Using Metamaterials to Defy Our Common Understanding of Light
Associate Chief Scientist
RIKEN Advanced Science Institute
‘Metamaterials’ — artificially created materials with nanostructures designed to control light — are attracting considerable attention for the development of a range of new technologies, such as very thin eyeglass lenses, optical microscopes that will allow the observation of atoms, optical fibers with no transmission loss, and technologies that could make a person invisible. The functions of metamaterials are beyond our common understanding of natural phenomena. Amid intensifying global competition over the development of metamaterials, Takuo Tanaka, Associate Chief Scientist at the RIKEN Advanced Science Institute, and his colleagues succeeded in February 2009 in developing a technology that uses light to fabricate metal into a three-dimensional nanoscale structure, thus making significant progress in metamaterials research. “Metamaterials will surely be able to expand the world of light,” says Tanaka. Attention is now being focused on research into how to use metamaterials to defy our common understanding of light.
Technology that will make a person invisible?
In 2006 John Pendry of the University of London, UK, and his colleagues published a paper stating that an object can be made invisible by covering it with a material having special refractive index properties. In their paper, they asserted that an object can be made invisible by diverting light coming from behind the object such that it is no longer blocked by the object. This proposal made headlines throughout the world as a possible technology for making a person invisible. However, the technology requires a material that can be used to arbitrarily control the traveling direction of light.
Light traveling through air changes direction when it enters a lens made of glass or plastic by a process called refraction. The degree of refraction, or the amount of deviation in the traveling direction of light at a boundary between two media, is determined by the refractive index (n), which is a constant for each medium. Light is an electromagnetic wave composed of interacting electric and magnetic components, which in turn interact with the medium through which the light is traveling. The degree of interaction between a material and the electric component is given by the relative permittivity (ε), while the interaction with the magnetic component is determined by the relative magnetic permeability (µ) of the material. The refractive index is then given by the product of the square roots of these two physical quantities: n = √ε√µ. It is common knowledge in optics, however, that at the wavelengths of visible light, natural substances only interact with the electric component of the electromagnetic wave, and not with the magnetic component; that is, the relative magnetic permeability of natural substances is fixed at unity, and the refractive index is determined solely by the relative permittivity.
“Creating metamaterials that interact with the magnetic wave of the light enables us to obtain various refractive indices,” says Tanaka. “‘Meta’ means ‘transcend’, and a metamaterial is a material that transcends the limits of our common understanding of optics.”
Using metamaterials with negative refractive index to observe atoms and molecules
How can we create metamaterials? “The principle is not difficult to understand. Just try to remember the phenomenon of electromagnetic induction you learned in your junior high school science class,” says Tanaka. Moving a magnet near a coil of wire causes an electric current to flow through the coil: that is the phenomenon of electromagnetic induction. “An electric current flows through the coil so that the changes in the magnetic field caused by the motion of the magnet can be canceled out. Use of this principle makes it possible to generate magnetism in a material in the form of coils even if the material is not itself magnetic. For example, nanoscale metal coils smaller than the wavelength of light could be incorporated into a transparent plastic or glass material. These coils would then interact with the magnetic waves of light to cause an electric current to flow through them in accordance with the principle of electromagnetic induction, changing the macroscopic magnetic permeability of the material.”
Metamaterials are now attracting worldwide attention, triggered by the paper published in 2000 by Pendry and his colleagues, who claimed, “A material with a negative refractive index would make it possible to use light to observe infinitesimally small objects.” Visible light has wavelengths of about 400–700 nm, and structures smaller than half the wavelength are not visually observable. This is the common understanding of light. However, the paper by Pendry and his colleagues has defied this common understanding; the use of light to observe living cells at the atomic or molecular level would greatly contribute to the development of life sciences. Furthermore, use of light to pattern a semiconductor circuit with a line width of several nanometers would dramatically improve the performance of computers.
