* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download JMG_Paper2_GainingSuperpowersThroughMetamaterials
Survey
Document related concepts
Silicon photonics wikipedia , lookup
Nonimaging optics wikipedia , lookup
Ellipsometry wikipedia , lookup
Ultraviolet–visible spectroscopy wikipedia , lookup
Surface plasmon resonance microscopy wikipedia , lookup
Harold Hopkins (physicist) wikipedia , lookup
Dispersion staining wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Thomas Young (scientist) wikipedia , lookup
Atmospheric optics wikipedia , lookup
Nonlinear optics wikipedia , lookup
Birefringence wikipedia , lookup
Refractive index wikipedia , lookup
Retroreflector wikipedia , lookup
Anti-reflective coating wikipedia , lookup
Transcript
Duke University Gaining Superpowers Through Metamaterials Justin Garfinkle Math 89s: Math of the Universe Professor Bray 3/28/16 The role of metamaterials is making the seemingly impossible possible. Metamaterials are artificially created materials that have properties often absent from materials occurring in nature. The stuff of science fiction turned to scientific reality, the first version of a legendary invisibility cloak was created right here at Duke University by engineering a metamaterial that is able to cloak an object so that both the metamaterial cloak and the object inside it are invisible to microwaves. There is a common game where people select which super power they wish to have. Invisibility is not the only super power being made possible with metamaterials. Is the goal x-ray vision? Metamaterials have been used to improve the resolution of MRI scans and to create a high-resolution terahertz lens that could be used in place of X-rays so that the patient is not exposed to potentially harmful radiation. If super-speed is the goal, metamaterials have the potential to revolutionize information technology through increased speed and energy efficiency of computers. Currently scientists are exploring the use of metamaterials to do everything from protecting buildings against earthquakes to creating houses that could be folded to fit into a backpack. In fact, the scientific community has only begun to explore all the “super powers” that could be made possible through the development of various metamaterials. The prefix “meta” is from the Greek word meaning “beyond” because metamaterials typically have properties that go beyond what is found in natural materials (Cai 13). Although not all sources agree on the origin of the term “metamaterial”, it was likely first used in 1999 by Professor Rodger Walser from the University of Texas, Austin. In an invitation to a seminar sponsored by the Defense Advanced Research Projects Agency (DARPA), Professor Walser stated that DARPA was “gathering information concerning the area of artificially constructed materials, or Meta-materials, which possess qualitatively new responses that do not occur in nature” (Ziolkowski 1). The US Department of Defense continues to be a major funding source for metamaterial research (Wagstaff). There is currently no consensus definition of the term “metamaterial”, but many experts continue to incorporate the concept of creating a material that has properties not found in nature. Professors Wenshan Cai and Vladimir Shalaev point out, however, that people should be “truly humble in the face of Mother Nature” (Cai 13). For example, a central focus of metamaterial research is to create materials with a negative refraction index (discussed below), but this has actually been observed in the compound eyes of some lobsters (Cai 13). More broadly, a metamaterial is an artificially created material that attains its properties from the unit structure itself and not any chemical reaction within the compound (Bargeron 7). In nature, materials are comprised of atoms and molecules. In a metamaterial, very tiny particles are used to create artificial “meta-atoms” and “meta-molecules” that are arranged in a particular geometric pattern within the host material to create an entirely new material with a unique set of properties (Cai 22). The tiny particles comprising the meta-atoms and metamolecules could be created using any metal, silicon or other natural material, depending upon the properties being sought in the resulting metamaterial. The meta-atoms and meta-molecules do not necessarily have to be the size of actual atoms and molecules, however. Since metamaterials are developed to have particular properties when interfacing with light, sound, or some other force, the meta-atoms and meta-molecules must simply be orders of magnitude smaller than the applicable wave or force. For example, optical metamaterials that are designed to respond in a particular manner to visible light could be constructed using meta-molecules that are bigger than actual molecules but smaller than visible light waves. Whereas molecules are less than a