Transformational Thermodynamics: Cloaks to Keep You Cool

Sébastien Guenneau (left) and Claude Amra (right)












Authors: Sebastien Guenneau and Claude Amra

Affiliation: Institut Fresnel, Centre National Recherche Scientifique, Aix-Marseille University and Ecole Centrale Marseille, France

 In 2006, two papers -- published in the same issue of 'Science' -- revolutionized the world of classical optics with the concept of transformation optics (TO). One was by Ulf Leonhardt of University of St Andrews, Scotland, UK [1] and the other was by John Pendry of Imperial College, London, UK and David Schurig and David Smith of Duke University, USA [2]. The concept of transformation optics allows coordinate changes in an isotropic homogeneous dielectric medium that can lead to a anisotropic heterogeneous metamaterial described by tensors of permittivity and permeability, a fact foreseen twenty years ago in two visionary papers on computational electromagnetic [3,4].

The conceptual breakthrough in 2006 was to note that one can blow-up a point into a finite region using a change of coordinates known from mathematicians working on inverse problems [5], in order to conceal this region from electromagnetic waves, and to demonstrate the feasibility of an invisibility cloak for microwaves [6], thereby bringing perhaps the most stunning electromagnetic paradigm of all times into reality!

However, similar changes of coordinates can also be applied to other wave equations, such as linear water waves propagating at the surface of a fluid [7], pressure waves propagating in a fluid [8], coupled pressure and shear waves propagating in a solid material [9,10], or flexural waves in thin elastic plates [11,12]. Such metamaterials designed using transformation acoustics (TA) could be used to protect regions from tsunamis (in general from ocean waves) or earthquakes (especially from surface elastic waves known as Rayleigh waves) on a larger scale! They could also be used to improve sound in opera theaters or to hide submarines from sonars (silence cloak).

But that’s not the end of the invisible story, as one can also leave the world of TO and TA and enter the brave new world of transformation thermodynamics (TT, or T²), whereby one now wishes to control diffusion processes, such as heat. Some precursory theoretical and numerical studies on the conduction equation in anisotropic media [5,13,14] have shown that one can control the diffusive heat flow in new ways in the static limit, a fact experimentally demonstrated this year [15].

Figure 1: [Click on the image to view high resolution version] Numerical simulation showing the distribution of temperature in a region heated from the left (temperature=100 degrees Celsius, red color). The temperature gradually decreases away from the source, until it reaches a temperature of 0 degree Celsius on the left hand side (blue color). Importantly, one sees that the temperature vanishes inside the inner disc of the thermal cloak (annulus containing an anisotropic heterogeneous conductivity). This thermal protection is achieved by curving the isothermal curves (black lines).

Our group at the Fresnel Institute in Marseille and the Ecole Centrale in Paris has shown, under the umbrella of the French National Center for Scientific Research (CNRS), that TT is a valid concept: one can control the flow of heat when time flows [16], and it is enough to use concentric layers with isotropic homogeneous conductivity to design invisibility cloaks (to protect a region from heat, see figure 1) and concentrators (to enhance heat exchange in a region). This opens unprecedented routes towards heat insulators -- for instance, for Green houses and also for heat harvesting in photovoltaics.

