Abhay S. D. Rajput
E-mail: abhaysdr@gmail.com
Recently, some groups of scientists have succeeded in developing certain artificially engineered composite materials which can have unusual optical or electromagnetic properties otherwise impossible for natural materials. These can have negative values of refractive index (n), electrical permeability (ε) and magnetic permittivity (μ) which are always positive for naturally occurring materials.
These materials are known as metamaterials (MTMs) and the term was first coined in 1999 by Rodger M. Walser of the University of Texas, Austin [1]. These exotic MTMs have such properties by virtue of an internal structure that resonates in the presence of waves [2].
According to Caloz et. al. [3], metamaterials are: ‘a new class of ordered composites that exhibit exceptional properties not readily observed in nature. These properties arise from qualitatively new response functions that are: (1) not observed in the constituent materials and (2) result from the inclusion of artificially fabricated, extrinsic, low dimensional inhomogeneities’.
In other words, MTMs are artificial effective structures exhibiting useful [electromagnetic-optical] properties not readily available in Nature [4]. They offer unlimited interesting scientific and technological possibilities.
The development of the first metamaterials with metal-lens antennas and metallic delay lenses in the late 1940s by W.E. Kock [1] initiated the MTM revolution. In 1967, the German Physcist Victor Veselago theoretically introduced the first Left-handed (LH) materials [5]. These materials have a negative refraction index (NRI), which Veselago [5] attributed to the simultaneous presence of negative permeability (ε) and negative permittivity (μ).
These MTMs are called ‘left-handed’ because they do not obey the conventional ‘right-hand rule’ for natural materials, and can direct the phase velocity of electromagnetic waves opposite to the power flow [6]. This anomalous behaviour is attributed to the special property of such materials to exhibit negative refractive index (NRI) and for that reason, these materials are also known as negative index materials (NIMs) or negative-phase-velocity (NPV) materials [6].
Due to their special and unusual properties and future technological possibilities, LH-MTMs or NIMs among MTMs have caught special attention of researchers. Therefore, most of the researchers are focusing their research on developing such materials and their noble new uses.
MTMs are all set to bring in another technological revolution. Scientists are busy developing MTMs with desired properties at nano level. Some interesting possibilities are: development of invisibility cloaks, acoustic cloaks, increasing the storage capacity of CDs and DVDs, advances in fibre-optic communications, more effective use of solar energy, super-lenses and better imaging technologies, microwave applications, agile antennas, etc.
J. B. Pendry of the Imperial College London, for the first time, theorized a practical way to make a left-handed metamaterial (LH-MTM) or NIM. He also predicted that LH-MTMs or NIMs can act as a superlens and their imaging resolution is not limited by the wavelength but rather by material quality [7].
Shelby, Smith and Schultz have also designed MTMs which can act as NIMs at microwave wavelengths [8].
Shalaev et. al. at Purdue University have experimentally made the first MTM that has a NRI in the wavelength of visible light [9]. They reported their research in the Journal Optics Letters [10], demonstrating a negative refractive index material for the optical range, specifically for wavelengths close to 1.5 μm (200 THz frequency) by using a metal–dielectric composite consisting of tiny parallel "nanorods" of gold. The rods being metal conduct current and create an effect called optical inductance (L). The dielectric medium between rods acts as an optical capacitance element (C). Thus the material consists of tiny LC circuits which by plasmon resonances result in the reversal of refraction. Using this material, they [10] succeeded in obtaining a negative refractive index of -0.3 at the optical communication wavelength of 1.5 μm.
Dr. George Eleftheriades experimentally demonstrated the first flat LH MTM lenses which can bend light negatively and can provide super-resolution by focusing the evanescent waves [11]. These super lenses enhance resolution by increasing the amplitude of evanescent waves (which carry an object’s sub-wavelength details), says Eleftheriades [11].
An MTM can affect electromagnetic waves only if it (MTM) has structural features smaller than the wavelength of the electromagnetic radiation it interacts with [1].
LH-MTMs show unique phenomena such as reversed Snell’s law of refraction, Doppler effect and Vavilov–Cerenkov radiation as well as subwavelength imaging diffraction [4]. This opens new doors of technological innovations.
Most of the MTMs being engineered are made up of periodic arrays of metallic resonant elements like Swiss-Roll, Split Ring Resonator and Capacitively Loaded Ring [12]. These consist of the inductor/conductor type structures organised in a special composite fashion.
Thus we can say that metamaterials are 1) artificial materials whose electromagnetic properties (ε, μ) are negative or can be controlled, 2) made up of periodic arrays of metallic resonant elements, and 3) both the size of the element and the unit cell are small relative to the wavelength [12].
References
1. (wikipedia). METAMATERIALS
2. http://www.audiodesignline.com/news/205602196
3. http://www.ee.duke.edu/~drsmith/about_metamaterials.html
4. Caloz, Christophe, et. al., New J. of Phys., 2005, 7, 167, available Online at http://www.njp.org/ doi:10.1088/1367-2630/7/1/167
5. Veselago, V. G., Sov. Phys.— Usp., 1968, 10, 509
6. Mackay, Tom G., and Lakhtakia, Akhlesh, Curr. Sci., 2006, 90, 641
7. As quoted by Shalaev, Vladimir M. et. al. in Optics Letters, 2005, 30, 3356. Available online: http://www.osa.org/news/pressroom/release/12.2005/Negative%20index%20of%20refraction%20paper%20OL%20Dec.%2015.pdf
8. Shelby, R. A., Smith, D. R. and Schultz, S., Science, 2001, 292, 77.
9. http://www.purdue.edu/UNS/html4ever/2005/051130.Shalaev.negative.html
10. http://www.osa.org/news/pressroom/release/12.2005/Negative%20index%20of%20refraction%20paper%20OL%20Dec.%2015.pdf
11. http://www.nserc.ca/news/2004/p040311_bio3.htm
12. http://www.serv.physik.uos.de/metamaterials/introduction.html
These materials are known as metamaterials (MTMs) and the term was first coined in 1999 by Rodger M. Walser of the University of Texas, Austin [1]. These exotic MTMs have such properties by virtue of an internal structure that resonates in the presence of waves [2].