nanometer in size, the meta-molecules could be a few tens of nanometers and will still behave as molecules relative to the visible light waves (Pendry). Any natural material, such as glass, water, or iron, will respond to light based upon the average response of many billions of atoms within the material that interact with the light wave. This averaging process will go on at the metamolecular level in a metamaterial as long as the meta-molecules are very small relative to the applicable wavelength (Pendry). The subwavelength scale of the materials used to create the meta-molecules results in an overall metamaterial that is uniform relative to the incoming wave, which is why the metamaterial is considered a “material” rather than a “device” (Cai 13). A metamaterial designed to work with radio waves, such as those used in satellites or MRI scanners, could be comprised of meta-molecules that are much larger than the ones designed for visible light since the size of the meta-molecule only has to be small relative to the applicable wave. Whereas visible light waves are typically between 380 nanometers and 700 nanometers, radio waves have the longest wavelengths in the electromagnetic spectrum that range between 1 millimeter and 100 kilometers. There are many applications for creating metamaterials that interact with radio waves, microwaves or infrared waves that are easier to create than applications for visible light because the meta-molecules created in the lab can be much larger and therefore much easier to work with and build (Smith). Since visible light and other forms of light like microwaves and radio waves are all electromagnetic radiation, they share many similar features with the exception of their wavelengths and related frequencies. Accordingly, these larger waves also can be used to test a metamaterial design that is then adapted for visible light waves if that is the goal. This is why the Duke invisibility cloak was created to cloak against microwaves rather than visible light waves in the initial demonstration that invisibility is possible (Smith). For acoustical metamaterials, the size of the meta-molecules would be determined relative to sound waves. In the field of mechanical metamaterials, the constituent elements are scaled relative to the links that hold the materials together. (Brown, quoting Professor Julia Greer). The size of the meta-atoms and meta-molecules is not the only feature that is important in creating a metamaterial that achieves a desired functionality. The nature and size of the metaatoms and meta-molecules, their respective shapes, their placement relative to each other, the composition of the host material, the geometric pattern of the constituent parts, and the overall structure of the compound are all important in the design of the metamaterial (Liu 57). Metamaterials are unique because, although they are artificially created, “they interact with forces at the nanoscale in ways that we can only explain by using quantum mechanics” (Brown, quoting Professor Julia Greer). In 1827, Scottish botanist Robert Brown observed the zig zag motion of a grain of pollen suspended in water. As discussed in my prior paper on Brownian motion, at the beginning of the twentieth century, German theoretical physicist Albert Einstein and French experimental physicist Jean-Baptiste Perrin were able to establish that the motion observed by Brown was the result of the grain of pollen interacting with the water molecules (Nott 43). To me, this research is the precursor to the modern efforts to use a nanoparticle as a synthetic molecule within the host substance in order to develop a useful metamaterial. Interestingly, in a recent patent application for a particular type of metamaterial, the inventors explained that “nanoparticles are subject to Brownian motion… [and this] structural force on nanoparticles driving them into the region of reduced order is used to produce metamaterials based on liquid crystal colloidal dispersions with prescribed shape of building blocks” (Musevic). Metamaterials were actually created thousands of years before the science behind their creation was likely understood (Alu). In 300 A.D., an ancient Roman artist created the Lycurgus Cup that is now on display at the British Museum in London. The Lycurgus Cup appears to be jade green if illuminated from the front and translucent red if illuminated from the back. This optical effect must have seemed magical at the time, but the effect was actually created by melting tiny amounts of precious silver and gold alloys, ten thousand times smaller than a grain of sand, into the glass. The color change occurs due to the interaction of light with these metallic nanoparticles. The precise size and characteristics of the nanoparticles used in the Lycurgus Cup and the precise spacing with which the nanoparticles were disbursed throughout the glass were