References:
[1] U. Leonhardt, “Optical Conformal Mapping”, Science 312, 1777 (2006). Abstract.
[2] J. B. Pendry, D. Schurig, and D. R. Smith, “Controlling Electromagnetic Fields”, Science, 312, 1780 (2006). Abstract. 
[3] A. Nicolet, J.F. Remacle, B. Meys, A. Genon and W. Legros, “Transformation methods in computational electromagnetic“, Journal of Applied Physics, 75, 6036-6038 (1994). Abstract.
[4] A.J. Ward and J.B. Pendry, “Refraction and geometry in Maxwell’s equations“, Journal of Modern Optics, 43, 773-793 (1996). Abstract.
[5] A Greenleaf, M Lassas and G Uhlmann, “On nonuniqueness for Calderon’s inverse problem“, Mathematical Research Letters, 10, 685–693 (2003). Full Article.
[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] 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.
[8] S. Zhang, C. Xia, and N. Fang, “Broadband Acoustic Cloak for Ultrasound Waves”, Physical Review Letters, 106, 024301 (2011). Abstract.
[9] 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.
[10] M. Brun, S. Guenneau and A.B. Movchan, "Achieving control of in-plane elastic waves". Applied Physics Letters, 94, 061903 (2009). Abstract.
[11] M. Farhat, S. Guenneau, S. Enoch, and A. B. Movchan, “Ultrabroadband Elastic Cloaking in thin Plates”, Physical Review Letters, 103, 024301 (2009). Abstract.
[12] N. Stenger, M. Wilhelm, and M. Wegener, “Experiments on Elastic Cloaking in Thin Plates”, Physical Review Letters, 108, 014301 (2012). Full Article. 2Physics Article.
[13] C. Z. Fan, Y. Gao, and J. P. Huang, “Shaped graded materials with an apparent negative thermal conductivity", Applied Physics Letters, 92, 251907 (2008). Abstract.
[14] T. Chen, C. N. Weng, and J. S. Chen, “Cloak for curvilinearly anisotropic media in conduction", Applied Physics Letters, 93, 114103 (2008). Abstract.
[15] Supradeep Narayan and Yuki Sato, “Heat flux manipulation with engineered thermal materials", Physical Review Letters 108, 214303 (2012). Abstract.
[16] S. Guenneau, C. Amra, and D. Veynante, ‘’Transformation thermodynamics: cloaking and concentrating heat flux’’, Optics Express, 20, 8207 (2012). Abstract.

Broadband Array of Invisibility Cloaks in the Visible Frequency Range

Author: Vera Smolyaninova 

Affiliation: Dept. of Physics Astronomy and Geosciences, Towson University, MD, USA

Ever since the first experimental demonstrations in the microwave and visible ranges [1,2], invisibility cloaks have stimulated progress in the fields of metamaterials and transformation optics. Very recently, Farhat and co-workers [3] suggested that arrays of invisibility cloaks may have interesting electromagnetic properties, and suggested some potential applications in noninvasive probing, sensing and communication. Our team, Vera Smolyaninova and Kurt Ermer from Towson University, and Igor Smolyaninov from the University of Maryland demonstrated the first experimental realization of an invisibility cloak array.

Our experiment is based on the recent demonstration of broadband invisibility cloak, which relies on a curved waveguide mimicking the metamaterial properties necessary for cloaking [4]. Since a gap between a gold-coated spherical lens touching a gold-coated planar glass slide provides a good approximation of the required waveguide shape, such geometry can be easily transformed into a large array of broadband invisibility cloaks using commercially available microlens arrays. This work is reported in the May issue of the New Journal of Physics [5]. In the experiments, conducted at Towson University, very large arrays of roughly 25000 cloaks were used to “hide” approximately 20% of the surface area. This is the first experimental arrangement which lets you study mutual interactions of a very large number of invisibility cloaks.

Past 2Physics article by Vera Smolyaninova:
June 06, 2009: "Large Broadband Invisibility Cloak for Visible Light"
by Vera Smolyaninova and Vlad Shalaev

Igor Smolyaninov of University of Maryland

Unlike the so-called “carpet cloaks” which hide objects on the metallic mirror background, every cloak in our array guides light around the cloaked area. While the former approach is akin to rendering bumps on a carpet invisible by allowing them to blend in with the carpet, classical cloaking concentrates on enabling light to flow around an object. Typically, such classical cloaks [1,2] require sophisticated metamaterial nanofabrication. Each material has its own refractive 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. Refraction occurs as electromagnetic waves 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.

Kurt Ermer of Towson University

Unlike natural materials, metamaterials are able to produce the index of refraction ranging from very large positive values of the order of 100 to less than zero. In particular, artificial metamaterials needed for cloaking must have the index of refraction, which varies from zero to one. Unfortunately, such artificial metamaterials have very large losses. In our cloak array the precisely tapered shape of the waveguide around each cloak alters the refractive index in the same way as in metamaterials, gradually increasing the index from zero to 1 along the curved surface of each microlens. Since light propagates mainly through the air gap, losses in this design are very low, and the resulting structure is broadband. It works across the whole visible light spectrum.

Theoretical work for the design was led by the University of Maryland, with Towson University leading work to fabricate the device and demonstrate its cloaking properties. The cloaking array device is formed by two gold-coated surfaces, one surface being a commercially available microlens array, and the other a flat glass slide. Individual cloaks in the array were separated by about 30 microns, or roughly the width of a human hair, so that a 5 by 5 millimeter squared microlens array would make approximately 25000 individual invisibility cloaks. Instead of being reflected as normally would happen, the light flows around each cloak and shows up on the other side, like water flowing around an array of stones.