According to Caloz et. al. [3], metamaterials are: ‘a new class of ordered composites that exhibit exceptional properties not readily observed in nature. These properties arise from qualitatively new response functions that are: (1) not observed in the constituent materials and (2) result from the inclusion of artificially fabricated, extrinsic, low dimensional inhomogeneities’.
In other words, MTMs are artificial effective structures exhibiting useful [electromagnetic-optical] properties not readily available in Nature [4]. They offer unlimited interesting scientific and technological possibilities.
The development of the first metamaterials with metal-lens antennas and metallic delay lenses in the late 1940s by W.E. Kock [1] initiated the MTM revolution. In 1967, the German Physcist Victor Veselago theoretically introduced the first Left-handed (LH) materials [5]. These materials have a negative refraction index (NRI), which Veselago [5] attributed to the simultaneous presence of negative permeability (ε) and negative permittivity (μ).
These MTMs are called ‘left-handed’ because they do not obey the conventional ‘right-hand rule’ for natural materials, and can direct the phase velocity of electromagnetic waves opposite to the power flow [6]. This anomalous behaviour is attributed to the special property of such materials to exhibit negative refractive index (NRI) and for that reason, these materials are also known as negative index materials (NIMs) or negative-phase-velocity (NPV) materials [6].
Due to their special and unusual properties and future technological possibilities, LH-MTMs or NIMs among MTMs have caught special attention of researchers. Therefore, most of the researchers are focusing their research on developing such materials and their noble new uses.
MTMs are all set to bring in another technological revolution. Scientists are busy developing MTMs with desired properties at nano level. Some interesting possibilities are: development of invisibility cloaks, acoustic cloaks, increasing the storage capacity of CDs and DVDs, advances in fibre-optic communications, more effective use of solar energy, super-lenses and better imaging technologies, microwave applications, agile antennas, etc.
J. B. Pendry of the Imperial College London, for the first time, theorized a practical way to make a left-handed metamaterial (LH-MTM) or NIM. He also predicted that LH-MTMs or NIMs can act as a superlens and their imaging resolution is not limited by the wavelength but rather by material quality [7].
Shelby, Smith and Schultz have also designed MTMs which can act as NIMs at microwave wavelengths [8].
Shalaev et. al. at Purdue University have experimentally made the first MTM that has a NRI in the wavelength of visible light [9]. They reported their research in the Journal Optics Letters [10], demonstrating a negative refractive index material for the optical range, specifically for wavelengths close to 1.5 μm (200 THz frequency) by using a metal–dielectric composite consisting of tiny parallel "nanorods" of gold. The rods being metal conduct current and create an effect called optical inductance (L). The dielectric medium between rods acts as an optical capacitance element (C). Thus the material consists of tiny LC circuits which by plasmon resonances result in the reversal of refraction. Using this material, they [10] succeeded in obtaining a negative refractive index of -0.3 at the optical communication wavelength of 1.5 μm.
Dr. George Eleftheriades experimentally demonstrated the first flat LH MTM lenses which can bend light negatively and can provide super-resolution by focusing the evanescent waves [11]. These super lenses enhance resolution by increasing the amplitude of evanescent waves (which carry an object’s sub-wavelength details), says Eleftheriades [11].
An MTM can affect electromagnetic waves only if it (MTM) has structural features smaller than the wavelength of the electromagnetic radiation it interacts with [1].
LH-MTMs show unique phenomena such as reversed Snell’s law of refraction, Doppler effect and Vavilov–Cerenkov radiation as well as subwavelength imaging diffraction [4]. This opens new doors of technological innovations.
Most of the MTMs being engineered are made up of periodic arrays of metallic resonant elements like Swiss-Roll, Split Ring Resonator and Capacitively Loaded Ring [12]. These consist of the inductor/conductor type structures organised in a special composite fashion.
Thus we can say that metamaterials are 1) artificial materials whose electromagnetic properties (ε, μ) are negative or can be controlled, 2) made up of periodic arrays of metallic resonant elements, and 3) both the size of the element and the unit cell are small relative to the wavelength [12].
References
1. (wikipedia). METAMATERIALS
2. http://www.audiodesignline.com/news/205602196
3. http://www.ee.duke.edu/~drsmith/about_metamaterials.html
4. Caloz, Christophe, et. al., New J. of Phys., 2005, 7, 167, available Online at http://www.njp.org/ doi:10.1088/1367-2630/7/1/167
5. Veselago, V. G., Sov. Phys.— Usp., 1968, 10, 509
6. Mackay, Tom G., and Lakhtakia, Akhlesh, Curr. Sci., 2006, 90, 641
7. As quoted by Shalaev, Vladimir M. et. al. in Optics Letters, 2005, 30, 3356. Available online: http://www.osa.org/news/pressroom/release/12.2005/Negative%20index%20of%20refraction%20paper%20OL%20Dec.%2015.pdf
8. Shelby, R. A., Smith, D. R. and Schultz, S., Science, 2001, 292, 77.
9. http://www.purdue.edu/UNS/html4ever/2005/051130.Shalaev.negative.html
10. http://www.osa.org/news/pressroom/release/12.2005/Negative%20index%20of%20refraction%20paper%20OL%20Dec.%2015.pdf
11. http://www.nserc.ca/news/2004/p040311_bio3.htm
12. http://www.serv.physik.uos.de/metamaterials/introduction.html