“Professor Pendry and his colleagues claimed in their paper that a lens made of a metamaterial with a negative refractive index could be used to amplify the ‘near-field light’ that includes the information of structures that are smaller than the wavelength of light, and make the light reach our eyes. Near-field light can be generated when an object is irradiated with light, but it immediately diminishes without reaching our eyes. Thus, the successful amplification of near-field light would be a landmark event.”
As early as 2000, a research group at the University of California at San Diego, USA, successfully used microwaves, which are longer in wavelength than visible light, to fabricate a metamaterial with a negative refractive index, prompting global competition over the development of metamaterials with negative refractive indices for visible-light applications.
Metamaterials that do not reflect light
“A microscope is an optical instrument that combines various optical techniques. Based on the microscope techniques, I have studied how to use light to observe and fabricate three-dimensional structures. When I joined RIKEN in 2003, I began exploring a new research theme, and I became interested in metamaterials around 2004.”
Tanaka and his colleagues started by theoretically confirming the feasibility of developing metamaterials that can interact with the magnetic component of visible light. In 2005 they demonstrated theoretically, for the first time, that a metamaterial with a negative refractive index throughout the entire visible-light wavelength region can be created by fabricating nanoscale silver coils and arranging them in a special pattern (Fig. 1). Tanaka and his colleagues soon started to explore theoretically what can be achieved with metamaterials.
“The creation of negative refractive index materials requires negative values in terms of both relative permittivity and relative magnetic permeability. However, the creation of negative refractive index materials is not the only application of metamaterials. Since global attention was focused on negative refractive index, we attempted to investigate what would happen if the relative magnetic permeability is increased to a value larger than 1.0.”
In 2006 Tanaka and his colleagues proposed an idea for a metamaterial that eliminates light reflection completely by introducing a relative magnetic permeability of larger than 1.0. A material that does not completely reflect light would contribute significantly to the development of light-based information technology. “Light can maintain 99.9% of its intensity even if it is transmitted through 1 km of optical fiber. However, it loses as much as 4% of its intensity at the ends of the optical fiber. If a metamaterial that does not reflect light is used at the ends of the optical fiber, all of the light can be made to enter and exit the optical fiber.”
A more familiar example is a dramatically thinner lens with a larger relative magnetic permeability. A metamaterial with a refractive index five times greater than glass can be used to create a lens five times thinner than conventional lenses. This will also lead to the development of small, lightweight lenses for optical equipment such as cameras. Furthermore, a significantly greater demand for metamaterials with larger refractive indices is expected in various fields.
Three-dimensional nanoscale metal fabrication
How is it possible to create metamaterials that can interact with the magnetic component of light? The answer is to create a multitude of nanoscale coils smaller than the wavelength of light passing through the material.
In August 2008 a research group at the University of California at Berkeley, USA, announced that it had successfully fabricated metamaterials with a negative refractive index at near-infrared and visible wavelengths. One of the metamaterials consists of multiple layers of two-dimensional net-like structures, thus forming a number of coils. “The metamaterial can interact only with the magnetic waves from a certain direction. Creating a metamaterial that can interact with magnetic waves coming from various directions requires a number of coils arranged in every direction. This cannot be satisfied by a metamaterial consisting of multiple layers of two-dimensional net-like structures. Use of semiconductor processing techniques makes it possible to fabricate metal into nanoscale structures, but only two-dimensional structures. Thus, three-dimensional processing techniques are required to arrange coils in every direction.”
In February 2009 Tanaka and his colleagues attracted public attention when they announced that they had successfully used a light-irradiation method to fabricate metal into nanoscale three-dimensional structures. In this process, a transparent material is first mixed with gold or silver ions, which can be reduced to metal atoms when excited by ultraviolet light (wavelength: 100–400 nm). Tanaka and his colleagues showed that near-infrared lasers with a wavelength of about 800 nm can be used to selectively metalize ions at the laser focal point, which can be moved progressively to fabricate a three-dimensional nanostructure (Fig. 2).