[Click on image to see a higher resolution version]

Building and studying the arrays of invisibility cloaks offers more refined experimental tools to test individual cloak performance. Compared to the characterization of individual cloaks, the angular performance of cloak arrays appears to be more sensitive to cloak imperfections. For example, cloak arrays perform better when light is sent in along the row directions. These findings may be useful in such related areas as acoustic and surface-wave cloaking, as well as in the potential practical applications listed above.

On the other hand, since light is “stopped” near each cloak, and the cloak radius depends on the light wavelength, the cloak array produced in our study may be used in the spectrometer on the chip applications. The “trapped rainbow” effect observed near each cloak [6] may find applications in such fields as biosensing and testing for genetic decease. Stopping light at the cloak boundary leads to considerable enhancement of fluorescence near each cloak in the array [6].

The work was funded by the National Science Foundation.

References
[1] “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, “Metamaterial electromagnetic cloak at microwave frequencies”, Science 314, 977-980 (2006). Abstract.
[2] “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.
[3] “Understanding the functionality of an array of invisibility cloaks”, M. Farhat, P.-Y. Chen, S. Guenneau, S. Enoch, R. McPhedran,C. Rockstuhl, and F. Lederer, Phys. Rev. B 84 235105 (2011). Abstract.
[4] “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 103, 213901 (2009). Abstract. 2Physics Article.
[5] “Experimental demonstration of a broadband array of invisibility cloaks in the visible frequency range”, V.N. Smolyaninova, I.I. Smolyaninov, and H.K. Ermer, New J. Phys. 14, 053029 (2012). Full Article.
[6] “Trapped rainbow techniques for spectroscopy on a chip and fluorescence enhancement” V.N. Smolyaninova, I.I. Smolyaninov, A.V. Kildishev, and V.M. Shalaev, Applied Physics B 106, 577-581 (2012).Abstract. arXiv:1101.336.

Pulsar Timing Arrays: Gravitational-wave detectors as big as the Galaxy

Rutger van Haasteren

[Rutger van Haasteren is the recipient of the 2011 GWIC (Gravitational Wave International Committee) Thesis Prize for his PhD thesis “Gravitational Wave detection and data analysis for Pulsar Timing Arrays” (PDF). -- 2Physics.com]

Author: Rutger van Haasteren 

Affiliation: 
Currently at Albert-Einstein-Institute (Max-Plack Institute for Gravitational Physics) in Hannover, Germany;
PhD research done at Leiden Observatory, Leiden University, The Netherlands.

Pulsars, rapidly rotating neutron stars that send an electromagnetic pulse towards the Earth with each revolution, are intimately connected to gravitational research and testing of Einstein's general theory of relativity. Besides the fact that neutron stars have very strong gravitational fields -- which is interesting from a general relativity point of view, their use as accurate clocks allows for a whole range of new gravitational experiments. Especially millisecond pulsars, recycled pulsars that have been spun-up by a companion star (see this video by John Rowe Animation/Australia Telescope National Facility, CSIRO, Australia), are very stable rotators due to their high spin frequency, relatively low magnetic field, and high mass. This makes millisecond pulsars most suitable as nearly-perfect Einstein clocks.

2Physics articles by past winners of the GWIC Thesis Prize:

Haixing Miao (2010): "Exploring Macroscopic Quantum Mechanics with Gravitational-wave Detectors"
Holger J. Pletsch (2009): "Deepest All-Sky Surveys for Continuous Gravitational Waves"
Henning Vahlbruch (2008): "Squeezed Light – the first real application starts now"
Keisuke Goda (2007): "Beating the Quantum Limit in Gravitational Wave Detectors"
Yoichi Aso (2006): "Novel Low-Frequency Vibration Isolation Technique for Interferometric Gravitational Wave Detectors"
Rana Adhikari (2003-5)*: "Interferometric Detection of Gravitational Waves : 5 Needed Breakthroughs"
*Note, the gravitational wave thesis prize was started initially by LIGO as a biannual prize, limited to students of the LIGO Scientific Collaboration (LSC). The first award covered the period from 1 July 2003 to 30 June 2005. In 2006, the thesis prize was adopted by GWIC, renamed, converted to an annual prize, and opened to the broader international community.

