Near-Infrared Metamaterials Go Beyond Metals

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











Authors: Gururaj V. Naik and Alexandra Boltasseva 

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

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

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

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

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

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

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

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

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

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

A Cloak for Elastic Waves in Thin Polymer Plates

[From Left to Right]  Nicolas Stenger, Manfred Wilhelm and Martin Wegener

Authors: Nicolas Stenger¹, Manfred Wilhelm² and Martin Wegener<sup>1

1</sup>Institute of Applied Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
²Institute for Technical Chemistry and Polymer Chemistry, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany  

 Since the 2006 pioneering theoretical works of Sir John Pendry et al [1] and Ulf Leonhardt [2], the ideas of transformation optics (TO) have been experimentally realized into new optical elements [3-5]. Among all the possible elements predicted by TO, the most demanding one in terms of fabrication is the cylindrical cloak or also called the free space cloak [6]. This device guides an impinging electromagnetic (EM) wave around an object without interacting with it, thus making the object and the cloak itself completely invisible to an external observer. The optical parameter (electric permittivity and magnetic permeability) values needed to guide EM waves in this manner are extreme and normally not available in Nature. To obtain such extreme values, we need to fabricate small resonant metallic elements, or “meta-atoms” which are much smaller than the wavelength of the impinging wave in order to act as an effective medium. This requires engineering meta-atoms with tens of nanometers sizes for visible EM waves and this is still difficult to reach with modern electron beam lithography. Moreover these resonant meta-atoms absorb a non-negligible part of the impinging light because they are usually dispersive, i.e., their optical response changes with frequency, thus strongly limiting the efficiency of the cloak.

Past 2Physics article by this Group:
June 19, 2011: "3D Polarization-Independent Invisibility Cloak at Visible Wavelengths" by Tolga Ergin, Joachim Fischer, Martin Wegener
April 11, 2010: "3D Invisibility Cloaking Device at Optical Wavelengths"
by Tolga Ergin, Nicolas Stenger, Martin Wegener

 Nevertheless David Shurig et al [6] fabricated such a structure for microwave frequencies (the wavelength is here of the order of a few millimeters) by using tailored U-shaped resonant meta-atoms also called split-ring-resonators (SRR). Since they used SRR, the efficiency of their cloaking device is restrained to a very narrow band of frequencies. However, this has been the only demonstration of a free-space cloak for EM waves so far.

 However, the ideas of TO are not restricted to EM waves and other groups have started to adapt TO to waves propagating in matter [7]. This led to two first experimental realizations of cloaking for acoustic waves propagating at the surface of a fluid [8] and for ultrasound pressure waves propagating in water [9]. These two cylindrical cloaks were working in a broader band of frequencies because the constituting elements and the meta-atoms used where less dispersive in comparison with their EM counterparts.

 Our group decided to investigate the possibility to fabricate a cylindrical cloak for waves propagating in elastic materials like waves propagating in guitar strings or at the surface of a drum. The main advantage of elastic materials lies in the fact that their properties, i.e., elastic modulus and the density, can show a very large contrast without frequency dependence for a broad range of frequencies. For example polyvinyl chloride (PVC) has an elastic modulus three orders of magnitude higher than polydimethylsiloxane (PDMS), a silicon rubber, and their densities are almost the same. This eliminates the need for resonant meta-atoms.

 Mohamed Farhat et al [10] showed theoretically the possibility to apply the ideas of TO to flexural waves propagating in thin elastic plates for frequencies of a few hundreds of Hertz. A flexural wave is a vertical displacement propagating in a thin plate or in an elastic membrane. The original design proposed by Mohamed Farhat et al for a cylindrical cloak consists of ten concentric rings made of six different materials. “Gluing” six different materials together still remains a technical challenge because polymer materials are usually repelling each others. We therefore decided to simplify the design by using 16 composites made only out of two materials [11], i.e., a hard material (PVC) and a very soft one (PDMS) (respectively white and black parts in Fig. 1a). By changing the PVC filling fraction from 0% to 100%, the effective elastic modulus can be tuned from the small PDMS value to the large PVC value (Fig. 1a). We then mapped the effective elastic moduli profile computed theoretically onto a local PVC filling fraction in our structure [11].

 To fabricate our cloak we mechanically machined small holes into a thin PVC plate with different volume filling fractions for each of the 20 concentric rings (Fig. 1b) [11] and then filled the holes with PDMS (not shown on Fig. 1b). Here the holes in the composite and the relative thicknesses of the concentric rings are much smaller than the wavelength of the impinging wave because we want our structure to appear as an effective material. The central region of the cloak was then clamped to zero amplitude and was used as the scattering object we want to hide.

Fig. 1: a) Blueprint of the free space cloak. The yellow color corresponds to the outside PVC part of the cloak with a constant filling fraction; the black (PDMS) and white (PVC) circular structure in the middle represents the cloak with 20 rings made of 16 different composites. The object we want to make invisible is symbolized by the red circle in the middle. b) Oblique view of our cloak after drilling holes in a PVC plate and before filling them with PDMS.