all necessary to create the optical effect of this ancient metamaterial (Alu). The modern challenge for optical metamaterials is to control light in even more fundamental ways than simply changing the perceived color of the observed material. Sir John Pendry, a theoretical physicist at Imperial College London, describes light as a “dance” between electricity and magnetism, since light is an oscillating magnetic field with an oscillating electric field perpendicular to it (Pendry). Many metamaterials are created using a “delicate” balance of metal and dielectric components in materials to interact in particular ways with electric and magnetic fields (Cia 36). For example, round split-ring copper wires have been used as metamolecules to create a circuit when it comes in contact with a magnetic field, causing the resulting metamaterial to have magnetic properties even though there is no iron, nickel or other intrinsically magnetic component in it (Pendry). “Plasmonic” metamaterials enable optical signals to be “squeezed into deep subwavelength scale, which helps to bridge length scale mismatch between typical optical systems and on-chip electronics” (Cia 36). Potential applications of plasmonics includes biochemical sensing and optical signal processing (Cia 36). The electromagnetic properties of any material will depend upon its permittivity and its permeability. When an electric field travels through matter, the resistance is known as its permittivity. On an atomic scale, each atom has electrons with a net negative charge circling the nucleus with a net positive charge. In a simple one-atom molecule of a material such as helium gas, the electrons will feel a force toward the positive charge electric field. The reaction to an electric field will depend upon the number of atoms in the molecules, the symmetry of the atoms and their relative position. When a magnetic field travels through matter, the magnetic flux is known as its magnetic permeability. Most materials are blind to the magnetic field of light at visible and infrared wavelengths (Pendry). Conventional materials known to be transparent have positive permittivity and positive permeability. Materials that have either a negative permittivity or a negative permeability are opaque. Noble metals at optical frequencies have negative permittivity but positive permeability, for example. In 1861, physicist and mathematician James Clerk Maxwell published equations that, together with the Lorentz force law, have formed the foundation of classic electrodynamics (Cia 19). Although not forbidden by Maxwell’s equations or Lorentz force law, there are certain combinations and levels of permittivity and permeability not found in nature (Cia 19). For example, there are no known natural non-magnetic materials with permeability greater than 1. In addition, there are virtually no known natural materials that have both negative permittivity and negative permeability. A “major focus” of metamaterial research is “to create artificial materials that enter regions of parameter space that are not forbidden by Maxwell’s equations but are not observed in conventional media, and to take advantage of this expanded parameter space for better control of electromagnetic waves” (Cia 19). The discovery in the year 2000 of a material with both negative permittivity and negative permeability was actually the catalyst for the modern explosion in metamaterial development. In 1968, a Russian physicist named Victor Veselago had written a paper titled “The electrodynamics of substances with simultaneously negative E [permittivity] & mu [permeability]” (Smith). In the intervening years between 1968 and 2000, Veselago’s paper received almost no attention because scientists thought it was impossible for there to be a material that had both negative permittivity and permeability (Smith). Among the radical predictions in Veselago’s paper was that a material with negative permittivity and negative permeability would be transparent and would have a negative refractive index (Smith). A negative refractive index would create very dramatic optical effects. Refraction is the amount that the light will bend at the interface between the two materials (Pendry). When light hits water, for example, the density of the water molecules causes a change in the velocity and/or trajectory of the light beam. Light travelling through a vacuum is assigned a refractive index of 1. This is the approximate refractive index for air. The refractive index for any other material is based on how much resistance light has to travelling through the medium relative to travelling through a vacuum. The refractive index of water is 1.333 because light travels about 1.333 times faster in air than it does in water (Pendry). Virtually all known natural materials have a positive refractive index (Pendry). When light moves from air to a denser material, if