We can accurately track a pulsar's trajectory with respect to the Earth by monitoring the arrival times of their pulses; given that for the most well-timed millisecond pulsars we can determine the time of arrival of a single averaged pulse up to 50 nanoseconds, we are effectively sensitive to variations in the Earth-pulsar distance up to only several dozens of meters. This is because light can only travel about one foot in a nanosecond. Because some pulsars are in very tight binary systems, such an accurate measurement of the orbit of a pulsar around a companion can be used to verify/falsify the predictions of general relativity. This was first done with the binary pulsar, PSR J1915+1606, also called the Hulse-Taylor binary, which was discovered in 1974 [1]. This system is a double neutron star system of which one of the two bodies is a pulsar. The two stars are close together: a full orbital period only takes 7.75 hours. For two such massive bodies in such a tight orbit general relativity predicts that the emission of gravitational waves is significant, which would cause a decrease in the orbital period due to loss of energy of the system (see see this video by John Rowe Animation/Australia Telescope National Facility, CSIRO, Australia). By closely tracking the dynamics of the Hulse-Taylor binary, this decrease in the orbital period was confirmed exactly (figure 1), confirming the existence of gravitational waves. This has resulted in Hulse and Taylor been awarded the Nobel prize in physics in 1993 [2].

Figure 1: Decreasing period of rotation of binary pulsar PSR J1915+1606

The confirmation of the existence of gravitational waves with the Hulse-Taylor binary is considered an indirect detection of gravitational waves, because it has been shown that the energy loss of a system is consistent with gravitational-wave emission. A direct detection would have to consist of evidence that the gravitational waves are present elsewhere than at the point of emission: by using a gravitational-wave detector.

Generally speaking, two approaches exist to directly detect gravitational waves:
1) A large body of mass is used as a resonator, where the gravitational waves are expected to excite the resonant frequencies of such a so-called resonant-mass detector.
2) A signal is sent from one place to another, where the gravitational waves are expected to perturb the propagation of the signal such that its arrival time slightly changes. In laser interferometry detectors (e.g. LIGO) this results in a changing interference pattern at the point of recombination of two laser beams.

Figure 2: Concept of a pulsar timing array [Image credit: David J. Champion]

As it turns out, millisecond pulsars can be used to 'construct' a gravitational-wave detector of the second kind, where the pulse propagation from the pulsar to the Earth is perturbed by astrophysical gravitational waves [3]. The very regular arrival times of a millisecond pulsar will arrive slightly early or late due to gravitational waves that pass through the Earth-pulsar system, which in principle makes the gravitational waves detectable. Because the typically observed millisecond pulsars for these purposes are several kpc away, a pulsar timing array is basically a gravitational wave detector of galactic scale (figure 2; Also see this video by John Rowe Animation/Australia Telescope National Facility, CSIRO, Australia)).

A pulsar timing array is sensitive to gravitational waves with frequencies of a few dozen to a few hundred nHz [4], which is the frequency range where tight supermassive black-hole binaries (SMBHB) are expected to be the dominant sources of continuous gravitational waves. A canonical SMBHB system that would contribute to the gravitational-wave signal would consist of two supermassive black holes with masses of close to one billion solar masses at a distance of a Gpc, with an orbital period of several months to years. Many such systems are expected to exist in the universe, which would results in an isotropic superposition usually called a stochastic background of gravitational waves [5]. Large-scale computer simulations of the evolution of the universe suggest that some of the individual sources might be eventually uniquely detectable, but the bulk of the signal would consist of such an isotropic stochastic background of gravitational waves [6]. Because the evolution of the universe is intimately connected to the SMBHB gravitational-wave signal, it is thought that measuring the stochastic gravitational-wave background and possibly single SMBHB sources would contribute greatly to our understanding of cosmology. This is a frequency band that is unreachable for any other type of gravitational-wave detector, which makes pulsar timing arrays a unique and complementary tool next to the other gravitational-wave detection programmes like the ground-based gravitational observatories.