Metallization usually occurs by ultraviolet irradiation, so how can metallization also be induced using a near-infrared laser? It is known that when near-infrared laser light is converted into ultra-short laser pulses of approximately 100 femtoseconds (1 femtosecond = 10–15 seconds) and focused into a very narrow beam using a microscope objective lens, the photon density at the focal point can be increased remarkably, triggering a phenomenon known as ‘two-photon absorption’ — the absorption of two photons (packets of light) instead of the usual one by each ion. As ultraviolet photons have twice the energy of near-infrared photons, the absorption of two near-infrared photons provides the same energy as a single ultraviolet photon, inducing metallization of the silver or gold ions.
“As far as I know, there are no facilities besides those at RIKEN that can provide techniques that make it possible to fabricate metal into three-dimensional nanoscale structures.” Thus, Tanaka and his colleagues have made a great step toward creating metamaterials that can interact with the magnetic component of light regardless of the incoming direction.
“However, we need to fabricate tens of trillions of microcoils for a cube of even 1 mm in size. There are many technical challenges to overcome, but I am really determined to fabricate metamaterials that can interact with the magnetic waves of light on my own.”
Video recording of a complete lifespan of 80 years
Tanaka and his colleagues are also working toward the development of optical disks with extremely large storage capacity by taking advantage of their light-based three-dimensional processing technology (Fig. 3). “A DVD is 1.2 mm thick, but only a layer with a thickness of about 1 µm is used for data storage. Thus, 99.9% of the disk is only a circular plastic board that supports the thin recording layer. If these thin layers are stacked into a single structure so that data can be stored three-dimensionally, we can possibly create a disk with extremely large storage capacity more than one million times that of a conventional DVD. In addition, the data retrieval speed will be drastically improved if information is stored three-dimensionally.”
Current DVDs have a data storage capacity of about 4.7 gigabytes, allowing for a maximum video recording time of about two hours. “If the data storage capacity is increased up to one million times that of the current DVD, we will be able to store a video recording with the same picture quality for a complete lifespan of 80 years, from birth to death. If the data storage capacity were increased by that amount it would eliminate the need to select the data to be stored. I think that the impact would be significant.”
Metaphotonics: beyond conventional light theory
Although Tanaka and his colleagues are currently working toward the development of optical disks with extremely large storage capacity, Tanaka also says, “I want to be fully engaged in science through research into metamaterials. I decided to engage in research into metamaterials because I thought that I could write a new textbook on optics. Conventional optics deal only with materials having a relative magnetic permeability of 1.0. This is a one-dimensional, very small world. Use of metamaterials with various relative magnetic permeabilities would greatly expand the world of light.”
Tanaka named the new field of academic study that deals with the wide world of optics ‘metaphotonics’, because the new field transcends the world of conventional optics and photonics (Fig. 4). “Conventional textbooks on optics are written on the assumption that materials do not interact with the magnetic waves of light. However, nobody knows what will happen if materials interact with the magnetic waves of light. Thus, I think we need to actually create metamaterials, and to verify the content of textbooks on optics. I am sure that we will be able to discover unknown phenomena and unexpected treasures buried in the wide world of light.”
Metaphotonics is emerging as a new field of study that takes advantage of metamaterials to explore the world of light.
About the researcher
Takuo Tanaka was born in Hyogo, Japan, in 1968. He graduated with a BSc from the department of Applied Physics, Faculty of Engineering, Osaka University, in 1991, and later obtained his MSc and PhD in Applied Physics from the same university in 1993 and 1996. He then joined the Department of Electrical Engineering, Faculty of Engineering Science, Osaka University, as Assistant Professor. In 2003 he moved to the RIKEN Nanophotonics Laboratory where he worked as a research scientist. He was promoted to Senior Research Scientist in 2005, and to Associate Chief Scientist in 2008, and is now working as head of the Metamaterials Laboratory in the RIKEN Advanced Research Institute. His research background is in three-dimensional microscopy, and his current research interests include nanophotonics, plasmonics and metamaterials, and their applications in functional photonics devices.