Introduction to Metamaterials Berhanu Kacha North Seattle College Seattle, WA [email protected] Copyright Edmonds Community College 2019; Permission granted for use and reproduction for educational purposes only.
Abstract Metamaterials have unique properties that are not found in natural materials. These materials have diverse potential applications from optical filters to high gain antennas and even to shielding buildings from earthquakes. This module presents an introduction to metamaterials, their basic electromagnetic and optical properties and some developing applications of these materials. We will cover basics definition of metamaterials, concept of metamaterials and their applications. We then present electromagnetic and acoustic metamaterials, negative refractive index, seismic metamaterial and potential applications in areas such soundproofing, antenna microwave and invisibility cloak. Module Objective: To provide a one lecture introduction to metamaterials that can be used to introduce the subject to students at the college level. Student Learning Objectives: The student will be able to: • Define what is metamaterial. • Describe metamaterials properties. • Explain metamaterials unique properties. • Describe metamaterials applications. Key Words: Metamaterials, negative refractive index, electromagnetic, acoustics, wave Type of Module: Interactive presentation to introduce concepts to be learned. Time required: one class period, 50 minutes Grade Level: college level
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Instructor background and notes
Definition of Metamaterials A metamaterial is a material that gains its properties from its structure rather than directly from its composition. It is engineered (at the atomic level) material that has unique properties not found in nature due to the arrangement and design of its constituents. They are typically man- made materials.
Properties of Metamaterials
Metamaterials have unique properties that are not found in natural materials. One such property is negative refractive index. The refractive index can be seen as the factor by which the wavelength and the speed of the radiation are reduced with respect to their vacuum values. The refractive index of a material is conventionally taken to be a measure of the optical density and is defined as n = c/v , where c is the speed of light in vacuum and v is the speed of an electromagnetic plane wave in the medium. From Maxwell’s equations the refractive index is given by, n2 = εμ where ε is the relative dielectric permittivity and μ is the relative magnetic permeability of the medium. This electrical permittivity (or dielectric constant) is a kind of measure to assess how easily or difficult the electrons can move in the material in response to the incident light. Most dielectrics materials have positive permittivities , ε > 0 . Metals will exhibit negative permittivity, ε < 0 at optical frequencies, and plasmas exhibit negative permittivity values in certain frequency bands. However, in each of these cases permeability of the materials remains always positive. A natural material that can achieve negative values for permittivity and permeability simultaneously has not been found, or discovered.
In these new artificially fabricated metamaterials the electrical permittivity and the magnetic permeability are the main determinants of a material's response to electromagnetic waves. In metamaterials, both these material parameters(ε< 0, μ< 0) are negative. Correspondingly, the refractive index of the metamaterials is also negative. Therefore, due to negative μ and negative ε the refractive index of the medium is calculated to be negative.
The key concept to metamaterials is based more on the arrangement of the constituents rather than their individual properties. The property that is designed is a macroscopic property that is seeming when the metamaterial is viewed as a uniform object. The central goal in designing metamaterials is wave manipulation unlike that of a natural material. When a wave, like a light or sound wave, moves from one medium to another, it undergoes refraction according to Snell’s Law. Metamaterials have a negative index of refraction and use this property to redirect waves around. For a metamaterial to be effective, it must be taken as a uniform material rather than an array of particles. This means the units that make up the metamaterial must be relatively small compared to the wavelength that is to be manipulated and redirected.
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The two major subcategories of metamaterials are electromagnetic and acoustic. For these categories, the central goal is wave redirection and manipulation. Electromagnetic metamaterials bend and manipulate electromagnetic waves like visible light waves, microwaves, and infrared waves. These are transverse waves. The main mechanism of electromagnetic metamaterials utilization of negative electric permittivity and negative magnetic permeability to control wave propagation. Whereas, acoustic metamaterials manipulate longitudinal waves associated with vibrations. One of specialized application of this is in seismic metamaterials which can redirect seismic waves. For their operation, acoustic metamaterials rely on a negative bulk modulus and negative mass density. Since the wavelengths for acoustic waves are much larger than those of electromagnetic waves, we realize that acoustic metamaterials are easier to construct.
For electromagnetic metamaterials negative refractive index is central. Therefore, it is important to understand what negative refraction of electromagnetic waves is. Normal materials refract waves with positive refractive index (n > 0). The definition of refractive index, n, is a function of magnetic permeability and electric permittivity which are properties relating to the material in the presence of magnetic and an electric field, respectively. Usually, a positive coefficient is assumed for most media, for metamaterials negative coefficient must be used. According to Snell’s Law, when a wave encounters a negative index material, the angle that the refracted wave makes with the normal plane is greater than that of materials with positive refractive index.
Application to Antennas
One example application of electromagnetic metamaterials is antennas. The negative refractive index indicates the wave associated with the antenna bents to a sharper angle. This efficiently increases the radiated power of the antenna and increase the frequency range. Another application of electromagnetic metamaterials is invisibility cloak, which is a method of rendering physical objects unnoticeable.