Pulsar timing array science is still relatively new, and a new international pulsar timing array (IPTA, [7]) collaboration has only recently been formed as an alliance between three ongoing pulsar timing array efforts: the European Pulsar Timing Array (EPTA, [8]), the North American Nanohertz Observatory for Gravitational waves (NANOGrav, [10]), and the Australian Parkes Pulsar Timing Array (PPTA, [9]). The PPTA uses a single radio telescope based in Parkes, Australia, with a 64m dish. NANOGrav uses the worlds two largest single-dish radio telescope: the 100m Green Bank Telescope, and the 305m Arecibo Observatory. The EPTA uses five radio telescopes spread throughout Europe: the Westerbork synthesis radio telescope in the Netherlands, the Lovell telescope in the UK, the Effelsberg telescope in Germany, the Nancay radio telescope in France, and the Sardinia radio telescope in Italy. These five European radio telescopes are currently being linked together to coherently combine their signals, effectively forming one big phased array called the Large European Array for Pulsars (LEAP, [8]). This improvement should boost sensitivity for pulsar timing array purposes.

The Effelsberg Radio Telescope near Effelsberg, Germany. This is the worlds second-largest fully steerable radio telescope, with a diameter of 100 meters. [Image credit: Gemma Janssen]

Between the ground-based gravitational-wave detectors and pulsar timing arrays, it is basically a scientific race focused on who will make the first detection, with both projects having good chances of being the first. Pulsar timing arrays have the advantage that the signal rms is expected to increase sharply with time. Even if pulsar timing arrays cannot reduce their noise with their always ongoing efforts, sensitivity will still gradually increase over time, making a detection possible. However, the theoretical predictions about the stochastic background amplitude and the event rates of single sources are less certain than for ground-based detectors. The big ground-based detectors are currently upgrading their instruments, which are expected to become operational somewhere in 2015.

The Parkes Radio Telescope near Parkes, Australia. This radio telescope with a diameter of 64 meters is the worlds leading radio telescope in discovery of radio pulsars. [Image credit: Aristeidis Noutsos]

Even without upgrading instruments, sensitivity of both types of gravitational-wave detectors can be increased with better data analysis methods which would allow more information to be extracted from the data. In order to do that, a Bayesian data analysis method for pulsar timing arrays has been developed that can theoretically extract all information of the signal that is present in the data. General relativity describes the gravitational-wave signal of the stochastic background as a both time correlated and spatially correlated signal between all the pulsars, which means that the data of the different pulsars cannot be treated individually. Extracting such a signal from the data is non-trivial, especially for non-uniformly sampled data with ill-understood noise like that of millisecond pulsars. The Bayesian analysis is suitable for such an analysis, and has been shown to work well for both stochastic background signals [11], and single sources like the gravitational-wave memory effect [12]. The developed Bayesian analysis has resulted in the most stringent upper-limit on the stochastic gravitational-wave background to date [11].

The Westerbork Synthesis Radio Telescope near Westerbork, The Netherlands. This radio telescope is composed of 14 dishes with a diameter of 25 meter, which combine into a radio telescope of similar sensitivity to that of the Effelsberg Radio Telescope. [Image credit: Cees Bassa]

In the coming years, the Chinese five hundred meter aperture spherical telescope (FAST, [13]), and the planned Square Kilometre Array (SKA, [14]) will provide a major leap in sensitivity. Especially the SKA, built by a collaboration of 20 countries, will dramatically change pulsar timing array science. It will be a phased array of many dishes located across South Africa, Australia and New Zealand [15]. With its vast collecting area of one million square meter it is expected to find nearly all the pulsars in the Galaxy. With all those pulsars a very sensitive gravitational-wave detector with possibly up to one hundred arms can be constructed. This should open up a new window to observe the universe, and provide unique insights into cosmology.