 We characterized our cloaking device with a home-built setup. We excited flexural waves with two loudspeakers attached to one end of the plate. A camera was placed vertically above the plate and recorded the vertical displacement created by the flexural wave. A diffuse stroboscopic illumination is then used to follow step by step the propagation of the wave in our structure. Unwanted reflections were reduced by placing absorbing foam material against the other end and the two sides of the plate.

 Figure 2 shows snapshots taken from the reference plate (column a) and the plate with the cloaking structure (column b) for two different frequencies (see also [11] for more frequencies and corresponding movies). The former plate is used as a baseline to quantify the scattering effect of the object on the impinging wave. The filling fraction of this plate is the same as the plate outside the cloaking structure in Fig. 1. For 200Hz (Fig. 2a top row) we can clearly see the effect of the central region leading to strong scattering in front of the object (standing wave) and to shadowing effect behind it. Furthermore the wave front is strongly distorted after the object. With the presence of the cloaking structure (Fig. 2b top row), symbolized by the black dashed circle, the scattering and distortion effects are strongly suppressed. We even recover a plane wave after the clamped region; an external observer will not be able to make the difference between a plane wave propagating in the reference plate without the object and another plate with a cylindrical cloak and the object in its center. The object is thus invisible.

Fig. 2: Measurement snapshots of the propagation of a flexural wave on a thin plate. A monochromatic wave is injected from the left side and propagates to the right. The left column (a) corresponds to the reference plate with a homogeneous filling fraction. The region clamped to zero amplitude (object) is represented by black circles. The right column (b) corresponds to the object plus the cloak. The dashed circles illustrate the outer radius of the cloak. The white scale bar in each image is 5 cm.

 Since our structure consists of non resonant elements we therefore expect our structure to be efficient on a wide range of frequency. This is indeed the case as shown for 400Hz (Fig. 2 bottom row), one octave above, where the cloak is still strongly reducing the scattering of the object. However increasing imperfections are visible with increasing frequency. Here the wavelength is small enough to see the cloak as a discrete structure and the continuous approximation made at the beginning is no more valid. Conceptually, our cloak should also work for frequencies below 200Hz down to 0Hz, however, we were not able to perform measurements in this range.

 To conclude, we have fabricated and characterized a broadband cylindrical cloak for elastic waves and its effect spans more than one octave. To our knowledge this is the largest bandwidth observed in any free-space cloaking device [11]. It is interesting to note this structure is rather easy to fabricate and quite inexpensive. Thus, it is perfectly well suited to convey the ideas of transformation optics. This structure can also be seen as a model experiment for seismic cloaks as discussed in [10].

References: 
[1] J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling Electromagnetic Fields”, Science 312, 1780 (2006). Abstract.
[2] U. Leonhardt, “Optical Conformal Mapping”, Science 312, 1777 (2006). Abstract.
[3] H. Chen, C. T. Chan, and P. Sheng, “Transformation optics and metamaterials”, Nature Materials 9, 387 (2010). Abstract.
[4] T. Ergin, N. Stenger, P. Brenner, J. B. Pendry, and M. Wegener, ”Three-Dimensional Invisibility Cloaking at Optical Wavelengths”, Science 328, 337 (2010). Abstract. 2Physics Post.
 [5] J. Fischer, T. Ergin, and M. Wegener, “Three-dimensional polarization-independent visible-frequency carpet invisibility cloak”, Optics Express 36, 2059 (2011). Abstract. 2Physics Post.
[6] 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 (2006). Abstract.
[7] G. W. Milton, M. Briane, and J. R. Willis, “On cloaking for elasticity and physical equations with a transformation invariant from”, New Journal of Physics 8, 248 (2006). Abstract.
[8] M. Farhat, S. Enoch, S. Guenneau, and A. B. Movchan, “Broadband Cylindrical Acoustic Cloak for Linear Surface Waves in Fluid”, Physical Review Letters 101, 134501 (2008). Abstract.
[9] S. Zhang, C. Xia, and N. Fang, “Broadband Acoustic Cloak for Ultrasound Waves”, Physical Review Letters 106, 024301 (2011). Abstract.
[10] M. Farhat, S. Guenneau, S. Enoch, and A. B. Movchan, “Ultrabroadband Elastic Cloaking in thin Plates”, Physical Review Letters 103, 024301 (2009). Abstract.
[11] N. Stenger, M. Wilhelm, and M. Wegener, “Experiments on Elastic Cloaking in Thin Plates”, Physical Review Letters 108, 014301 (2012).Full Paper.

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).

Negative Index Materials Reverse the Optical Doppler Effect

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Extremely High Refractive Index Terahertz Metamaterial

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

Author: Bumki Min

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Alexandra Boltasseva

Authors:
Alexandra Boltasseva¹ and Harry A. Atwater²

Affiliations:

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

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

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

Harry A. Atwater

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

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

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

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

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

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

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

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

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

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.