Acoustic waves are longitudinal waves which use parameters pressure and particle velocity to describe them. For acoustic metamaterials, their refractive index defined by bulk modulus and density, which are component parameters. For some frequency bands, the bulk modulus and effective mass density may become negative. This leads to a negative refractive index result.
Among the metamaterial applications, high impedance surfaces (HISs) and artificial magnetic conductors (AMCs) are the ones which are most related to antenna applications, since they can lead to the design of compact and low profile antenna systems. In such a case, metamaterial designs are placed around or close to the antennas, although metamaterials could also be used in the feeding part of the antenna system, or even as a part of the antenna structure.
The use of metamaterials could enhance the radiated power of an antenna. Negative permittivity and permeability of these engineered structures can be utilized for making electrically small antenna, highly directive, and reconfigurable antennas. Their purpose is to launch energy into free space as with any electromagnetic antenna.
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Conventional antennas that are very small compared to the wavelength reflect most of the signal back to the source. A metamaterial antenna behaves as if it were much larger than its actual size, because its novel structure stores and re-radiates energy. Some applications for metamaterial antennas are wireless communication, space communications, GPS, satellites, space vehicle navigation, airplanes and micro-sensors and portable ground-penetrating radars to search for geophysical features.
Metamaterial Absorbers for Photovoltaics
Another potential application area of metamaterial is perfect absorption, which is crucial for solar energy utilization applications. Perfect absorption can be obtained when the transitivity and the reflectivity of metamaterials are simultaneously reduced to zero so that all the incident electromagnetic radiation is absorbed by the material.
The parameters, permittivity and permeability, specify the properties of the metamaterial absorbers. These parameters can be arranged so that the effective impedance of the absorber becomes same with the impedance of the free space. As a result, the reflected and transmitted waves minimized simultaneously, and the perfect absorption is achieved.
The perfectly absorptivity property of the metamaterials can be utilized to convert solar energy directly into the electric energy in the photovoltaic cell. For photovoltaic response, generally an active material is needed to absorb and convert the absorbed energy into the useful forms. Active material is a semiconducting material where the light energy are transformed into electron hole pairs. These pairs are then separated to the electrodes and finally the energy can be taken as the form of electric current from these electrodes.
During the energy conversion process there are losses which occurred within or out of the solar cell. This reduce their performance and cause the inefficiently converting the solar energy into the usable forms. These losses includes the energy lost in the metallic parts. This condition leads to the generation of heat which is a side effect for photovoltaic application. Another disadvantage could be the thickness of the active material which result with an inefficient working. As the metamaterials are compact structures and they are small in dimension, they can help the preventation of these loses occurred within the cell. Therefore, metamaterials are very important materials for the utilization of solar energy as a renewable source.
Metamaterial are expected to have a great impact in a wide range of future technology, especially, for technologies where electromagnetic radiation are used. This will provide a flexible stage for technological development. From those metamaterials, negative refractive index materials (left- handed materials) have drawn special attention in microwaves. Metamaterial characteristics allows to reduce in size as compared to other materials. This results in applying them for reconfigurability and multiband operation of microwave devices and antennas. Also, they have application as an absorber and sensors for humidity, soil moisture measurement. The future applications of metamaterials lie in the field of optical and medical devices which is thoroughly linked to advancements of nanotechnology.
Optional Student Activity Using the internet, search metamaterials properties and applications demonstration videos or literatures supported by figures. What are the electromagnetic and optical applications of these
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materials? Identify the concepts related to wave manipulation by the metamaterial. How the metamaterial manipulates the wave?
Further study 1. Identify electromagnetic metamaterials. What properties are enhanced? 2. Research negative refractive index, super-lens and hyper-lens metamaterial use on future technologies. 3. Investigate transformation optics, antenna, microwave and invisibility cloak applications. 4. What are the advantages using meta-surfaces and phase engineering? References 1. Metamaterials: http://people.ee.duke.edu/~drsmith/ 2. Metamaterials(Overview): https://nanohub.org/resources/19767/download/2013.10.29- ECE695S-L13.pdf 3. Vladimir M. Shalaev (2013), "ECE 695S: Nanophotonics and Metamaterials," https://nanohub.org/resources/19272. 4. Multifunctional Metamaterial Designs for Antenna Applications, https://upcommons.upc.edu/bitstream/handle/2117/95748/TPJFG1de1.pdf?sequence=1 5. A multi-band metamaterial absorber design for solar cell applications, http://etd.lib.metu.edu.tr/upload/12620288/index.pdf
Evaluation: Student evaluation questions (discussion or quiz): 1. What is metamaterial? 2. What are the properties of metamaterial? 3. How to achieve negative index of refraction? 4. What gives metamaterials their unique properties? 5. How are meta-devices created and what are their applications? Course evaluation questions (for the students) 1. Was the module clear and understandable? 2. Was the instructor’s explanation comprehensive and thorough? 3. Was the instructor interested in your questions? 4. Was the instructor able to answer your questions? 5. What was the most interesting thing that you learned?
This work is part of a larger project funded by the Advanced Technological Education Program of the National Science Foundation, DUE #1400619
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