References:
[1] R.A. Hulse, J.H. Taylor, "Discovery of a pulsar in a binary system", The Astrophysical Journal, 195, L51 (1975). Full text.
[2] http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/
[3] Frank B. Estabrook and Hugo D. Wahlquist, "Response of Doppler spacecraft tracking to gravitational radiation", General Relativity and Gravitation, 6, 439 (1975). Abstract.
[4] R.S. Foster, D.C. Backer, "Constructing a pulsar timing array", The Astrophysical Journal, 361, 300 (1990). Abstract.
[5] E.S. Phinney, "A Practical Theorem on Gravitational Wave Backgrounds", eprint arXiv:astro-ph/0108028v1 (2001).
[6] A. Sesana, A. Vecchio, C. N. Colacino, "The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays", Monthly Notices of the Royal Astronomical Society, 390, 192 (2008). Abstract.
[7] G Hobbs, A Archibald, Z Arzoumanian, D Backer, M Bailes, N D R Bhat, M Burgay, S Burke-Spolaor, D Champion, I Cognard, W Coles, J Cordes, P Demorest, G Desvignes, R D Ferdman, L Finn, P Freire, M Gonzalez, J Hessels, A Hotan, G Janssen, F Jenet, A Jessner, C Jordan, V Kaspi, M Kramer, V Kondratiev, J Lazio, K Lazaridis, K J Lee, Y Levin, A Lommen, D Lorimer, R Lynch, A Lyne, R Manchester, M McLaughlin, D Nice, S Oslowski, M Pilia, A Possenti, M Purver, S Ransom, J Reynolds, S Sanidas, J Sarkissian, A Sesana, R Shannon, X Siemens, I Stairs, B Stappers, D Stinebring, G Theureau, R van Haasteren, W van Straten, J P W Verbiest, D R B Yardley and X P You,"The International Pulsar Timing Array project: using pulsars as a gravitational wave detector", Classical and Quantum Gravity, 27, 084013 (2010). Abstract.
[8] R D Ferdman, R van Haasteren, C G Bassa, M Burgay, I Cognard, A Corongiu, N D'Amico, G Desvignes, J W T Hessels, G H Janssen, A Jessner, C Jordan, R Karuppusamy, E F Keane, M Kramer, K Lazaridis, Y Levin, A G Lyne, M Pilia, A Possenti, M Purver, B Stappers, S Sanidas, R Smits and G Theureau, "The European Pulsar Timing Array: current efforts and a LEAP toward the future", Classical and Quantum Gravity, 27, 084014 (2010). Abstract.
[9] G. Hobbs, D. Miller, R. N. Manchester, J. Dempsey, J. M. Chapman, J. Khoo, J. Applegate, M. Bailes, N. D. R. Bhat, R. Bridle, A. Borg, A. Brown, C. Burnett, F. Camilo, C. Cattalini, A. Chaudhary, R. Chen, N. D’Amico, L. Kedziora-Chudczer, T. Cornwell, R. George, G. Hampson, M. Hepburn, A. Jameson, M. Keith, T. Kelly, A. Kosmynin, E. Lenc, D. Lorimer, C. Love, A. Lyne, V. McIntyre, J. Morrissey, M. Pienaar, J. Reynolds, G. Ryder, J. Sarkissian, A. Stevenson, A. Treloar, W. van Straten, M. Whiting and G. Wilson, "The Parkes Observatory Pulsar Data Archive", Publications of the Astronomical Society of Australia, 26, 103 (2009). Full Text.
[10] P. B. Demorest, R. D. Ferdman, M. E. Gonzalez, D. Nice, S. Ransom, I. H. Stairs, Z. Arzoumanian, A. Brazier, S. Burke-Spolaor, S. J. Chamberlin, J. M. Cordes, J. Ellis, L. S. Finn, P. Freire, S. Giampanis, F. Jenet, V. M. Kaspi, J. Lazio, A. N. Lommen, M. McLaughlin, N. Palliyaguru, D. Perrodin, R. M. Shannon, X. Siemens, D. Stinebring, J. Swiggum, W. W. Zhu, "Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves", eprint arXiv:1201.6641 (2012)
[11] R. van Haasteren, Y. Levin, G. H. Janssen, K. Lazaridis, M. Kramer, B. W. Stappers, G. Desvignes, M. B. Purver, A. G. Lyne, R. D. Ferdman, A. Jessner, I. Cognard, G. Theureau, N. D’Amico, A. Possenti, M. Burgay, A. Corongiu, J. W. T. Hessels, R. Smits and J. P. W. Verbiest, "Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data", Monthly Notices of the Royal Astronomical Society, 414, 3117 (2011). Abstract.
[12] Rutger van Haasteren and Yuri Levin, "Gravitational-wave memory and pulsar timing arrays", Monthly Notices of the Royal Astronomical Society, 401, 2372 (2010). Abstract.
[13] R. Smits, M. Kramer, B. Stappers, D.R. Lorimer, J. Cordes, and A. Faulkner, "Pulsar searches and timing with the square kilometre array", Astronomy & Astrophysics, 505, 919 (2009). Abstract.
[14] T. Joseph W. Lazio, "The Square Kilometre Array", in Panoramic Radio Astronomy: Wide-field 1-2 GHz Research on Galaxy Evolution (2009). Full Text.
[15] http://www.skatelescope.org/news/dual-site-agreed-square-kilometre-array-telescope/

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.