride layer with negative refraction.

The method could in principle be used to construct optical microscopes for imaging things as small as molecules, and even to create cloaking devices for rendering objects truely invisible (none of the previous ideas about 'invisible' cloak had to do with visible light!).

Reference:
"Negative Refraction at Visible Frequencies",
by Henri J. Lezec, Jennifer A. Dionne, Harry A. Atwater,
Science Express (online), Link to Abstract.
Will be published in the journal 'Science' on a future date.