the ray approaches the material at an angle, it will bend toward the boundary. This results can be demonstrated by viewing a straw placed in a glass of water. The straw will appear to be bent due to the refraction of the light when it moves from air to water. When light moves from a denser medium to a less dense medium, it will cause an even more unusual optical illusion, such as a person seeing a mirage in the desert. Since cold air is denser than hot air, as the sunlight passes down from cool air in the desert sky to hot air near the desert ground, it gets bent upward so that the observer will perceive the refracted image of the sky on the ground. The image of the sky on the ground will appear to be a pool of water to the desert traveler. The optical illusion is even starker if there is a negative refractive index. A material with a negative refractive index would bend light away from the boundary when it meets the interface between air and the negative index material. If fish were swimming in a pool of some type of liquid with a negative refractive index, the fish would appear to an external observer be flying above the pool (Pendry). Creating a metamaterial with a negative refractive index therefore opens dramatic opportunities in optics. For example, a typical microscope is not able to have resolution beyond a half wavelength. If the lens were made of a metamaterial with a negative refractive index, it could convey a perfect image at the molecular level to the observer in the same way that the fish appears to be swimming above the “negative index” pool (Pendry). Therefore, discovery of a negative index metamaterial opens up the possibility to create a “perfect lens” (Pendry). According to recent market research, the current metamaterials industry went from relative obscurity in the year 2000 to an industry that is projected to reach $643 million within the next several years (Markets). Many people credit David R. Smith of Duke University for bringing “major attention” to the metamaterials field (Markets). Professor Smith was part of the team that created the first invisibility cloak, which worked at microwave frequencies in 2006 and then “perfected” this cloaking device in 2012 by controlling for absorption and other interference (Knapp). Perhaps more significant, however, Professor Smith was part of the team that discovered the first negative refraction metamaterial in 2000. The metamaterial had negative refraction because it was created to have negative permittivity and negative permeability. This double negative metamaterial was able to bend light in a way that was previously not thought possible. Professor Smith, Professor Pendry, and several other European scientists won the 2005 Decartes Research Prize for the discovery of what is sometimes called a “left-handed metamaterial” because the way in which the light bends backwards (Duke News Release). Ironically, Professor Smith’s discovery of a metamaterial that responds to light in a revolutionary way happened when he was trying to verify a finding in a different area (Smith). In fact, he did not initially even comprehend the importance of what he had discovered or its vast practical applications (Smith). Professor Smith was originally only trying to prove that he had created artificial magnetism with negative permeability at microwave frequencies, as had been previously postulated by Professor Pendry (Smith). Since two negatives make a positive, Professor Smith incorporated a material with negative permittivity to see if the compound would be transparent, as with double positive, rather than opaque as with one positive and one negative with respect to permittivity and permeability. This process worked to prove that Professor Smith had created a material with negative permeability, but as part of his test he had actually created a material with both negative permeability and permittivity and therefore a negative refraction index (Smith). As discussed above, this was believed impossible by the scientific community. Professor Smith then re-discovered Veselogo’s paper discussed above on the theoretical possibility of a double negative substance and all of its potential uses (Smith). This marked the true beginning of the current effort to control waves in very special and unique ways. Professor Smith developed the invisibility cloak with the new techniques he had discovered for controlling light. The theory behind the invisibility cloak is that it creates an optical illusion that literally enables an observer to see behind an object without even seeing the shadow of the object (Pendry). The way a person observes an object is that light interacts with an object and is either reflected, refracted or absorbed. If the light, instead, moves around an object and returns to its original path, the observer will receive the same image as if the object were never there (Pendry). Professor Pendry suggests this is just like a river rushing around a rock but then returning to its original path, wherein a downstream observer would not know that the water had ever encountered the rock (Pendry). Professor Smith was able to bend microwaves around a metamaterial cloak that surrounded a concealed object and refract the microwaves around the cloaked object to return the trajectory of the waves to their original path (Pendry). Other teams working on invisibility have used different techniques. For example, one team of scientists was able to create a metamaterial invisibility cloak by sending waves from the cloaking device that are exactly opposite to the incoming waves and thus countering their effect. The problem with this technique is that it only works for the specific object being cloaked (Pendry). One of Professor Pendry’s most interesting observations is that the only way to truly make something invisible is through Einstein’s theory of general relativity and not through metamaterials. If a cloak were massive enough to bend space-time, light coming toward the cloaked object would travel around the object and then return to its original trajectory. Since this is not possible as a practical matter, Professor Smith’s invisibility cloak attempts to simulate this effect by bending light around the object through carefully structuring the refractive indices of the metamaterial cloaking device (Pendry). Researchers are now working on creating metamaterials that would cloak buildings against seismic waves to protect them from earthquakes (Wogan). As techniques for creating metamaterials are expanded and perfected, the possibilities for using metamaterials for different useful purposes could truly change the world around us. Bibliography Alu, Andrea “On the Quest to Invisibility: Meta-materials and Cloaking” TedxAustin (February 20, 2013) (http://tedxtalks.ted.com/video/On-The-Quest-To-Invisibility-Me) (text: http://users.ece.utexas.edu/~aalu/index_htm_files/Alu_TedxAustin_Text_Figs.pdf) Brown, Alan “Forging Metamaterials: Labs Craft Invisibility Cloaks, Perfect Lenses and Nanostructures (Kavli Roundtable)” Space.com (February 25, 2015) (http://www.space.com/28660-forging-metamaterials-for-invisibility-cloaks-andnanostructures.html) Cai, Wenshan, and Vladimir Shalaev, Optical Metamaterials: Fundamentals and Applications. Springer-Verlag New York, 2010. Chu, Jennifer “New Metamaterial Lens Focuses Radio Waves: Device Could Improve Satellite and Molecular Imaging” MIT News (November 14, 2012) (http://news.mit.edu/2012/newmetamaterial-lens-focuses-radio-waves-1114) Duke University, Pratt School of Engineering. News Release: Smith Shares Descartes Award for Artificial Material that Reverses Light’s Properties(December 3, 2005) (http://pratt.duke.edu/news/smith-shares-descartes-award-artificial-material-reverses-lightsproperties) Kapp, Alex “Duke Researchers Perfect the Original Invisibility Cloak” Forbes (November 14, 2012) (http://www.forbes.com/sites/alexknapp/2012/11/14/duke-researchers-perfect-the-originalinvisibility-cloak/#4f80a85272a8) Liu, Na “Coupling Games in Metamaterials: How to Design Metamaterial Structures for Desired Optical Properties and Resonant Behavior” Physik Journal 9 (2010): 57-60 (file:///C:/Users/carlag/Downloads/ISSART65920DE.PDF ) “Markets and Markets Research Insight” marketsandmarkets.com (2016) (http://www.marketsandmarkets.com/PressReleases/metamaterials.asp) Musevic, Igor, Miha Skarabot, Slobodan Zumer, and Miha Ravnik. Metamaterials and Resonant Materials Based on Liquid Crystal Dispersions of Colloidal Particles and Nanoparticles. Patent EP 1975656 B1. 8 June 2011. (http://www.google.com/patents/EP1975656B1?cl=en) Nott, Mick “Molecular Reality: The Contributions of Brown, Einstein and Parrin” School Science Review (June 2005): 39-46 (https://www.ase.org.uk/.../SSR317...) Pendry, Sir John “The Schrodinger Lecture 2012 – Metamaterials: New Horizons in Electromagnetism” YouTube (Imperial College London 2012) (https://www.youtube.com/watch?v=ogNKrQCH1Kk) Smith, David “The Story of Metamaterial and Negative Index: A Personal Perspective” Meta Group: Novel Electromagnetic Media - Duke University, Pratt School of Engineering (http://people.ee.duke.edu/~drsmith/metamaterials/metamaterials_history_4.htm) Wagstaff, Jeremy “Metamaterials Do Way More than Just Invisibility, Scientists Say” Huffpost Science (February 25, 2014) (http://www.huffingtonpost.com/2013/12/26/metamaterials-morethan-invisibility-scientists_n_4501918.htm) Wogan, Tim “Seismic Cloak Could Minimize Earthquake Damage” Physics World (March 15, 2014) (http://physicsworld.com/cws/article/news/2014/mar/25/seismic-cloak-could-minimizeearthquake-damage) Ziolkowski, RW, “Metamaterials: The Early Years in the USA” EPJ Applied Metamataterials (EDP Sciences, 2014): 1-5 (http://epjam.edp-open.org/articles/epjam/pdf/2014/01/epjam140003.pdf)